A variety of methods have been developed for assessing body composition, including isotopic determination of total body water, whole body 40 K 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.
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Determination of Body Composition
