Summary: Bioelectrical impedance provides an affordable assessment of the body's water compartments: total body water (TBW), extracellular (ECW) and intracellular water (ICW). However, little is known of its validity in athletes. We aim to validate TBW, ECW, and ICW obtained by multi-frequency bioelectrical impedance (MFBIA), using dilution techniques as reference (REF) in athletes. Thirty-six elite athletes (17males and 19females) with a mean age of 18.5±4.1 years were evaluated. TBW, ECW, and ICW were assessed by MFBIA (Tanita MC-180MA) and dilution techniques. TBWREF and ECWREF were obtained by deuterium and bromide dilution techniques, respectively, while ICWREF was calculated as their difference. MFBIA underestimated ECW (~1.3kg) and overestimated TBW (~1kg) and ICW (~3.5kg), in relation to criterion. MFBIA explained 96%, 81%, and 83% of the total variability of results obtained from the reference for TBW, ECW, and ICW respectively. Standard error of the estimates ranged between 1.6 kg (ECW) and 2.9 kg (ICW). Bland-Altman analyses revealed 95% confidence intervals:-3.1 to 5.1kg for TBW,-5.0 to 2.3kg for ECW, and-2.5 to 9.6kg for ICW. Although methods are highly associated, these findings highlight that the MFBIA used is of limited accuracy at an individual level to estimate TBW, ECW and ICW, in an athletic population.
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