Comparison of different bromide equilibrium times to measure extracellular water in athletes: Analysis before and after a 4-day intervention to increase water intake

BACKGROUND: Bromide (Br-) dilution is the criterion to determine extracellular water. While 3 h equilibrium is typically used in athletes, the optimal equilibrium time has not been assessed. The study aimed to determine the optimal time of equilibrium of Br- in athletes before and after increasing water intake (WI). METHODS: Thirty-one athletes [21.5 (0.8) years; 41.9% females] from the H2OAthletes randomized controlled trial (clinicaltrials.gov ID: NCT05380089) were included in this longitudinal sub-analysis. Athletes were assessed before and after a 4-day intervention to increase WI (experimental group) or maintain WI (control group). Br- dilution was assessed through saliva before and 3, 4, and 5 h post-dose. Total body water (TBW) and water turnover (rH2O) was measured through deuterium dilution. Body composition was determined through the 4-compartment model. RESULTS: At baseline, most athletes achieved equilibrium at 3 h (45.2%) or 4 h (48.4%). After the intervention, most athletes (70.4%) achieved peak concentration at 3 h, with fewer reaching at 4 h (14.8%) or 5 h (14.8%). No differences in body composition, TBW, or rH2O were observed between athletes grouped according to the time of peak, both before and after the intervention. Time of peak was not associated with the intervention group [x2 (1) = 1.439; p = 0.487] or sex both before [x2 (1) = 0.230; p = 0.891] and after the intervention [x2 (1) = 0.728; p = 0.695]. CONCLUSION: Br- equilibrium appears to occur 3 h-4 h post-dosing, suggesting it is unnecessary to extend the collection to 5 h, reducing the burden on athletes. Future studies should include larger samples to confirm the current findings.