Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Background: Bioelectrical impedance spectroscopy (BIS) provides an affordable assessment of the body's various water compartments: total body water (TBW), extracellular water (ECW) and intracellular water (ICW). However, little is known of its validity in athletes. Aim: To validate TBW, ECW and ICW by BIS in elite male and female Portuguese athletes using dilution techniques (i.e. deuterium and bromide dilution) as criterion methods. Subjects and methods: Sixty-two athletes (18.5 +/- 4.1 years) had TBW, ECW and ICW assessed by BIS during their respective pre-season. Results: BIS significantly under-estimated TBW by 1.0 +/- 1.7 kg and ICW by 0.9 +/- 1.9 kg in relation to the criterion methods, with no differences observed for ECW. The values for the concordance correlation coefficient were 0.98 for TBW and ECW and 0.95 for ICW. Bland-Altman analyses revealed no bias for the various water compartments, with the 95% confidence intervals ranging from -4.8 to 2.6 kg for TBW, -1.5 to 1.6 kg for ECW and -4.5 to 2.7 kg for ICW. Conclusions: Overall, these findings demonstrate the validity of BIS as a valid tool in the assessment of TBW and its compartments in both male and female athletes.402152156Portuguese Foundation for Science and Technology [PTDC/DES/69495/2006, PTDC/DES/098963/2008]Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Portuguese Foundation for Science and Technology [PTDC/DES/69495/2006, PTDC/DES/098963/2008]CAPES [BEX 2018/10-1]