chapter 8

1. chapter8 chapter 8 Assessing Body Composition Author name here for Edited books

2. Objectives • Understand the importance of measuring body composition • Identify standard techniques for body composition assessment for skinfolds, hydrostatic weighing, bioelectrical impedance, and anthropometry • Differentiate between two-component (2C) and multi-component models • Recognize similarities and differences between two densitometric methods • Correctly classify body fat levels

3. Classification and Use of Body Composition Measures • Relative body fat (%BF) used for classification of body fatness • Minimal, average, and obesity fat values vary with age, gender, and activity status (continued)

4. Classification and Use of Body Composition Measures (continued) • Body composition assessment is also good for the following: • Estimating healthy body weight • Formulating nutritional recommendations and exercise prescriptions • Estimating competitive weight for athletes • Monitoring growth • Identifying those at risk because of under- or overfatness • Assessing changes associated with aging, malnutrition, and certain diseases • Assessing effectiveness of nutrition and exercise interventions in counteracting changes identified above

5. Table 8.1

6. Body Composition Models • Two-component (2C) model • Categorizes total body mass into fat and fat-free body (FFB) • Fat-free body is comprised of water, muscle (protein), and bone (mineral) • Serves as foundation of hydrodensitometry (under-water weighing) • Siri and Brozek are two popular formulas for converting body density (Db) into %BF

7. Two-Component Model • 2C model assumptions: • Density of fat is 0.901 g/cc. • Density of the FFB is 1.100 g/cc. • Densities of fat and the FFB components (water, protein, mineral) are the same for everyone. • Densities of FFB components are constant for an individual, and their proportion remains constant. • Person being measured differs from reference body only in the amount of fat. • Reference FFB assumed to be 73.8% water, 19.4% protein, and 6.8% mineral. (continued)

8. Two-Component Model (cont.) • 2C models work well if underlying assumptions about FFB are met. • FFB density (FFBd) varies depending mainly on the relative proportion of water and mineral. • FFBd varies with age, gender, ethnicity, level of body fatness, and physical activity level. • Higher than assumed Db (1.10 g/cc) can produce negative %BF. • %BF of those with lower than assumed Db will be overestimated using 2C model equations.

9. Multicomponent Models • Account for mineral and or water contribution to FFB • Improve estimation of %BF • Avoid systematic errors in %BF estimation through use of population-specific reference bodies that take into account the age (e.g., for children, elderly persons), gender, and ethnicity of the individual

10. Reference Methods • Commonly used methods: densitometry and dual-energy X-ray absorptiometry (DXA) • For densitometric methods, Db is estimated from the ratio of body mass to body volume (Db = BM/BV) • Two methods of densitometry: hydrodensitometry (hydrostatic weighing, HW) and air displacement plethysmography (ADP) • Densitometry measures BV from which Db is calculated

11. Hydrostatic Weighing • Relies on Archimedes’ principle and total body submersion to determine BV. • BV must be corrected for residual lung volume (RV) and gastrointestinal air (GV). • GV assumed to be 100 ml or 0.1 L or 0.1 kg. • BV must also be corrected for water density. • Db is a function of the muscle, bone, water, and fat in the body. • Db is converted to %BF using best conversion formula for the person being assessed. • Best results occur if you follow standardized techniques. (continued)

12. Hydrostatic Weighing (continued) • HW is a valid, reliable, and widely used laboratory method. • Precision with HW is excellent (predictive error ≤1% BF) when RV is measured. • Precision with HW decreases substantially (predictive error ±2.8 to 3.7 %BF) when RV is estimated.

13. Air Displacement Plethysmography • Another densitometric method • Utilizes displacement of air within a closed chamber (Bod Pod) and pressure–volume relationships (Boyle’s Law) to estimate BV • Less time-consuming than HW and requires less technician skill • One assumption is that the Bod Pod controls the isothermal effects of clothing, hair, thoracic gas volume, and body surface area in the enclosed chamber. • Clients are tested while wearing minimal clothing (a swimsuit) and a swim cap. (continued)

14. Air Displacement Plethysmography (continued) • Research is divided as to whether ADP produces significantly different Db when compared to HW. • Compared to multicomponent body composition models, the Bod Pod and HW methods have similar predictive accuracy. • The Bod Pod is more accommodating than HW. • It may be more suitable in clinical settings or with hydrophobic clients.

15. Dual-Energy X-ray Absorptiometry • DXA yields estimates of bone mineral, fat, and lean soft-tissue mass. • DXA is safe, rapid, requires minimal client cooperation, and accounts for individual variability in bone mineral content. • The basic principle is that the attenuation of X rays with high and low photon energies is measurable and dependent on the thickness, density, and chemical composition of the underlying tissue. • Attenuation ratios for the two X-ray energies are thought to be constant for all individuals. (continued)

16. Dual-Energy X-ray Absorptiometry (continued) • Body composition results vary with manufacturer, model, and software version. • Experts reviewing DXA studies have called for more standardization among manufacturers. • No consensus exists that DXA is better than HW. • Current investigations indicate DXA estimates of %BF are within 1% to 3% of reference measures from multicomponent model. • Further research is needed before DXA can be firmly established as the best reference method.

17. Field Methods of Body Composition Assessment • They are more practical for estimating body composition compared to laboratory methods. • You must closely follow standardized testing procedures. • You must practice in order to perfect your measurement techniques for each method. • Common tests: • Skinfold (SKF) • Bioelectrical impedance analysis (BIA) • Anthropometry

18. Skinfold Method • SKFs indirectly measure the thickness of subcutaneous adipose tissue. • Assumptions: • SKF is a good measure of subcutaneous fat. • Distribution of fat subcutaneously and internally is similar for all individuals within each gender. • The sum of several SKFs (ΣSKF) can be used to estimate total body fat. • There is a relationship between ΣSKF and Db. • Age is an independent predictor of Db for adults. (continued)

19. Skinfold Method (continued) • Population-specific or generalized equations are needed to convert Db to %BF. • Population-specific %BF prediction equations are based on a linear relationship between SKF fat and Db (linear model). • However, there is a curvilinear relationship (quadratic model) between SKFs and Db across a large range of body fatness. • Population-specific equations tend to underestimate %BF in fatter subjects and overestimate it in leaner subjects. (continued)

20. Figure 8.7

21. Skinfold Method (continued) • Generalized are equations developed using heterogeneous sample, diverse in age, %BF. • Only one equation is needed to estimate Db. • Most equations use 2 or 3 SKFs to predict Db. • Db is converted to %BF using appropriate population-specific conversion formula. • You can accurately estimate the %BF of your clients within ±3.5% BF. • Nomograms exist to estimate %BF for some SKF prediction equations. (continued)

22. Skinfold Method (continued) • Technician skill is key. • Follow and practice standardized SKF technique. • Be meticulous about SKF site identification. • Be attentive to span of thumb and index finger, direction of fold, and placement of caliper jaws. • Continuously work on interpersonal communication skills. • Errors occur due to technician, caliper, and client factors.

23. Figure 8.9

24. Bioelectrical Impedance Method • BIA is rapid, noninvasive, and relatively inexpensive. • A low-level electrical current is applied and tissue opposition to the current is used to estimate body composition. • You can estimate total body water (TBW) and subsequently FFM with BIA. • Extent of hydration or dehydration determines resistance of tissues to flow of electrical current. • When water reduces resistance and current moves easily it means tissue leaner. • When dehydration or adipose tissue slows current it means tissue is fatter. (continued)

25. Bioelectrical Impedance Method (continued) • Assumptions of BIA: • The human body is shaped like a perfect cylinder with a uniform length and cross-sectional area. • Assuming the above, at a fixed signal frequency, the impedance (Z) is directly related to the length (L) of the conductor (height) and inversely related to its cross-sectional area. • Biological tissues act as conductors or insulators; the flow of current through the body follows the path of least resistance. • Impedance is a function of resistance and reactance, where Z = √(R2 + Xc2). (continued)

26. Bioelectrical Impedance Method (continued) • Several methods: • Traditional, ipsilateral, tetrapolar whole body analysis via either single- or multiple-frequency analyzers • Upper-body impedance analysis via hand-to-hand analyzers • Lower-body impedance analysis via foot-to-foot analyzers • Vertical, bilateral, whole-body analysis via multiple-frequency analyzers (continued)

27. Bioelectrical Impedance Method (continued) • Use caution when using %BF displayed on analyzer; you must know what equation was used. • Accuracy of BIA is similar to that of SKF. • Advantages of BIA: • Does not require a high degree of technician skill • More comfortable • Less invasion of client’s privacy • Can be used to estimate body composition of obese individuals (continued)

28. Bioelectrical Impedance Method (continued) • Sources of error: • Instrumentation • Client factors • Technician skill • Environmental factors • Prediction equation used to estimate FFM

29. Other Anthropometric Methods • Anthropometry is the measurement of the size and proportion of the human body. • These measures are relatively simple, inexpensive, and well suited for large epidemiological surveys and for clinical purposes. • Minimal requirements are needed for technical skill and training. (continued)

30. Other Anthropometric Methods (continued) • Circumferences: affected by fat mass, muscle mass, and skeletal size; they are related to fat mass and lean body mass. • Bony diameters: Skeletal size directly relates to lean body mass. • Body mass index: body weight divided by height squared; relationship of BMI to body fat varies with age, gender, and ethnicity. (continued)

31. Other Anthropometric Methods (continued) • Anthropometric prediction equations estimate Db, %BF, and fat-free mass (FFM) from combinations of weight, height, skeletal diameters, and circumferences. • Anthropometric equations are based on either population-specific or generalized models. (continued)

32. Other Anthropometric Methods (continued) • Generalized equations include body weight or height, along with two or three circumferences, as predictors of Db or %BF. • BMI, WHR, waist circumference, waist–height ratio, and SAD are used to assess regional fat distribution and to identify at-risk individuals. • Existing standardized techniques must be followed.

33. Body Mass Index • Easily calculated (body weight ÷ height squared) • Widely used to identify at-risk individuals • Does not account for composition of the body • Possible misclassifications of underweight, overweight, and obese status • BMI cutoff to define obesity (≥30 kg/m2) may not be appropriate

34. Table 8.7

35. Waist Circumference • Indirect assessment of abdominal adiposity • Waist circumference (WC) alone may predict obesity-related health risk better than the combination of BMI and waist circumference. • Gender-specific circumference cutoff values are used to classify obesity.

36. Waist-to-Hip ratio (WHR) • An indirect measure of lower- and upper-body fat distribution • Calculated as waist circumference (cm) ÷ hip circumference (cm) • Young adults with WHR values >0.94 for men and >0.82 for women are at high risk for adverse health consequences • Location of waist site is not universally standardized

37. Waist–Height Ratio (WHTR) • WHTR = waist circumference at the umbilical level at standing height • May be better indicator of adiposity and health risks than waist circumference alone • A cutoff boundary value of WHTR >0.50 indicates an increased health risk for men and women • As a rule, waist circumference should be less than half the height

38. Sagittal Abdominal Diameter • SAD measures anteroposterior thickness of the abdomen at the umbilical level. • Excellent indirect measure of visceral fat. • SAD is more strongly related to risk factors for cardiovascular and metabolic diseases in adults. • Procedures to assess SAD are not standardized.

39. Frame Size • Used to classify frame size to improve validity of height–weight tables for evaluating body weight • Helps differentiate weight due to a large musculoskeletal mass from weight due to a large fat mass • Possible causes of errors: • Instrumentation • Client factors • Technician skill