Hydration of Fat-Free Mass: Mechanistic Basis of Classic Body Composition Constant

The widely used two-component hydrometry model of body composition rests on the assumption that fat-free mass hydration (total body water, TBW/FFM) is stable at 0.732. Deviations from this constant have been reported, but their basis is unknown. The current study applied Wang’s 2008 mechanistic FFM hydration model to establish factors underlying this variation. Healthy participants (407 males, 469 females) had TBW, total body potassium (TBK), body density, and bone mineral (Mo) measured; FFM was derived with a five-component model. Two key model terms, extracellular-to-intracellular water ratio (ECW/ICW) and extracellular solids-to-TBW ratio (ECS/TBW), were evaluated across discrete age-sex groups and modeled as continuous functions of age, followed by causal mediation analysis. Group mean FFM hydration ranged from ∼0.73 and 0.75, and within that range increased significantly with age (0.00018/yr, P=0.003) after adjusting for sex and BMI. ECW/ICW increased (0.00463/yr, P<0.001) and ECS/TBW decreased (-0.00018/yr, P<0.001) with age, two main factors contributing to age-related increase in FFM hydration. Mediation analysis indicated near-complete mediation by ECW/ICW ( P<0.001; direct effect, P=0.862) and partial mediation by ECS/TBW (56.2%), the latter differing by sex. ECS/TBW demonstrated near-complete mediation in females (97.6%) but no significant total age effect on TBW/FFM in males, reflecting an increase rather than decrease in ECS/TBW with age in males. Although FFM hydration occupies a narrow absolute range in adults, it rises systematically with age, mechanistically attributable to relative expansion of ECW and secondarily to loss of ECS. The practical importance of this shift is likely minimal for two-component densitometric applications, but potentially meaningful for bioimpedance and other methods that embed a fixed hydration constant.