We aim to understand the effects of hydration changes on athletes’ neuromuscular performance, on body water compartments, fat-free mass hydration and hydration biomarkers and to test the effects of the intervention on the response of acute dehydration in the hydration indexes. The H2OAthletes study (clinicaltrials.gov ID: NCT05380089) is a randomised controlled trial in thirty-eight national/international athletes of both sexes with low total water intake (WI) (i.e. < 35·0 ml/kg/d). In the intervention, participants will be randomly assigned to the control (CG, n 19) or experimental group (EG, n 19). During the 4-day intervention, WI will be maintained in the CG and increased in the EG (i.e. > 45·0 ml/kg/d). Exercise-induced dehydration protocols with thermal stress will be performed before and after the intervention. Neuromuscular performance (knee extension/flexion with electromyography and handgrip), hydration indexes (serum, urine and saliva osmolality), body water compartments and water flux (dilution techniques, body composition (four-compartment model) and biochemical parameters (vasopressin and Na) will be evaluated. This trial will provide novel evidence about the effects of hydration changes on neuromuscular function and hydration status in athletes with low WI, providing useful information for athletes and sports-related professionals aiming to improve athletic performance.

We assessed total body water (TBW) and extracellular water (ECW) in thirty-four non-ascitic cirrhotics and twenty healthy controls by 2H2O and Br dilution. In the same subjects, bioelectric impedance (BI) was recorded at multiple frequencies. Body hydration was similar for controls (mean 55·6 (SD 6·7)), less-severe cirrhotics (Child-Pugh classification A; CPA; n 21, mean 56·2 (SD 6·2)) and moderately-severe cirrhotics (Child-Pugh classification B; CPB; n 13, mean 57·2 (SD 5·4)). However, intracellular water standardized per litre TBW was significantly higher in CPB subjects (mean 27·0 (SD 7·5); P<0·01) compared with CPA (mean 21·3 (SD 10·6)) and control subjects (mean 18·0 (SD 9·8)). Published formulas for predicting TBW and ECW from BI at multiple frequencies were applied to the cirrhotics. These formulas gave accurate predictions of TBW and ECW, although standard errors of estimates were higher for CPB subjects (TBW ≤ 2·5 and ECW ≤ 2·11) than those for CPA (TBW ≤ 2·0 and ECW ≤ 1·81) and control (TBW 1·4 and ECW 0·9 1) subjects

The relationship between total body water (TBW) and extracellular water (ECW), measured by deuterium oxide dilution and bromide dilution respectively, and impedance and impedance index (height2/impedance) at 1, 5, 50 and 100 kHz was studied. After correction for TBW, ECW was correlated only with the impedance index at 1 and 5 kHz. After correction for ECW, TBW was best correlated with the impedance index at 100 kHz. The correlation of body-water compartments with impedance values obtained with modelling programs was lower than with measured impedance values. Prediction formulas for ECW (at 1 and 5 kHz) and TBW (at 50 and 100 kHz) were developed. The prediction errors for ECW and TBW were 1·0 and 1·7 kg respectively (coefficient of variation 5%). The residuals of both ECW and TBW were related to the ECW/TBW value. Application of the prediction formulas in a population, independently measured, revealed a slight overestimation of TBW and ECW, which could be largely explained by differences in the validation group in body-water distribution and in body builds. The ratio of impedance at 1 kHz to impedance at 100 kHz was correlated with body-water distribution (ECW/TBW). The relation is however not strong enough to be useful as a predictor. It is concluded that an independent prediction of ECW and TBW, using impedance at low and high frequency respectively, is possible, but that the bias depends on the body-water distribution and body build of the measured subject.