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

Water is an essential component of living organisms, and in adult mammals the fraction of fat-free body mass (FFM) as water is remarkably stable at ∼0.73. The stability of FFM hydration is a cornerstone of the widely used water isotope dilution method of estimating total body fat. At present, the only suggested means of studying FFM hydration is by experimental total body water (TBW) and FFM measurements. Although deviations from the classical hydration constant are recognized, it is unknown if these are explainable physiological aberrations and/or methodological errors. Moreover, many questions related to hydration stability prevail, including body mass and age effects. These unresolved questions and the importance of the TBW-fat estimation method led us to develop a cellular level FFM hydration model. This physiological model reveals that four water-related ratios combine to produce the observed TBW-to-FFM ratio. The mean and range of FFM hydration observed in adult humans can be understood with the proposed physiological model as can variation in the TBW-to-FFM ratio over the human life span. An extension of the model to the tissue-organ body composition level confirms on a theoretical basis a small but systematic decrease in hydration observed in mammals ranging in body mass by a factor of 105. The present study, the first to advance a physiological hydration model, provides a conceptual framework for the TBW-fat estimation method and identifies important areas that remain to be studied.