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)/Ht2 (m2)
– Skinfolds
– Bioelectrical Impedance Analysis (BIA)
Indirect Measurements of Body
• All of the methods listed except for hydrostatic
weighing are indirect measurements of body
• The results of the measurements are used with
regression equations to predict body density
• Body density is then used to predict body
• This piggy-backing of regression equations leads
to larger error in the predictions
Body Mass Index
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
• BIA can also be used to test total body
water content
– For example pre and post an exercise bout in
the heat
• Water with electrolytes acts a conductor of
electrical current
• Water and electrolytes make up much of fatfree 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
• 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
• 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
• 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
• 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
• Tends to overpredict body fat in lean and
athletic subjects (even greater overprediction
in African American athletes)
• Tends to underpredict body fat in obese
• Not accurate to predict small changes in body
• 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 ) - 4.142] 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
• 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
– 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