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

<p dir="ltr">This study investigated the effect of running in a hot compared to temperate environment on exogenous carbohydrate oxidation, whilst maintaining a state of euhydration. Ten trained runners (24 ± 6 y; 72.7 ± 8.3 kg; V̇O 2 peak: 63 ± 6 mL/kg/min) completed 2 trials [100 minutes of steady state running at ~65% V̇O₂ peak 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- 13 C] glucose (145 ± 2 δ‰ vs. PDB). Expired breath (analysed for 13 C: 12 C) 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 ( P = 0.111) was not significantly different between trials resulting in a greater contribution from endogenous sources in HOT vs 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</p>