Estimation of total body water and extracellular water with bioimpedance in athletes: A need for athlete-specific prediction models.

PURPOSE: Bioelectrical impedance analysis (BIA) equations can predict total body water (TBW) and extracellular water (ECW) in non-athletic healthy populations. This study aimed: a) to develop BIA-based models for TBW and ECW prediction based on dilution methods in a sample of national level athletes; and b) to validate the new models with a cross-validation approach in a separate cohort using dilution methods as criterion.
METHODS: Two hundred and eight highly trained athletes (21.3 ± 5.0 years) were evaluated during their respective competitive seasons. Athletes were randomly split into development (n = 139) and validation groups (n = 69). The criterion method for TBW was deuterium dilution and for ECW was bromide dilution, where ICW was the respective difference between both. Resistance (R) and reactance (Xc) were obtained with a phase-sensitive 50 kHz BIA device and used for the estimation of TBW and ECW.
RESULTS: Athletic BIA-based models were developed for TBW and ECW [TBW = 0.286 + 0.195*S(2)/R + 0.385*Wt + 5.086*Sex; ECW = 1.579 + 0.055*S(2)/R + 0.127*Wt + 0.006*S(2)/Xc + 0.932*Sex, where sex is 0 if female or 1 if male, Wt is weight (kg), S is stature (cm), and R and Xc are in ohm (Ω)]. Cross validation revealed R(2) of 0.91 for TBW and R(2) 0.70 for ECW and no mean bias.
CONCLUSIONS: The new equations can be considered valid, with no observed bias, thus affording practical means to quantify TBW and ECW in national level athletes.