Body Composition Continued BMI, BIA, and Skinfolds

Ways to Determine Body Density Direct Measurements –Hydrostatic Weighing –Plethysmography (Bod Pod) Indirect Measurements –Standard Tables –Body Mass Index (BMI): Wt (kg)/Ht 2 (m 2 ) –Skinfolds –Bioelectrical Impedance Analysis (BIA)

Indirect Measurements of Body Density All of the methods listed except for hydrostatic weighing are indirect measurements of body density The results of the measurements are used with regression equations to predict body density Body density is then used to predict body composition This piggy-backing of regression equations leads to larger error in the predictions

Low risk Moderate risk High risk Overweight Obese Morbidly Obese Normal Body Mass Index Underweight

Bioelectrical Impedance Analysis A simple method of predicting body density using the electrical properties of the body Often used in scales and hand held devices that provides estimates of body fat percentage BIA can also be used to test total body water content –For example pre and post an exercise bout in the heat

Impedance Water with electrolytes acts a conductor of electrical current Water and electrolytes make up much of fat-free mass Adipose tissue acts as a resistor to electrical current thus impeding the current

Bioelectrical Impedance Subject lies flat on a nonconducting surface connected to an electrical source by electrodes A painless electrical current is passed through the body and the resistance to that current is measured Impedance can be converted to body density through a regression equation

Confounding Variables with BIA Hydration State Room/Body Temperature Difficulties Associated with Regression

Hydration BIA gives a good idea of total body water content because water with electrolytes acts as the conducting substance Changes in body fluid content (hydration status) then can affect the amount of resistance measured Thus, body composition measurement can be different based on hydration of the subject –Dehydrated subjects tend to have lower body composition estimates because electrolyte concentration is higher –Hyperhydrated subjects tend to have higher body composition estimates

Temperature Skin temperature (influenced by ambient conditions) affects whole-body resistance Body fat predictions are lower in a warmer environment because moist skin produces less impedance to electrical flow

Other Problems BIA less accurate than hydrostatic weighing or skinfolds Tends to overpredict body fat in lean and athletic subjects (even greater overprediction in African American athletes) Tends to underpredict body fat in obese subjects Not accurate to predict small changes in body composition

Skinfolds Take skinfold measurements at different sites using calipers: –Biceps, triceps, subscapular, chest, abdomen, suprailium, thigh, calf, midaxillary Calculate body density using one of a number of regression equations derived from densitometry –Varies with number of sites (1, 3, 4, 7, 12) –How many should you include? –Varies with population % Fat = [(4.570/body density ) ] x 100

Skinfolds – Measurement 3-4% error compared to densitometry May overestimate % body fat in very lean people and underestimate in obese people –Accuracy varies with age, gender, race, ethnic background Most beneficial for tracking an individual’s % body fat over time

Skinfolds – Measurement To improve accuracy of measurement: –take three measurements at each site and use mean or median value of these measurements –person taking measurements should be trained so he/she can consistently repeat measurement –same person should administer successive tests so fair comparison to earlier results can be made

Skinfolds – Assumptions Compressed double layer of skin and adipose tissue is representative of an uncompressed single layer of adipose tissue –implies that skin thickness is constant or negligible skin thickness comprises greater proportion of thin skinfold than thick skinfold, so it cannot be constant skin thickness varies between individuals and between sites, so it cannot be negligible Limited number of skinfold sites represents remaining subcutaneous adipose tissue throughout body Limited number of subcutaneous sites represents fat deposited in other storage sites, such as viscera, bone marrow, interstitial spaces, and intramuscular triglyceride