Definition Body Composition Analysis (Estimate of Percentage Body Fat

Skinfold analysis measures the thickness of a double fold of skin and subcutaneous adipose tissue
at various body locations [44]. The use of skinfold calipers is the most widely used method for
determining obesity. The advantages of this method are its relative simplicity, quickness, and
nonevasive nature. In addition, very little space is needed. Various equations have been developed
in the prediction of body composition [45–52]. In overweight children, a two-site formula using triceps
and subscapular developed by Slaughter et al. [53] has been used and shown to be highly correlated
with both underwater weighing and dual-energy x-ray absorptiometry (DEXA) measurements [54].
Bioelectrical impedance may be used to measure the density of lean and adipose tissue in
relation to hydration [49,51,52,55]. Studies have shown total body water (TBW) and electrical
impedance to be related [56,57]. Further studies also report the utility and reliability of body
impedance measurements in the assessment of total body fat [55,58]. Both bioelectrical impedance
and skinfolds provide a reasonably accurate, time- and cost-effective method of assessing body fat
in overweight children [49–51]. However, care should be taken to select a formula specific to the
age, race, and gender of the child, as accuracy varies by study [54,59,60]. More accurate methods
are available but are typically reserved for severe overweight conditions or research studies. These
include underwater weight and DEXA.
For many years, underwater, or hydrostatic, weighing has been the “gold standard” for the
measurement of body fat [44]. The use of underwater weighing is based on Archimedes’ principle.
Simply stated, Archimedes reasoned that an object submerged in water is buoyed up by a counterforce
equaling the weight of the displaced water. An object “loses weight in water.” Therefore, if an object
weighs 50 kg in air and 3 kg when submerged, the loss of weight in water, 47 kg, equals the weight
of displaced water. We can then calculate the volume of water displaced because the density of water
at any temperature is known. More recently, DEXA has been used to determine percentage of fat,
lean body mass, and bone mineral density in overweight children [49,51,52,61]. DEXA uses an
x-ray source to generate photons to scan subjects [62]. Bone-mineral content measurements previously
calibrated against secondary standards with ashed bone sections are used to help calculate fatfree
mass [63,64]. Percentage of fat and fat-free body can be predicted with accuracy by observing
the ratio of absorbance of the different-energy-level photons, which are linearly related to the percentage
of fat in the soft tissues of the body [64]. The coefficient of variation of fat-free tissue measurement
has been calculated at 2%, which is comparable to that obtained by hydrodensitometry [65].