TY  - JOUR
AB  - 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; V?o2peak: 63?±?6 mL/kg/min) completed two trials [100 min of steady state running at ?65% V?o2peak 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 ?? 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; P = 0.006) and 18% (HOT: 0.67?±?0.10 vs. TEMP: 0.81?±?0.11 g/min; P = 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 versus TEMP (2.10?±?0.35 vs. 1.86?±?0.30 g/min; P = 0.020). Gastrointestinal temperature and heart rate (P < 0.001) were greater in HOT. Even with adequate hydration, running in a hot environment reduced exogenous carbohydrate.NEW & NOTEWORTHY 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.
AU  - Mougin, L
AU  - Horner, M
AU  - Edwards, D
AU  - Nickels, M
AU  - Taylor, L
AU  - James, LJ
AU  - Mears, SA
DA  - 2025/12/01
DO  - 10.1152/japplphysiol.00873.2025
EP  - 1446
JO  - J Appl Physiol (1985)
PB  - American Physiological Society
PY  - 2025/12/01
SP  - 1436
TI  - Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration.
VL  - 139
Y1  - 2025/12/01
Y2  - 2026/06/13
ER  -