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ACSM's Body Composition Assessment
by Timothy Lohman, Laurie A. Milliken and American College of Sports Medicine
200 Pages
ACSM’s Body Composition Assessment delves into the methodology for a number of techniques, including DXA, BIA, ultrasound, underwater weighing, ADP, total body water, multicomponent models, anthropometry (including skinfolds and circumferences), and BMI. The text uncovers the sources of error inherent in each measurement technique, and it identifies populations to whom these techniques can be applied with accuracy. Researchers and clinicians alike will benefit from descriptions of methods for use in both laboratory and field settings, protocols for the standardization of each method, and advantages and limitations for each method.
The text thoroughly examines the health implications of body composition by looking at the relationships between chronic disease and total body fat, fat distribution, muscle mass, and bone density. It also facilitates the reader’s ability to assess changes in body composition over time and to understand special considerations in assessing body composition in athletes, children, older adults, the overweight population, and clinical populations.
ACSM’s Body Composition Assessment is supplemented with a web resource containing audio-narrated PowerPoint slides to support a deep understanding of the content. The slides walk readers through key points and assessments in each chapter, and select photos and tables from the book are included to facilitate learning and retention.
ACSM’s Body Composition Assessment will help alleviate errors in body composition assessment, making it an ideal reference for practicing fitness, health, and medical professionals; nutrition specialists; and exercise physiologists.
Timothy G. Lohman, PhD; Laurie A. Milliken, PhD, FACSM; and Luis B. Sardinha, PhD
Errors in Body Composition Measurement and Assessment
Validation and Cross-Validation Studies
Body Composition Terms and Concepts
Summary
Chapter 2. Body Composition Models and Reference Methods
Jennifer W. Bea, PhD; Kirk Cureton, PhD, FACSM; Vinson Lee, MS; and Laurie A. Milliken, PhD, FACSM
Levels of Human Body Composition
Models of Human Body Composition
Total Body Potassium Counting and Neutron Activation Analysis
Imaging Methods
Summary
Chapter 3. Body Composition Laboratory Methods
Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Densitometry
Total Body Water
Total Body Potassium Counting
Dual-Energy X-Ray Absorptiometry
Ultrasound
Summary
Chapter 4. Body Composition Field Methods
Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
Skinfolds
Circumferences
Bioelectric Impedance Analysis
Use of Weight and Height Indexes to Estimate Body Composition
Summary
Chapter 5. Assessing Measurement Error
Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Types of Measurement Error
Intra- and Interobserver TEM/CV of Various Body Composition Measurement Methods
Reducing Error Associated With Field Methods
Summary
Chapter 6. Estimation of Minimum Weight
Timothy G. Lohman, PhD; and Kirk Cureton, PhD, FACSM
Estimating Minimum Weight in Wrestlers
Laboratory Methods for Estimating Minimum Weight
Field Methods for Estimating Minimum Weight
Summary
Chapter 7. Applying Body Composition Methods to Specific Populations
Jennifer W. Bea, PhD; Timothy G. Lohman, PhD; and Laurie A. Milliken, PhD, FACSM
Laboratory Methods
Field Methods
Summary
Chapter 8. Body Composition Applications
Vanessa Risoul-Salas, MSc, RD; Alba Reguant-Closa, MS, RD; Luis B. Sardinha, PhD; Margaret Harris, PhD; Timothy G. Lohman, PhD; Nuwanee Kirihennedige, MS, RD; and Nanna Lucia Meyer, PhD, FACSM
Nutritional Status
Competitive Sports and Exercise Training
Body Composition and Eating Disorders
Body Composition and Weight Loss
Body Composition, Chronic Disease, and Aging
Other Applications
Summary
The American College of Sports Medicine (ACSM), founded in 1954, is the largest sports medicine and exercise science organization in the world. With more than 50,000 members and certified professionals worldwide, ACSM is dedicated to improving health through science, education, and medicine. ACSM members work in a range of medical specialties, allied health professions, and scientific disciplines. Members are committed to the diagnosis, treatment, and prevention of sport-related injuries and the advancement of the science of exercise. The ACSM promotes and integrates scientific research, education, and practical applications of sports medicine and exercise science to maintain and enhance physical performance, fitness, health, and quality of life.
Timothy G. Lohman, PhD, is a professor emeritus at the University of Arizona and is widely considered a leading scientist in the field of body composition assessment. His research includes serving as principal investigator (PI) of both the TAAG (Trial of Activity for Adolescent Girls) study—a collaborative multicenter study focused on physical activity of adolescent girls—and the Bone Estrogen Strength Training (BEST) study. He was co-PI of the Pathways Study, a collaborative study (by the National Heart, Lung, and Blood Institute; four field centers; and a coordinating center) designed to prevent obesity in Native American children. Lohman served as a consultant to the Women’s Health Initiative (WHI) Vanguard Center and Health ABC study of long-term aging, and he was an advisor on youth fitness for the Cooper Institute. He previously served as the director of the Center for Physical Activity and Nutrition at the University of Arizona. He is a member of the American College of Sports Medicine.
Lohman’s additional works, published by Human Kinetics, include his co-edited Human Body Composition, Second Edition; his authored monograph, “Advances in Body Composition Assessment”; and his co-edited Anthropometric Standardization Reference Manual. His research in body composition helped to establish the chemical immaturity of children using the multicomponent model.
Laurie A. Milliken, PhD, FACSM, is an associate professor and former chair of the exercise and health sciences department at the University of Massachusetts at Boston. In the New England chapter of the American College of Sports Medicine (NEACSM), she has served as a state representative, an executive committee member, the Continuing Education Committee chair, and president, and she has been an active member since 1998. Nationally, she has served on the ACSM Research Awards Committee and is also an editorial board member of ACSM’s Health & Fitness Journal. She is currently a peer reviewer for leading scientific journals such as Medicine & Science in Sports & Exercise, the Journal of Applied Physiology, and the European Journal of Applied Physiology. She has been a member of ACSM since 1994 and has presented her research at many annual meetings. Her research interests include the regulation of body composition in response to exercise throughout the lifespan. She has received NIH funding for her work and is also a fellow of the American College of Sports Medicine.
"This is the only current, comprehensive review of body composition that has been published in nearly 15 years."
—© Doody’s Review Service, 2020, Anthony Ewald, MD, Indiana University School of Medicine (5-star review)
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
Specific skinfold techniques for the triceps
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject’s arm above the mark, with the thumb and fingers directed inferiorly.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.
Types of measurement errors
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value.
By Vinson Lee, MS; Leslie Jerome Brandon, PhD, FACSM; and Timothy G. Lohman, PhD
Systematic and Random Error
Systematic errors are measurement biases in one direction, which lead to measured values that are consistently higher or lower from the actual value. All measurements are prone to systematic errors, often of different types. Sources of systematic error may be imperfect calibration of measurement instruments, changes in the environment that interfere with the measurement process, and imperfect protocols or methods of observation. An example of systematic measurement error is when a protocol calls for a specific measurement of abdominal circumference and a different protocol is followed, leading to consistently higher or lower results. Systematic errors can be reduced by ensuring that all equipment is properly calibrated and taking meticulous measurements using standardized protocols.
Random errors are errors caused by the lack of predictability (uncertainty) that is characteristic of the measurement process and variation in the variable being measured. These errors fluctuate around the true value and, unlike systematic errors, are unavoidable. Random errors can be reduced by performing repeated measurements. Both systematic and random errors are inherent to the errors discussed in the next two sections.
Practical Insights
Systematic errors occur when a bias is introduced in the measurement, causing it to be consistently higher or lower than the actual value. An example of how this could occur is when a technician incorrectly reads a scale on a skinfold caliper as each line equaling 2 cm when each line really equals 1 cm. Random errors occur equally in both directions and do not cause a bias in the measurement. They are not consistently in one direction or another. Better training and more practice can reduce both kinds of errors. It is also recommended that your measurements be certified against a trained professional's measurements so that you are sure you are making accurate assessments.
Intra- and Interobserver Variability
Intraobserver variability is the error or difference obtained by the same investigator when completing the same assessment with the same participant using the same equipment and employing the same techniques. These errors are assessed two ways for this discussion: repeated assessments within the same day (within day) and assessments between days (interday). When assessments are made on different days, other sources of variation, such as hydration status of the participant, may contribute to intraobserver measurement error, whereas fewer sources of error are present when repeated assessments are performed within the same day.
Interobserver variability or error is a measure used to assess the degree of agreement among investigators. It provides a score that indicates the consensus among investigators, often called objectivity of the measurement. Assessment of interobserver variability is necessary because different investigators typically experience measurement error regardless of their efforts to ensure a high degree of precision. This is especially true with inexperienced investigators completing body composition measurements. To better understand and effectively compare body composition assessments of people from different research laboratories across the world, standardized procedures that yield minimum errors are needed.
Following standardized protocols should yield acceptable intra- and interobserver variation for a given body composition measurement method.
Using the dilution principle for total body water (TBW)
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man.
By Robert M. Blew, MS; Luis B. Sardinha, PhD; and Laurie A. Milliken, PhD, FACSM
Water is the most abundant component in the human body comprising about 60% of body mass in the reference man. Because it is mostly found in the fat-free body in a relatively constant amount, assessment of body water has been of interest as a method of body composition assessment for almost 100 years. Unlike the other molecular body components, the water component consists of a single molecular species (H2O), which simplifies the task of its measurement. Water's characteristic as a singular molecular species offers itself to the use of the dilution principle, which in its simplest form, states that the volume of the component is equal to the amount of isotope added to the component divided by the concentration of the isotope in that component.
In 1915, the dilution principle was first used in the study of human body composition when the use of a red dye to measure the plasma volume was extrapolated. The investigators verified that the concentration of the dye after mixing was not constant because it “disappeared” from blood plasma. Using a mathematical approach, a reasonable estimate was made to calculate the volume of plasma in which the dye was first diluted. Following this investigation and using the same principle, tracer material was injected intravenously and allowed to reach a uniform distribution, and from the dilution achieved at equilibrium, the constituents of the body were measured. Both radioactive and stable isotopes were thus used to measure the potassium and sodium of the body.
Tritiated water was first described by Pace et al. as an isotope for measuring TBW. The main advantage of using tritium (3H), the radioactive isotope of hydrogen, is that it is readily available and easily assayed by scintillation counting. On the other hand, a large amount of tritiated water must be administered to obtain adequate precision, eliminating its use in cases where the use of radionuclides is restricted.
Currently, deuterium (2H) and oxygen-18 (18O), which correspond to nonradioactive stable isotopes, are the most commonly used isotopes for the measurement of TBW. Oxygen-18 has the advantage that its dilution space more closely approximates TBW, but it can be adequately measured only by isotope ratio mass spectrometry, and the cost of 18O-labeled water is about 15 times more than that of deuterium. Thus, deuterium is the most frequently used isotope to estimate TBW because it is a stable isotope and easy to obtain and has lower costs than tritium or oxygen-18 with no radioactivity exposure. Deuterium can be measured by infrared spectrometry but preferably by mass spectrometry. Greater technical errors have been found using the infrared approach.
When using isotope dilution, particularly deuterated water, two body fluid samples from urine, blood, or saliva are collected: one just before administration of the deuterium dose to determine the natural background levels and the second after allowing enough time for penetration of the isotope. If the amount of isotope is known and the baseline and equilibration concentrations are measured, the volume in which the isotope has been diluted can be calculated.
There are four basic assumptions that are inherent in any isotope dilution technique.
- The isotope is distributed only in the exchangeable pool. None of the commonly used isotopes are distributed only in water. But tracer exchanges with nonaqueous molecules are minimal, and consequently, the volume of distribution or dilution space of the isotope can be determined, albeit slightly greater than the water pool. Deuterium exchange with nonaqueous molecules is estimated at 4.2% in human adults.
- The isotope is equally distributed within the pool. Isotopic tracers are identical to body water, except for differences in molecular weight, which can lead to isotopic fractionation. Isotopic fractionation corresponds to the process that accounts for the relative abundances of isotopes and consequent redistribution of isotopes within the body. Samples collected from plasma, urine, and sweat do not show fractionation, whereas samples from water vapor do.
- Isotope equilibration is achieved relatively rapidly. The equilibration time corresponds to the point where all body fluid compartments have the same proportion of the isotope. The rate of equilibration as a function of the route of deuterium oxide administration has been investigated by Schloerb et al., who observed that equilibration was reached 2 h after intravenous administration and 3 h after subcutaneous or oral administration. Wong et al. verified that the time to equilibration was approximately 3 h regardless of whether plasma, breath carbon dioxide, breath water, saliva, or urine was sampled. Schoeller et al. detected less TBW at 3 h than at 4 h after oral isotope administration. Considering these findings, equilibration time for TBW was set at 4 h, with the exception of patients with expanded extracellular water compartments, where 5 h equilibration time is required. Considering TBW assessment, urine has demonstrated a low isotope enrichment relative to venous plasma water. Still, it is important to consider voids after tracer administration. Three voids are recommended after the dose when urine is used as the biological sample.
- The tracer is not metabolized during the equilibration time. Body water is in a constant state of flux. In temperate climates, the average fractional turnover rate in adults is 8% to 10% each day. This turnover comprises inputs of water from beverages, food, metabolic water produced during the oxidation of fuels, and water exchange with atmospheric moisture. The inputs are balanced by an output of water in the form of urine, sweat, breath water, or transdermal evaporation. This constant turnover has led to two approaches when assessing TBW: the plateau method and the back-extrapolation, or slope-intercept, method. For body composition research, the plateau method is the usual approach. Deuterated water is administered, samples are collected for 3 to 5 h, and TBW is calculated from the samples collected before and after the enrichment has reached a plateau, or a constant value.
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