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

Plain-language summary

This study looked at how starting a workout well-hydrated or under-hydrated affects sweating and thinking during moderate exercise. Thirty active men were tested in one state or the other, with hydration confirmed by fasting urine readings over three weeks, in groups of 16 and 14. They cycled at a steady pace in comfortable conditions until they lost 3% of body weight or reached two hours, while core temperature, sweat rate and thinking were tracked. The well-hydrated men sweated more and were far more likely to reach the 3% mark. Core temperature early on was similar in both, and the under-hydrated men were slower on a test of focus and control.

Key findings

• The hydrated group sweated more (mean +0.30 L/h, peak +0.34 L/h) and were more likely to reach 3% body-mass loss (10/16 vs. 1/14).
• Core temperature early in the workout was similar between groups.
• The less-hydrated group showed slower reaction times on a test of focus and control (+63 ms).