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Nutrition for Sport, Exercise, and Health
by Marie Spano, Laura Kruskall and D. Travis Thomas
488 Pages
Nutrition for Sport, Exercise, and Health includes applied content and research-based guidelines to help students distinguish between nutrition recommendations backed by science and the plethora of misinformation available in the field. This comprehensive resource blends nutrition and exercise science with practical information to provide a clear understanding of how nutrition affects sports, exercise, and overall health.
Nutrition for Sport, Exercise, and Health covers the basics of nutrition, including the functions of and daily allowances for carbohydrate, fat, and protein, as well as micronutrient recommendations; the importance of hydration and electrolyte balance; nutrition in health and disease prevention; population-based nutrition considerations for training and sports; and practical information on measuring and altering body composition. The accessible presentation of material keeps students from getting too bogged down in research, and the text offers real-world applications. Students will also discover career opportunities available to them, including qualifications and job responsibilities for each position.
The full-color text includes more than 70 photos and more than 140 illustrations alongside digestible, engaging writing. Concepts are presented in a user-friendly manner, and each chapter includes a number of features that enhance understanding:
• Chapter objectives provide a roadmap to ease students into upcoming content.
• Key terms help students focus on important vocabulary. The key terms are identified at the beginning of the chapter, appear in boldface within the chapter, and are included within the glossary, where they are defined.
• Putting It Into Perspective sidebars contain compact vignettes that help college students relate to the content and apply the concepts to their own lives.
• Do You Know? sidebars are short callouts that provide key insights and easy takeaways for students.
• Review questions help students identify areas they may need to revisit as well as reinforce key concepts.
Content is organized in a logical sequence, with each chapter building upon the information previously presented. In part I, the reader is provided with an overview of the role nutrition plays in overall well-being throughout a person's life. Part II focuses on each macronutrient and its role in health and disease, as well as dietary recommendations that support health and an active lifestyle. The role of micronutrients in health and performance is covered in part III. Part IV provides information on the application of nutrition to sport, exercise, and health.
Instructors will find a full suite of ancillaries that will be helpful in their teaching. The instructor guide and presentation package plus image bank will help in preparing for class, while the test package and chapter quizzes will help assess student learning.
Students and professionals alike will benefit from the broad coverage found in Nutrition for Sport, Exercise, and Health. Armed with accessible, research-based application, readers will have the tools they need to improve athletic performance, exercise outcomes, and general well-being.
Part I: The Big Picture
Chapter 1: Optimizing Health and Well-Being Throughout the Lifespan
Chapter 2: Energy Metabolism
Part II: Role of Energy-Yielding Macronutrients
Chapter 3: Carbohydrate
Chapter 4: Fat
Chapter 5: Protein
Part III: Role of Micronutrients, Water, and Nutritional Supplements
Chapter 6: Vitamins
Chapter 7: Minerals
Chapter 8: Water and Electrolytes
Chapter 9: Nutritional Supplements and Drugs
Part IV: Application of Nutrition for Health and Fitness
Chapter 10: Body Weight and Composition
Chapter 11: Nutrition for Aerobic Endurance
Chapter 12: Nutrition for Resistance Training
Chapter 13: Changing Weight and Body Composition
Chapter 14: Special Nutrition Concerns
Marie A. Spano, MS, RD, CSCS, CSSD, is one of the country’s leading sports nutritionists and the sports performance nutritionist for the Atlanta Hawks, Atlanta Braves, and Atlanta Falcons. She combines science with practical experience to help athletes implement customized nutrition plans to maximize athletic performance, recovery, and career longevity. Also a nutrition communications expert, Spano has appeared on CNN as well as on NBC, ABC, Fox, and CBS affiliates. She has authored hundreds of magazine and trade publication articles in addition to book chapters in NSCA's Essentials of Personal Training and Essentials of Strength Training and Conditioning. She is coeditor of NSCA's Guide to Exercise and Sport Nutrition.
A three-sport collegiate athlete, Spano earned her master’s degree in nutrition from the University of Georgia, where she worked in the athletic department as a graduate assistant running the sports nutrition program, and her bachelor’s degree in exercise and sports science from the University of North Carolina at Greensboro, where she also ran Division I cross country. Her experiences as a college athlete provide her an effective perspective when working with athletes of all levels, especially student athletes, by giving her a firsthand understanding of how the demands of athletics, psychological aspects of injury, sleep, recovery, and nutrition can affect an athlete’s overall well-being and performance.
Laura J. Kruskall, PhD, RDN, CSSD, LD, FACSM, FAND, is an associate professor and director of nutrition sciences at University of Nevada, Las Vegas. She oversees the nutrition sciences academic degree programs and serves as program director for the ACEND-accredited dietetic internship. She has held numerous leadership positions at the local, state, and national levels, including serving on the board of trustees of the American College of Sports Medicine (ACSM) and as the cochair of the committee that authored “Standards of Practice and Standards of Professional Performance for Registered Dietitian Nutritionists (Competent, Proficient, and Expert) in Sports Nutrition and Dietetics,” published in the Journal of the Academy of Nutrition and Dietetics in 2014. She is currently serving as the chair of the nutrition track for the ACSM Health and Fitness Summit and Exposition and is a member of the editorial board for ACSM’s Health & Fitness Journal. Dr. Kruskall has been given fellow status by both ACSM and the Academy of Nutrition and Dietetics for her leadership and contributions to the profession.
Dr. Kruskall’s areas of teaching and practice expertise are sports nutrition, weight management, and medical nutrition therapy. Her research interests include the effects of nutrition and exercise interventions on body composition and energy metabolism and the role of vitamin D in bone health and performance in collegiate athletes. In addition to her academic duties at the university, she is a nutrition consultant for Canyon Ranch SpaClub and Cirque du Soleil in Las Vegas.
Dr. Kruskall earned her PhD in nutrition from Penn State University. She also holds a certificate of training in Adult Weight Management Level 2 from the Commission on Dietetic Registration, is certified as an exercise physiologist by ACSM, holds the Exercise Is Medicine Credential II from ACSM, and is a Board Certified Specialist in Sports Dieteticss (CSSD).
D. Travis Thomas, PhD, RDN, CSSD, LD, FAND, is an associate professor of clinical and sports nutrition in the College of Health Sciences and director of the undergraduate certificate program in nutrition for human performance at the University of Kentucky. Dr. Thomas is a Board Certified Specialist in Sports Dietetics (CSSD). He has held multiple national leadership positions for SCAN (a dietetic practice group of the Academy of Nutrition and Dietetics) and received honors as the lead author on the position statement “Nutrition for Athletic Performance” (March 2016, published in three journals).
Dr. Thomas became a registered dietitian in 2001 and worked as a clinical dietitian in Greensboro, North Carolina, managing medical nutrition therapy for patients in general medicine, cardiac, rehabilitation, and intensive care units. Dr. Thomas then completed his PhD in nutrition at the University of North Carolina at Greensboro and a postdoctoral fellowship at the University of Kansas. Throughout his doctoral and postdoctoral studies, he expanded his clinical knowledge base to include aspects of exercise physiology, sports nutrition, body composition, and hormone physiology.
Dr. Thomas has more than nine years of experience conducting human studies involving nutrition and exercise interventions across the lifespan. Over the last six years, Dr. Thomas has served as an investigator on several research projects that focused on nutrition issues associated with the preservation and enhancement of skeletal muscle function and performance. These studies have included examining the relationship between vitamin D and muscle metabolic function, studies on nutrition and physical function in aging and athletic populations, nutrition interventions to reduce symptoms in patients with advanced heart failure, and investigating nutrition strategies to preserve physical performance and lean body mass in patients with head and neck cancer. Collectively, this work has resulted in multiple publications and has led to NIH funding to examine the contribution of vitamin D to muscle metabolic function.
Dr. Thomas is married and has two children, Averie and Collin. In his free time, he enjoys traveling, staying physically active, cooking, and gardening. He is an avid fan of collegiate sports.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
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Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
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Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
Learn more about Nutrition for Sport, Exercise, and Health.
Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
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Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
Learn more about Nutrition for Sport, Exercise, and Health.
Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
Learn more about Nutrition for Sport, Exercise, and Health.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
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Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
Learn more about Nutrition for Sport, Exercise, and Health.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
Learn more about Nutrition for Sport, Exercise, and Health.
Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
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Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
Learn more about Nutrition for Sport, Exercise, and Health.
Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
Learn more about Nutrition for Sport, Exercise, and Health.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
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Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
Learn more about Nutrition for Sport, Exercise, and Health.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
Learn more about Nutrition for Sport, Exercise, and Health.
Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
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Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
Learn more about Nutrition for Sport, Exercise, and Health.
Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
Learn more about Nutrition for Sport, Exercise, and Health.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
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Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
Learn more about Nutrition for Sport, Exercise, and Health.
Nutrition Before Resistance Training
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger.
The main purpose of the preexercise meal, consumed within an hour or less before resistance training, is to hydrate the athlete, top off glycogen stores, and decrease hunger. In addition, adding protein to one's preexercise meal or snack might be advantageous if the athlete hasn't consumed protein for several hours prior, or if their daily protein needs are high and the preexercise meal represents an important opportunity to help them meet their daily protein needs. For instance, a 280-pound male athlete who is trying to gain 20 pounds will have substantial daily protein needs and might find it challenging to consume the quantity of food (including protein) necessary to gain weight, particularly if he is training for several hours a day (more time spent training means less available hours to eat).
Hydration
Staying hydrated is important for resistance training. Hypohydration compromises resistance-training performance and recovery. However, methodological considerations in study design make it difficult to clearly distinguish the mechanism through which hypohydration affects strength, power, and high-intensity exercise. Cardiovascular strain such as decreases in maximal cardiac output and blood flow to muscle tissue (and thus a decline in the delivery of nutrients and metabolite removal) are plausible contributing factors. The studies to date indicate dehydration of 3 to 4 percent body weight loss reduces muscle strength by approximately 2 percent, muscular power by approximately 3 percent, and high-intensity endurance activity (maximal repeated activities lasting >30 seconds but
Hypohydration also impacts the hormonal response to exercise. In one resistance training study, increasing levels of hypohydration (from 2.5% of body weight to 5% of body weight) led to progressive increases in the stress hormones cortisol and norepinephrine, and a subsequent increase in blood glucose, presumably to cope with increased physiological demands (the stress response leads to greater energy availability). These results suggest hypohydration significantly enhances stress from resistance exercise and could impair training adaptations. Over time, these changes could decrease training adaptations to resistance training if consistently performed in a hypohydrated state.
Given the impact of hypohydration on strength, power, and repetitive activity lasting more than 30 seconds but less than 2 minutes, athletes engaging in resistance-training sessions or participating in sports that require these variables, such as American football, soccer, wrestling, ice hockey, and rugby, should ensure they are adequately hydrated prior to training or competition. There are no specific pretraining guidelines for resistance exercise because any recommendations depend on hydration status prior to exercise.
Carbohydrate
Carbohydrate, from circulating blood sugar and muscle glycogen, is the primary source of fuel used during resistance training. In addition, maintaining adequate glycogen stores can help attenuate muscle breakdown during exercise and keep both the immune and nervous system functioning normally. Low carbohydrate intake can acutely suppress immune and central nervous system functioning.
Do You Know?
Muscle breakdown increases when resistance training is performed on an empty (fasted) stomach.
Consuming carbohydrate prior to training can help decrease muscular fatigue, particularly in fast-twitch muscle fibers, which tire quickly compared to slow-twitch muscle fibers; spare the use of protein as a source of energy; and perhaps also improve performance. This strategy is particularly important for athletes who exercise first thing in the morning after an overnight fast; those who haven't consumed enough carbohydrate in the time period since their last training session; and those who are lifting weights right after speed work, endurance exercise, or any other type of training that requires a significant amount of carbohydrate for energy. Like many aspects of nutrition, there is a caveat to the need for carbohydrate prior to resistance training. The body can adapt to sustained alterations in the intake of energy-yielding macronutrients (carbohydrate, protein, fat), and thus individuals on a low-carbohydrate diet might not experience any negative effects or performance decrements once adapted to this diet, provided their diet contains enough protein to build and repair muscle and energy to help spare protein losses (from protein breakdown in muscle as a source of energy). However, there is a paucity of data on this topic, so at this time low-carbohydrate diets are not recommended for those engaging in a resistance-training program.
In one study, six trained men were given either a carbohydrate supplement (1 g of carbohydrate per kg body mass before exercise and 0.17 g of carbohydrate per kg body mass every 6 minutes during the session) or a placebo sweetened with saccharin and aspartame (nonnutritive sweeteners). They performed a series of static contractions of the quadriceps at 50 percent maximum contraction with 40 seconds of rest between sets until muscle failure (i.e., they couldn't do anymore). Time to exhaustion and force output were significantly higher in the group receiving carbohydrate compared to the group consuming the placebo.
Carbohydrate loading is a technique endurance athletes have used for several decades to super-compensate glycogen stores and improve performance. In general, the athlete will taper their training program for a specified period of time - from a few days to weeks prior to an event - while consuming a higher-carbohydrate diet, generally between 8 and 10 grams of carbohydrate per kilgram body weight each day. Few studies have looked at the effect of carbohydrate loading on resistance exercise performance. However, in one study, healthy young men were randomized to receive either a moderate-carbohydrate diet (4.4 g of carbohydrate per kg body weight) or higher-carbohydrate diet (6.5 g of carbohydrate per kg body weight) for 4 days before a resistance exercise test, including 4 sets of 12 repetitions of maximal-effort jump squats with a load of 30 percent of 1 repetition maximum (1RM) and a 2-minute rest period between sets. Power performance didn't differ between groups, indicating a higher-carbohydrate diet did not enhance power-endurance performance over four sets of exercise. However, it isn't clear if an even greater intake of carbohydrate, reaching 8 to 10 grams per kilogram body weight, would have led to a performance difference or if the diet used would have made a difference over the course of several sets.
Bodybuilders might carbohydrate-load prior to competition to increase muscle size, a practice that makes sense physiologically (particularly if they consume a lower-carbohydrate diet for a few days, followed by carbohydrate loading), yet only one study to date has examined this practice. There was no change in muscle size following a low- versus high-carbohydrate diet; however, study subjects consumed the same amount of total energy during the period of low-carbohydrate intake (3 days of 10% energy from carbohydrate) as they did during the 3-day period of carbohydrate loading (80% energy from carbohydrate). Thus it is possible that a difference might have been noted if they also increased total energy intake.
Do You Know?
Muscle size and strength do not increase together in a direct linear fashion.
Protein and Amino Acids
Research shows that protein or essential amino acids (EAAs) consumed prior to resistance training will stimulate muscle protein synthesis, and prolonged supplementation can improve lean mass, body fat percentage, and muscle hypertrophy. Protein can be taken pre- or postexercise to enhance acute muscle protein synthesis.
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Protein in Exercise and Sport
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
Gluconeogenesis
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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Nutrients
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories).
There are six groups of nutrients: carbohydrate, lipids, protein, vitamins, minerals, and water. Alcohol is not a nutrient, but it does contain energy (calories). Carbohydrates, fats (including fatty acids and cholesterol), protein (including amino acids), fiber, and water are macronutrients, which are required in the diet in larger amounts. Carbohydrates, fats, and proteins are also referred to as energy-yielding macronutrients becausethey supply the body with energy. Vitamins and minerals are micronutrients, which are required in the body in smaller amounts in comparison to macronutrients. A common misconception is that vitamins and minerals are energy nutrients. These do not contain energy, though they play essential roles in the production of energy. Deficiencies of certain vitamins and minerals can lead to fatigue. Macronutrients and micronutrients work together for optimal physiological function. The unit of energy in food is called a kilocalorie, commonly referred to as a calorie or kcal. A kilocalorie is the amount of heat it takes to raise the temperature of one kilogram of water by one degree Celsius. A person's energy requirements refer to the number of kilocalories needed each day. Food labels list calories per serving of the item. Both carbohydrate and protein contain 4 calories per gram, while lipidsprovide 9 calories per gram, making lipids more energy dense - that is, they contain more calories permass or volume than do carbohydrate or protein.
Nutrition Tip
Counting your carbs? Note that while all vegetables contain primarily carbohydrate, their content per serving is not the same. Vegetables can be categorized as starchy or nonstarchy. Starchy vegetables, such as potatoes, corn, and peas, contain approximately 15 grams of carbohydrate in a half-cup serving. Nonstarchy vegetables, such as broccoli, beets, and asparagus, contain considerably less carbohydrate - about 5 grams per half-cup serving. So if you want to reduce your intake of carbohydrate, choose nonstarchy vegetables.
Dietary Reference Intakes
The Institute of Medicine (IOM) of the United States Department of Agriculture (USDA) developed the Dietary Reference Intakes (DRIs), a set of recommendations based on the latest "scientific knowledge of the nutrient needs of healthy populations" (figure 1.1). The DRIs include:
- Estimated Average Requirement (EAR). The EAR is the estimated mean daily requirement for a nutrient as determined to meet the requirements of 50 percent of healthy people in each life stage and gender group (different amounts are provided based on age ranges and life stages, such as pregnancy and lactation). The EAR is based on the reduction of disease and other health parameters. It does not reflect the daily needs of individuals but is used to set the RDA and for research purposes.
- Recommended Dietary Allowance (RDA). The RDA is set to meet the needs of nearly all (97 - 98%) healthy people in each gender and life stage. This is the amount that should be consumed on a daily basis. The RDA is two standard deviations above the EAR based on variability in requirements, or if the standard deviation is not known, the RDA is 1.2 times the EAR.
- Adequate Intake (AI). The AI is the recommended average daily nutrient level assumed to be adequate for all healthy people. The AI is based on estimates - observed or experimentally determined approximations - and used when the RDA cannot be established because of insufficient data.
- Tolerable Upper Intake Level (UL). The UL is the highest average daily intake considered safe for almost all individuals. The UL represents average daily intake from all sources, including food, water, and supplements. Lack of a published UL does not indicate that high levels of the nutrient are safe. Instead, it means there isn't enough research available at this time to establish a UL.
- Acceptable Macronutrient Distribution Range (AMDR). The AMDR is a range given as a percentage of total calorie intake - including carbohydrate, protein, and fat - and is associated with a reduced risk of chronic disease and adequate intake of essential nutrients.
- Estimated Energy Requirement (EER). The EER is the average daily energy intake that should maintain energy balance in a healthy person. Factors such as gender, age, height, weight, and activity level are all considerations when calculating this value.
Figure 1.1 Dietary Reference Intakes. The AI or RDA describes the recommended daily amount of a nutrient, while the UL describes the amount not to exceed. Too little or too much of a nutrient can increase the risk of undesirable effects.
Energy-Yielding Macronutrients
The primary function of carbohydrate is to provide energy. Carbohydrate becomes increasingly important when exercising at a high intensity. High-intensity exercise increases energy (calorie) demands, and carbohydrate is a fast source of energy - the body can quickly access it - whereas fat, another source of energy, is much slower in meeting the body's demand for energy during high-intensity exercise. Carbohydrate can be stored, to an extent, in the human body as an energy reserve in the form of glycogen in the liver and muscle. The AMDR for carbohydrate is 45 to 65 percent of kilocalories for both males and females ages 19 and older. Common sources of carbohydrate are rice, pasta, wheat products, and grains such as corn, beans, legumes, fruits, vegetables, and milk. Some carbohydrates are considered nutrient-dense because they contain nutrients important for good health, including vitamins, minerals, and dietary fiber. A type of carbohydrate, fiber is discussed in detail in chapter 3.
Dietary fats and oils are examples of lipids. Dietary fats and oils provide energy and aid in the absorption of fat-soluble vitamins and food components. Fat can be stored in limitless quantities in the body, serving as an energy reserve. Fat can be an important energy source during long-duration activities, such as an ultra-endurance race. The AMDR for fat is 20 to 35 percent for both males and females ages 19 years and older. Common sources of fats include meat, nuts, seeds, oils, dairy products, and vegetable spreads.
Protein can be used for energy, but its primary function is to support cell and tissue growth, maintenance, and repair. Unlike carbohydrate and lipids, the primary purpose of protein is not to supply energy, so it is important to get an adequate amount on a regular basis. The RDA for protein is 56 grams per day for males aged 19 and over, and 46 grams per day for females aged 14 and over. The RDA for pregnant and lactating women of all ages is 71 grams per day. Despite these recommendations, considerable evidence suggests that the RDA for protein is too low to support muscle growth and maintenance, particularly for athletes and older adults. The AMDR for protein is 10 to 35 percent of calories for both males and females ages 19 and older. Common sources of protein include poultry, beef, fish, eggs, dairy foods, and some plant foods, particularly soy foods, nuts, and seeds.
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Dehydration and Hypohydration
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe.
Whereas dehydration refers to the process of losing body water, hypohydration is the uncompensated loss of body water. Depending on the amount of body fluid lost, hypohydration can be mild, moderate, or severe. Symptoms of mild to moderate hypohydration include thirst, dry mouth, low urine production, dry and cool skin, headache, muscle cramps, and dark urine (not to be confused with bright yellow or orange urine from B vitamins, carotene, or certain medications). Signs and symptoms of severe hypohydration include
- increased core body temperature,
- decreased blood pressure (hypotension),
- decreased sweat rate,
- rapid breathing,
- fast heartbeat,
- dizziness or lightheadedness,
- irritability,
- confusion,
- lack of urination,
- sunken eyes,
- shock,
- unconsciousness,
- delirium,
- dry, wrinkled skin that doesn't "bounce back" quickly when pitched,
- reduced stroke volume and cardiac output,
- reduced blood flow to muscles,
- exacerbated symptomatic exertional rhabdomyolysis (serious muscle injury), and
- increased risk of heat stroke and death.
Nutrition Tip
Your hydration level might affect your ability to study and do well on tests. A body water loss of just 1 to 2 percent (1.5 - 3 lbs. for a 150-lb. person) can impair concentration and short-term memory, while increasing reaction time, moodiness, and anxiety.
During exercise, hypohydration occurs when fluid intake doesn't match water lost through sweat. Risk for hypohydration is greater in hot, humid environments and at altitude. Clothing, equipment, heat acclimatization, exercise intensity, exercise duration, body size, and individual variations in sweat rates all affect risk of hypohydration. With repeated exposures to hot environments, the body adapts to heat stress, and cardiac output and stroke volume returns to normal, sodium loss is conserved, and the risk for heat-related illness is reduced.
Sickle cell trait, cystic fibrosis, diabetes medications, diuretics, and laxatives increase risk of dehydration. Children and the elderly also have an increased risk of dehydration.
Will Supplements Make Me Dehydrated?
Reflection Time
Certain ingredients commonly found in detox products, cleanses, and weight-loss supplements will increase water lost through urine. Some of these products include uva ursi, dandelion (Taraxacum officinale), burdock root, horsetail, and hawthorn. Though many people believe creatine increases dehydration, no research supports this.
Several studies suggest children are at greater risk than adults for dehydration and heat illness. Children have more body surface area relative to their body weight, leading to greater heat gain from the environment. They have a lower sweat rate, and thus a decreased ability to dissipate heat through sweat (though this conserves body water) and higher skin temperature. Children also take longer to acclimatize to heat. Some studies suggest that children rehydrate as well as, if not better than, adults, whereas other studies show children, like adults, do not drink enough to adequately replace fluid losses in warm temperatures, even when they have sufficient access to fluids. However, sweat sodium concentration is lower in children than in adults, a factor that may help children retain fluid. Signs and symptoms of heat illness are presented in figure 8.4.
Figure 8.4 Heat illness.
Based on Binkley 2002.
Older adults do not have a sensitive thirst mechanism, so they generally drink less fluid than younger adults. In addition, the kidneys do not conserve water as well with age. Older individuals on certain medications might have increased fluid requirements or increased fluid losses. Some elderly people have limited access to food and fluids because of impaired motor skills, injuries, diseases, or surgeries that limit mobility. All these factors put the elderly at greater risk for heat stress, dehydration, and hypohydration.
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