Water intake in athletes: Agreement between food records and isotope-dilution methods

Background: Although water intake (WI) is commonly estimated through self-reporting, its inaccuracy is reported. Water turnover (rH₂O)-derived WI is the reference method; however, it is costly. The study aimed to validate alternative methods for determining WI in a group of athletes.

Methods: Eighty-two athletes (20.4 ± 5.1 years; 28 females) were included in this cross-sectional validation study. Doubly labelled water (DLW) determined rH₂O and rH₂O-derived WI by subtracting metabolic, transcutaneous, and inspired water from rH₂O (reference). The rH₂O components were determined by alternative approaches: i) average values of macronutrients instead of individual food records (DLW-derived WI); ii) estimating total energy expenditure (TEE) and rCO₂ through a predictive equation instead of DLW (²H-derived WI); iii) estimating rH₂O by prediction equation (equation-derived WI). Food records were employed to determine WI (food record-derived WI).

Results: Only DLW-derived WI showed no significant differences compared to reference in both sexes (females: 2 mL/day, p = 0.183; males: -4 mL/day, p = 0.118). ²H-derived WI showed significant but small differences (females: 23 mL/day, p = 0.001; males: 32 mL/day, p < 0.001) while food record-derived WI showed the largest differences (females: -924 mL/day, p < 0.001; males: -1504 mL/day, p < 0.001). DLW-derived and ²H-derived WI showed good performance [r² > 0.987 and concordance correlation coefficient (CCC) > 0.993], while food record-derived WI showed poor performance [r² < 0.031 and CCC < 0.133)].

Conclusion: Although some alternative approaches provide adequate estimates, DLW- and ²H-derived WI are the only valid methods to determine WI in our sample of competitive athletes. These approaches are less costly and could contribute to the development of WI guidelines for athletes. Food records underestimate WI, but evidence-based recommendations could reduce misrecordings.