Pre-Exercise Hydration Modulates the Sweat Rate and Executive Control during Moderate Exercise to 3% Dehydration under Thermoneutral Conditions

Dehydration impairs endurance and may compromise cognition, yet the impact of pre-exercise hydration status on thermoregulation and cognitive function during continuous exercise is not well defined. Thirty physically active men were randomized to be tested either well-hydrated (HYD; n = 16) or insufficiently hydrated (HYP; n = 14), classified by fasting urine specific gravity (USG; HYD 1.018) verified over three weeks. On the experimental day, participants cycled at 50% maximal power output (Wmax) under thermoneutral conditions (22°C, 45% RH) until 3% body-mass loss or for 120 min. Core temperature (Tc) was recorded continuously, the sweat rate (SR) was derived from nude body-mass change, and executive function (Stroop Interference) along with visuospatial working memory (Corsi Block-Tapping) were assessed pre- and post-exercise. Participants in the HYD group produced a higher SR (ΔSRmean +0.30 L·h⁻¹, 95% CI 0.18–0.43; ΔSRpeak +0.34 L·h⁻¹, 95% CI 0.04–0.63) and were more likely to reach 3% mass loss (10/16 vs. 1/14; p = 0.002). Despite greater sudomotor output, early Tc burden (0–90 min) did not differ between groups. Hydration status selectively affected executive control: Stroop naming interference showed a time × status interaction (F(1,27) = 4.57, p = 0.042), driven by slowing in HYP participants (≈+63 ms, p = 0.029), whereas Corsi indices were unchanged. These findings support targeting euhydration before prolonged exercise (e.g., morning USG <1.018), as inadequate baseline hydration may impair inhibitory control even when early Tc responses are comparable. Accordingly, for training or competition requiring rapid decision-making and attentional control, pre-exercise hydration strategies (planned fluid intake and/or USG-based monitoring) may be warranted to mitigate decrements in executive function.