Hydration of fat-free body mass: new physiological modeling approach

Hydration of fat-free body mass: new physiological modeling approach. Am. J . Physiol. 276 (Endocrinol. Metab. 39): E995-E1003, 1999.-Water is an essen tial component of living organisms, and in adult mammals the fraction of fa t-free body mass (FFM) as water is remarkably stable at similar to 0.73. Th e stability of FFM hydration is a corner-stone of the widely used water iso tope dilution method of estimating total body fat. At present, the only sug gested means of studying FFM hydration is by experimental total body water (TBW) and FFM measurements. Although deviations from the classical hydratio n constant are recognized, it is unknown if these are explainable physiolog ical aberrations and/or methodological errors. Moreover, many questions rel ated to hydration stability prevail, including body mass and age effects. T hese unresolved questions and the importance of the TBW-fat estimation meth od led us to develop a cellular level FFM hydration model. This physiologic al model reveals that four water-related ratios combine to produce the obse rved TBW-to-FFM ratio. The mean and range of FFM hydration observed in adul t humans can be understood with the proposed physiological model as can var iation in the TBW-to-FFM ratio over the human life span. An extension of th e model to the tissue-organ body composition level confirms an a theoretica l basis a small but systematic decrease in hydration observed in mammals ra nging in body mass by a factor of 10(5). The present study, the first to ad vance a physiological hydration model, provides a conceptual frame-work for the TBW-fat estimation method and identifies important areas that remain t o be studied.