Determination of Body Composition
Introduction
A variety of methods have been developed for assessing body composition, including
isotopic determination of total body water, whole body 40K counting, radiography, electrical
conductance and impedance, etc. Two of the most common methods of assessing body composition,
however, are hydrostatic weighing and determination of skinfold thicknesses. Although we won’t be
doing hydrostatic weighing as part of the lab activities, the method is important for you to
understand.
The hydrostatic or underwater weighing method is based upon the assumption that the body
is composed of two components or compartments. The components are fat-free or lean mass (FFM),
which is assumed to have a density of 1.10 kg/L, and a fat component, which is assumed to have a
density of 0.90 kg/L. The density of the whole body, therefore, will depend upon the relative size of
these two components. If the body density is known, it is possible to convert this to a % body fat
using the following equation, which was derived by Siri:
% fat= (495/body density)-450
Although the concept involved in determining body composition from body density is
relatively simple, actually measuring body density can be difficult. By definition, density is the
mass of an object divided by its volume (D=M/V). Although it is easy to determine the mass of an
object using scales, it is very difficult to determine the volume of an object that has an irregular
shape such as the human body. It is possible to measure the volume of the human body by
submerging a person in water, and measuring their weight. The decrease in their body weight when
submerged is equal to the weight of water they displace. By knowing the density of water, it is then
possible to calculate the volume of water they displace, and therefore determine the volume of their
body.
This relatively simple concept is complicated by the fact that the volume of the whole body
also includes certain volumes of gas, which have a very low density and therefore tend to cause a
person to float. The two most important of these are the air in the lungs and the air in the
gastrointestinal tract. The first of these is corrected for by having the subject exhale as much air as
possible from their lungs before determining their weight underwater, and by measuring or
estimating the amount of air they cannot exhale (residual volume). The air in the GI tract is assumed
to be approximately 0.1 L when a person is fasted.
Skinfolds
Determination of body composition from skinfold measurements is based on the fact that a
large proportion of total body fat is stored directly underneath the skin. Therefore, by measuring
body composition by underwater weighing in a large number of people and by obtaining skinfold
measurements in these same subjects, it is possible to develop equations to predict % body fat from
simple skinfold measurements. There are some issues using skinfolds to estimate % body fat for a
number of reasons, including distribution of body fat varies with age, sex, race, and athletic activity.
Reasonably accurate prediction of % body fat from skinfolds therefore requires that the equation
used for a given person to have been developed using a similar subject population. This has led to a
huge number of “population specific” equations, many of which are probably based on too few
subjects to be of any value.
In an attempt to remedy this problem, Jackson and Pollock and Jackson, Pollock, and Ward
have provided generalized equations that have been validated for various age groups and both
athletic and non-athletic populations. These equations are:
Men: D=1.1125025-0.0013125(x) + 0.0000055(x2) – 0.000244(y)
Women: D=1.089733-0.0009245(x) + 0.0000025(x2) – 0.0000979(y)
Where x=sum of triceps, chest, and subscapular skinfolds (in mm) for men, and the sum of triceps,
suprailium, and abdominal skinfolds for women, and y =age in years.
Bioelectrical Impedance
Bioelectrical impedance analysis to estimate body composition is based on the following
simple principle. A small, alternating current flowing between two electrodes passes more rapidly
through hydrated fat-free body tissues and extracellular water compared with fat or bone tissue
because of the greater electrolyte content of the fat-free component. Impedance (resistance) to the
electrical current flow relates to the amount of body water, which in turn relates to the amount of fat
and FFM. With BIA, the person receives a painless, localized electrical current. The impedance
measurement is determined based on the resistance to the current flow. This can then be converted
to body density, adding body weight and height, gender, age, and sometimes race, level of fatness,
and some girth measurements. We will be using the Tanita body composition scale to take
measurements of impedance.
Bioelectrical impedance analysis requires measurement under standardized conditions,
particularly hydration status, recent food and beverage intake, skin temperature, and recent physical
activity. Hydration affects BIA accuracy. For example, impedance is lower when there is
significant body water loss as a result of exercise of fluid restriction. This would produce a lower
body fat percentage. Hyperhydration would have the opposite effect. Skin temperature also affects
impedance measurements. A warm environment may result in less impedance to electrical flow, and
thus, a lower percent body fat.
Even under optimal conditions, BIA measurements may be questionable compared to
underwater weighing. BIA tends to overpredict body fat in lean and athletic subjects, and
underpredict body fat in obese subjects.
Girth Measurements
Girth measurements offer an easily administered, valid and attractive alternative to skinfolds.
Girth measurements should be taken at the following sites using the Gulick tape. The sites
commonly used for girth measurements are: upper arm (biceps), forearm, abdomen, hips (buttocks),
thigh, and calf. The equations we will use to estimate % body fat using girth measurements are
designed for young and old men and women, provided the individual’s physical characteristics
resemble the original validation group. The equations should not be used for individuals who appear
excessively fat or thin, or who participate regularly in strenuous sports or resistance training that
often increases girth without changing subcutaneous body fat. Along with predicting % body fat,
girth measurements can also be used to analyze patterns of body fat distribution.
Goal Body Fat Percentage and Target Weight
The following method can be used to determine goal body fat percentage (GBF%) and target weight
(TW).
1. Multiply total body weight (TBW) by the body fat percentage (BF%) to
determine fat weight (FW).
2. Subtract FW from TBW
3. The remaining weight is the fat free mass (FFM)
4. Determine an appropriate and reasonable GBF%
5. Divide the LBM by the (1-GBF%)
6. The answer will be the TBW at the predetermined GBF%
7. Subtract the TW from the TBW to determine the amount of weight loss (WL) required to
achieve GBF%
Step 1: TBW x BF%-FW
Step 2: TBW – FW=LBM
Step 3: LBM/(1-GBF%) = TW
Step 4: TBW – TW = WL
Lab Activity:
1. With a partner, practice using the skinfold calipers at sites listed in your ACSM book. Take
the measurements on the right side of the body.
a. Place the calipers 1 cm away from the thumb and finger, perpendicular to the skinfold
and halfway between the crest and base of the fold.
b. Maintain the pinch while reading the calipers
c. Wait 1-2 seconds before reading the calipers
d. Take duplicate measures on each site
e. Retest if measurement do not fall within 1-2 mm
f. Rotate through the sites
g. Record the measurements on a data sheet
h. Calculate your subject’s estimated percent body fat using the 7-site formula and one
of the 3 site formulas listed on page 63 of the ACSM book
For this activity, you should practice all of the skinfold sites, as demonstrated in lab. You may work
at a facility that uses these sites for the estimation of body fat, so you should be familiar with all
skinfold sites (described on page 62-63 of ACSM Guidelines book).
2. With a partner, measure body weight in kg, height in cm, and bioelectrical impedance using
the Tanita body composition scale or the handheld BIA.
3. Measure waist and hip circumference of your partner. Calculate Waist-to-hip ratio (WHR).
a. Waist: Narrowest part of the torso (above the umbilicus and below xiphoid process)
b. Hip: At the maximal girth of the hips or buttocks region above the gluteal fold
4. Utilize girth measurements to estimate body composition. Each measurement should be
taken in duplicate and an average measurement value should be calculated for each site.
Utilize the tables in Appendix (provided) to determine the constants for each site.
a. Pull the Gulick tape to proper tension (so it is snug) without pinching the skin
b. Check that tape is neither indenting the skin nor loose enough to leave gaps between
the tape and skin
c. Retest if the measurements do not fall within 1 cm
d. Record the measurements
e. For Young Men (18-26 years):
Percent Fat = Constant A + Constant B – Constant C – 10.2
A=upper arm, B=abdomen, C=forearm
For Older Men (27-50 years):
Percent Fat = Constant A = Constant B –Constant C – 15.0
A=buttocks, B=abdomen, C=forearm
f. For Young Women (18-26 years):
Percent Fat = Constant A + Constant B – Constant C – 19.6
A=abdomen, B=thigh, C=forearm
For Older Women (27-50 years):
Percent Fat = Constant A + Constant B – Constant C – 19.6
A=abdomen, B=thigh, C=calf
Again, you should practice and be familiar with all girth measurements, as demonstrated in lab and
described on pages 60-61 in ACSM Guidelines book.
Write Up: After each of you have practiced, conduct a body composition assessment on your partner
using the above methods in the form of a lab report. Include the following sections: introduction,
results, discussion/implications.
1. Utilize the skinfolds, bioelectrical impedance, and girth measurements above to calculate % body
fat and lean body mass. Do these numbers agree with one another? If not, can you suggest some
reasons why?
2. Utilize measurements of body weight and height to calculate body mass index (weight in
kg/(height in meters)2).
3. Identify the Disease Risk Classification for BMI and WHR from measurements (ACSM, p. 58).
Compare the results of your subject to norms (if norms are provided).
4. Indicate the results of the Goal Body Fat Percentage and Target Weight for your subject.
5. Identify the positive and negative components of the various body composition methods in
relation to cost, time, validity, ease of test, and overall practicality.