Effects of during-exercise oral hydration strategies on endurance performance and thermophysiological strain: a systematic review with meta-analysis

Background The effects of during-exercise drinking may differ by strategy and outcome construct. Methods: Six databases were searched from database inception to 1 June 2026 for randomized trials in healthy adults completing at least 60 min of continuous endurance exercise. Two reviewers independently screened records, extracted data, and assessed risk of bias and certainty. Construct-specific random-effects meta-analyses used restricted maximum likelihood and modified Hartung–Knapp intervals. Results Twenty-four studies were retained (225 study-reported participants: 170 men, 20 women, 35 with sex unreported); 21 contributed to pairwise meta-analysis. Oral hydration versus no or minimal fluid showed no clear time-trial/fixed-duration benefit (−0.1%, 95% CI − 3.6 to 3.5%; four studies). The time-to-exhaustion estimate favored hydration (34.1%, 11.9–60.7%; three studies, 21 participants), but its prediction interval included no benefit (−21.4 to 128.8%). Core temperature (−0.38 °C, −0.74 to −0.02; nine studies) and heart rate (−6.82 beats/min, −11.16 to −2.48; 12 studies) favored hydration on average, with high heterogeneity and prediction intervals crossing the null. Programmed/individualized versus self-regulated drinking did not clearly improve performance (0.7%, −4.4 to 6.2%) but reduced measured body-mass loss (−1.47 percentage points, −1.96 to −0.99), an expected consequence of greater intake. Volume-matched sodium evidence was sparse; plasma-volume change was imprecise (1.78 percentage points, −5.24 to 8.81; two studies). All critical/important estimates were very low certainty. Conclusion Physiological and exhaustion-test averages favored hydration, but self-paced performance and optimal fluid or sodium dosing remain uncertain. Small samples, incomplete source coverage and post hoc analytical decisions limit inference. Systematic review registration https://www.crd.york.ac.uk/PROSPERO/view/CRD420261460972 .