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Biologic Regulation of Physical Activity
224 Pages
The benefits of a healthy lifestyle are well documented, yet many people continue to struggle with sedentary behavior and obesity. In Biologic Regulation of Physical Activity, Dr. Thomas W. Rowland posits a distinct possibility of the existence of a central biologic controller of activity. If harnessed, this mechanism could lead to breakthroughs in health science professionals’ quest for more effective ways of helping people be more active and, as a result, healthier.
Rowland is one of the most well-respected pediatric cardiologists in the United States. He has authored three other books and more than 150 journal articles and has served in several key national leadership positions in pediatric medicine. In Biologic Regulation of Physical Activity, Rowland uses his expertise, along with numerous references and direct quotes from expert witnesses, to provide a detailed account of how current research may support the existence of a biologic regulator—a mechanism in the brain that involuntarily controls biological processes—associated with physical activity. Rowland proposes a possible mechanism for such a control and explores the implications of this theory. This developing area of research and theory offers a new lens through which health professionals and those who research issues related to obesity, physical activity adherence, and sedentary behaviors can view their work.
The book moves methodically through the research, rationale, and implications of a biologic regulator of physical activity. In part I, Surveying the Evidence, readers are guided through a litany of research—both on humans and on animals—that provides support for the existence of a biologic regulator. This section synthesizes evidence from an interdisciplinary perspective, covering research on topics such as behavioral disorders, brain damage, lifetime activity patterns, and sex differences.
Part II, Rationale and Mechanisms details the possible biologic explanation for control of energy output through activity and proposes a mechanism by which it might function in order to maintain an energy in–energy out balance. The hypothesis presented in this section is that the body has a need for energy balance that leads to activity regulation, similar to how the body regulates appetite.
In part III, Implications of Biologic Regulation of Activity, some clear implications from current research, which may help health science professionals in their treatment and prevention efforts against patients’ obesity and inactivity, are discussed. Rowland also poses some critical questions for further research, if indeed a biologic controller of activity exists, such as how much effect a biologic controller might have on activity level as compared to environmental factors and whether this biologic regulator could be altered.
This book will initiate further discussion, examination, and research into the idea that physical activity may be, at least in part, controlled by a central biologic regulator. Further study may lead to a widespread realization that there is an involuntary biologic regulator of activity that, once fully understood, could lead researchers to discover alternative interventions in the fight against inactivity and obesity.
Part I. Surveying the Evidence
Chapter 1. Nature of Physical Activity
Measuring Physical Activity
Categorizing Physical Activity
Chapter 2. Physical Activity Through the Life Span
Human Beings
Physical Activity of Animals
Chapter 3. Effects of Sex
Sexual Maturation
Sex Differences in Infancy
Chapter 4. Neurochemical Models
Dopamine
Other Neurochemical Mediators
Chapter 5. Perturbations of Brain Function
Lesions in Animal Brains
Craniopharyngioma
Anorexia Nervosa
Restless Legs Syndrome
Attention Deficit Hyperactivity Disorder
Chapter 6. Organized Variability
Animal Circadian Rhythms
Human Circadian Rhythms
Other Variability
Chapter 7. Genetic Influences
Familial and Twin Studies
Animal Selection
Genetic Markers
Epigenetic Influences
Chapter 8. Physical Activity Play
Function of Physical Play
Neurological Basis
Part II. Rationale and Mechanisms
Chapter 9. Activity Regulation and the Need for Energy Balance
Energy Balance as a Biological Need
Role of Physical Activity in Energy Balance
Biologic Origin of Other Contributors to Energy Balance
Parallel Decline With Aging
Compensatory Responses in Energy Balance
Chapter 10. Mechanisms for Biologic Control
Feedback Systems
Proposed Biologic Control System for Habitual Physical Activity
Activity-Stat Versus Energy-Stat
Part III. Implications of Biologic Regulation of Activity
Chapter 11. Responses to Activity Interventions
Compensatory Changes in Physical Activity
Compensatory Changes in Caloric Intake
Long-Term Changes in Physical Activity Habits
Implications for Health Promotion
Chapter 12. Understanding Obesity: The Biologic Perspective
First Law of Thermodynamics
Obesity as an Error in Energy Balance
Behavioral Explanations for Energy Imbalance
Genetic Explanations for Energy Imbalance
Implications for Treatment and Prevention
Chapter 13. Altering the Biologic Control of Activity
Plasticity of Biologic Set Points
Can Cognitive Will Override Biologic Control?
Can Hedonistic Behavior Override Biologic Control?
Role of Spontaneous Physical Activity and NEAT
Pharmacological Manipulation of Physical Activity Regulators
Thomas W. Rowland, MD, is a pediatric cardiologist at the Baystate Medical Center in Springfield, Massachusetts. He is a professor of pediatrics at Tufts University School of Medicine and was a past adjunct professor of exercise science at the University of Massachusetts at Amherst. A graduate of the University of Michigan Medical School, Rowland is board certified in pediatrics and pediatric cardiology by the American Board of Pediatrics.
Rowland, who has had more than 150 journal articles published, is the author of three books: Children’s Exercise Physiology, Second Edition; Tennisology: Inside the Science of Serves, Nerves, and On-Court Dominance; and The Athlete’s Clock. He has served as editor of the journal Pediatric Exercise Science and president of the North American Society for Pediatric Exercise Medicine (NASPEM) and was on the board of trustees of the American College of Sports Medicine (ACSM). He is past president of the New England chapter of the ACSM and received the ACSM Honor Award in 1993.
Rowland is a competitive tennis player and distance runner. He and his wife, Margot, reside in Longmeadow, Massachusetts.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
Physical activity levels: Familial and twin studies
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual’s daily energy expenditure by motor activity.
The premise to be weighed in the pages that follow is this: There exists within the central nervous system an inherent control center that serves to regulate an individual's daily energy expenditure by motor activity. According to this concept, such an activity governor within the brain is involuntary and acts to influence levels of activity beneath the level of consciousness, differentiating it from motor centers within the cerebral cortex responsible for purposeful muscular activity. This governor is a shared function throughout the animal kingdom. It may act as a means of maintaining the body's energy balance, and its existence is consistent with other feedback regulatory centers in the brain critical to maintaining homeostasis, including controllers of temperature, pH, body fluid content, and blood glucose levels.
The credibility of this concept is strengthened by a considerable body of observational and interventional data from diverse sources in both human and animal models supporting the existence of such an involuntary biologic influence on activity energy expenditure. And, in providing a scientific foundation for such a brain function, that information raises significant questions regarding the quantitative importance and plasticity of a biologic controller in response to extrinsic manipulation.
The role of a deterministic biological control of habitual physical activity needs to be considered in the context of a real-world universal model explaining human motor activity. As depicted schematically in figure I.1, daily physical activity, as with most human behaviors, reflects the causal inputs of a variety of physical, psychological, social, and environmental factors. The central argument in the pages that follow holds that (a) such central control exists and (b) its potential influence on activity behaviors should not be ignored.
Figure I.1 The basic schema by which multiple determinants might act to dictate physical behavior for health outcomes in humans.
One of the most compelling pieces of evidence for the existence of biological control of physical activity comes from the observation that levels of daily energy expenditure through physical activity in both humans and animals (adjusted for body size) steadily decline during the course of a lifetime. This fall in daily activity with increasing age is observed in virtually all animal species and is consistent regardless of measurement technique.
As further supporting evidence, experimentally induced lesions in particular parts of the brain in animals can produce reduced or increased levels of physical activity. Changes in physical activity have been observed in human beings with certain brain tumors (i.e., craniopharyngioma). And disease states characterized by increased motor activity (anorexia nervosa, restless legs syndrome, attention deficit hyperactivity disorder) are considered to reflect catecholamine imbalance in the central nervous system. Biochemical effects on the brain that alter habitual activity levels have been described for a variety of administered drugs and chemical agents in both humans and animals. Similarly, certain deficiency and toxic states have been associated with changes in physical activity habits.
If a biologic controller of physical activity exists, activity levels should be expected to demonstrate some evidence of a genetic effect. In fact, from twin and family studies, heritability levels of daily physical activity have been reported to range from 30% to 60%. Recent advances in molecular genetics have permitted identification of specific gene loci that are associated with activity energy expenditure. The observation that spontaneous physical activity demonstrates a temporal rhythmicity expected of biological systems provides further evidence for such an intrinsic central brain controller.
The demonstration that physical activity levels are linked to a level of biological (sexual) development in youth supports a biological influence on regular motor activity. That boys are consistently found to be engaged in greater levels of habitual activity than girls during the pediatric years has traditionally been explained by sociocultural influences. However, such sex differences in motor activity have been documented in very early infancy when the effects of such influences are expected to be minimal, suggesting some biological basis.
Save
Save
Learn more about Biologic Regulation of Physical Activity.
Compensatory changes in physical activity
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
Learn more about Biologic Regulation of Physical Activity.
Plasticity of biologic set points
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations.
Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.
Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.
As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.
In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.
Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.
These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.
Learn more about Biologic Regulation of Physical Activity.
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