Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration

This study investigated the effect of running in a hot environment compared with a temperate environment on exogenous carbohydrate oxidation, while maintaining a state of euhydration. Ten trained runners (24 ± 6 yr; 72.7 ± 8.3 kg; V̇o_2peak: 63 ± 6 mL/kg/min) completed two trials [100 min of steady state running at ∼65% V̇o_2peak in either a temperate (19°C; TEMP) or a hot environment (34°C; HOT)]. Water was provided every 20 min to replace ∼90% of body mass losses (TEMP: 0.8 ± 0.2 L; HOT: 1.7 ± 0.4 L). In each trial, participants consumed 60 g/h (bolus every 20 min) of a 35% dextrose solution enriched with [U-¹³C] glucose (145 ± 2 δ‰ vs. PDB). Expired breath (analyzed for ¹³C:¹²C) and blood samples were collected every 20 min during exercise. Average (40-100 min) and peak exogenous carbohydrate oxidation rates were 20% (HOT: 0.43 ± 0.09 vs. TEMP: 0.54 ± 0.12 g/min; <i>P</i> = 0.006) and 18% (HOT: 0.67 ± 0.10 vs. TEMP: 0.81 ± 0.11 g/min; <i>P</i> = 0.002) lower in HOT than in TEMP, respectively. Total carbohydrate oxidation (<i>P</i> = 0.111) was not significantly different between trials, resulting in a greater contribution from endogenous sources in HOT versus TEMP (2.10 ± 0.35 vs. 1.86 ± 0.30 g/min; <i>P</i> = 0.020). Gastrointestinal temperature and heart rate (<i>P</i> < 0.001) were greater in HOT. Even with adequate hydration, running in a hot environment reduced exogenous carbohydrate.<b>NEW & NOTEWORTHY</b> This study showed that exogenous carbohydrate oxidation is reduced by ∼20% during running in the heat, even while controlling fluid intake to maintain euhydration, highlighting that heat stress alone impairs exogenous carbohydrate use. These findings suggest a lower exogenous carbohydrate oxidation and a greater reliance on endogenous stores when exercising in the heat, independently of the effects of dehydration.