chapter 88 Assessing Body Composition Author name here

chapter 88 Assessing Body Composition Author name here for Edited books

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 (2 C) and multicomponent models • Recognize similarities and differences between two densitometric methods • Correctly classify body fat levels

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)

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

Table 8. 1

Body Composition Models • Two-component (2 C) 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 (underwater weighing) – Siri and Brozek are two popular formulas for converting body density (Db) into %BF

Two-Component Model • 2 C 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)

Two-Component Model (cont. ) • 2 C 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 2 C model equations.

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

Reference Methods • Commonly used methods: densitometry and dualenergy 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

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)

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.

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)

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.

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)

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.

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

Skinfold Method • SKFs indirectly measure 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)

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)

Figure 8. 7

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 populationspecific 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)

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.

Figure 8. 9

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)

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 = √(R 2 + Xc 2). (continued)

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 multiplefrequency analyzers (continued)

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)

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

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)

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)

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)

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.

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/m 2) may not be appropriate

Table 8. 7

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

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

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

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.

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