Hydration of fat-free body mass: review and critique of a classic body-composition constant

The assumed "constancy" of fat-free body mass hydration is a cornerstone in the body-composition research field. Hydration, the observed ratio of tota l body water to fat-free body mass, is stable at approximate to 0.73 in mam mals and this constancy provides a means of estimating total body fat in vi vo. This review examines both in vitro and in vivo data that support the hy dration constancy hypothesis and provides a critique of applied methodology . Biological topics of interest are then examined and critical areas in nee d of future research are identified. These are important issues because wat er 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).