Skeletal Muscle Mass Is Independently Associated With Water Turnover in Male Athletes

ABSTRACT Fat‐free mass (FFM) is a key component of body composition that contributes moderately to water turnover (WT). However, FFM consists of multiple components, and the extent of its contribution to WT, especially in relation to skeletal muscle mass (SMM) in athletes, remains unclear. This study aimed to examine the association between WT and body composition in athletes. Thirty male participants (20 ± 2 years old) were members of college sports clubs or athletic organizations. WT was determined using the deuterium dilution method for 1 week. Total body water (TBW), intracellular water (ICW), and extracellular water (ECW) were measured using deuterium and sodium bromide dilution. Bone mineral content was measured using DXA, and four‐component models were used to calculate FFM and total body protein (TBP). MRI and MRS were used to evaluate SMM and muscle glycogen. WT was positively associated with TBW ( r = 0.563), ICW ( r = 0.549), SMM ( r = 0.548), and ambient temperature ( r = 0.519). To overcome mathematical coupling and multicollinearity, regularized regression analyses (LASSO and Elastic Net) were performed. Because TBW and TBP are algebraically dependent on the deuterium‐derived water space from which WT was calculated, they were excluded from the candidate predictors. Both models retained SMM, ICW, and ambient temperature as predictors of WT, whereas ECW, bone mineral content, muscle glycogen, and relative humidity were not retained. The modest cross‐validated performance (leave‐one‐out R 2 = 0.20–0.23) suggests that WT in athletes is also influenced by behavioral and unmeasured environmental factors. Functional body composition compartments, specifically SMM and ICW, were associated with WT in athletes beyond the mathematical dependency between WT and TBW; SMM, measured independently by MRI, was an independent correlate, whereas the association of ICW—derived in part from TBW—was not fully independent of the WT measurement.