Body-composition research is a branch of human biology that has three interconnecting areas: body-composition levels and their organizational rules, measurement techniques, and biological factors that influence body composition. In the first area, which is inadequately formulated at present, five levels of increasing complexity are proposed: I, atomic; II, molecular; III, cellular; IV, tissue-system; and V, whole body. Although each level and its multiple compartments are distinct, biochemical and physiological connections exist such that the model is consistent and functions as a whole. The model also provides the opportunity to clearly define the concept of a body composition steady state in which quantitative associations exist over a specified time interval between compartments at the same or different levels. Finally, the five-level model provides a matrix for creating explicit body-composition equations, reveals gaps in the study of human body composition, and suggests important new research areas.
WC is more closely linked to cardiovascular disease risk factors than is BMI.
The assumed "constancy" of fat-free body mass hydration is a cornerstone in the body-composition research field. Hydration, the observed ratio of total body water to fat-free body mass, is stable at Ϸ0.73 in mammals and this constancy provides a means of estimating total body fat in vivo. This review examines both in vitro and in vivo data that support the hydration constancy hypothesis and provides a critique of applied methodology. Biological topics of interest are then examined and critical areas in need of future research are identified. These are important issues because water dilution is the only method currently available for estimating body fat in all mammals, which range in body mass by a factor of 10 4 . Am J Clin Nutr 1999;69:833-41. KEY WORDSHydration, fat-free body mass, total body water, body composition, mammals, dual-energy X-ray absorptiometry INTRODUCTIONOne of the primary aims of body-composition research is to identify quantitative relations between components that are relatively constant under most circumstances. These stable relations form the basis of many widely used body-composition methods and the origin of their constancy is of fundamental scientific interest (1).Water, the largest chemical component in mammals, plays a central role in nutrient transport, waste removal, maintenance of cell volume, and thermal regulation. The water content or hydration of fat-free body mass (FFM) is among the best known and most widely applied of the body-composition constants. More than 5 decades ago, Pace and Rathbun (2) first proposed that total body water (TBW) is a constant fraction of FFM (x -± SD: 0.724 ± 0.021) on the basis of experiments in guinea pigs. Subsequent chemical analysis of mature animals supported a hydration magnitude of Ϸ0.73 with a range of between 0.70 and 0.76 for several mammal species that range in body size from that of mice to cattle-a body mass difference of 10 4 (3). Additional strong support for the observed FFM hydration magnitude in mammals is provided by whole-body chemical analysis of 9 human cadavers with a mean TBW:FFM of 0.737 ± 0.036 and a range of between 0.684 and 0.808 (Table 1).The in-depth study of FFM hydration not only provides insight into basic biological processes but also enhances our understanding and application of the TBW method of quantifying total body fat. The relative stability of FFM hydration between species led to the wide use of the in vivo method, that is, Fat = body mass Ϫ FFM = body mass -TBW/0.73 (1) TBW can be measured by 3 H 2 O, 2 H 2 O, or H 2 18 O dilution (10). At present, no other body-composition measurement method is capable of providing fat estimates in mammals that range in body size from a few grams, such as the shrew, to several thousand kilograms, such as the elephant.The importance of the TBW-based method of body-composition measurement led Sheng and Huggins in 1979 (3) to critically review available literature on methodology. Since then, many additional FFM hydration studies have been published, although no synthe...
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