INTRODUCTION: Exogenous carbohydrate oxidation (i.e., from drinks) is reduced in hot conditions^1,2. Increased thermal and cardiovascular strain and reduced gastrointestinal (GI) integrity³ may impair glucose uptake, gastric emptying, and absorption. Dehydration resulting from heat exposure, can also contribute to these impairments by reducing blood volume and altering blood flow distribution. As previous studies in hot conditions have not controlled hydration status, it remains unclear whether increased exogenous carbohydrate oxidation was due to increased core temperature or dehydration. Therefore, this study investigated the effect of running in a hot compared to temperate environment on exogenous carbohydrate oxidation, whilst maintaining a state of euhydration.

METHODS: Ten trained runners (24 ± 6 y; 72.7 ± 8.3 kg; V̇O₂peak: 63 ± 6 mL/kg/min) completed a preliminary session (V̇O₂peak and sweat rate testing) and two experimental trials [100 minutes of steady state running at ~65% V̇O₂peak in either a temperate (19°C; TEMP) or a hot environment (32°C; HOT)]. Water was provided every 20 min to replace ~90% of body mass losses (TEMP: 795 ± 213 mL; HOT: 1665 ± 437 mL). 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‰ enrichment). Expired breath (analysed for ¹³C:¹²C), blood samples and subjective scales of GI comfort were collected at rest and every 20 min during exercise.  Data were analysed using linear mixed models (significance at P < 0.05). Results presented as mean ± SD. Institutional ethical approval was granted (LEON 16408).

RESULTS: 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 (HOT: 2.72 ± 0.40 g/min vs. TEMP: 2.57 ± 0.34; P = 0.111) was not different between trials resulting in a greater contribution from endogenous sources in HOT (2.28 ± 0.38 vs. 2.03 ± 0.33 g/min; P = 0.020). Gastrointestinal temperature (HOT: 39.2 ± 0.4°C; TEMP: 37.9 ± 0.3°C; P < 0.001) and heart rate (HOT: 166 ± 14 bpm; TEMP: 137 ± 16 bpm; P < 0.001) at the end of trials were greater in HOT. In both trials body mass loss remained in a state of euhydration (± 1% body mass loss⁴) but was greater in HOT (-0.47 ± 0.51% vs. -0.04 ± 0.33%; P = 0.004). No difference was reported for changes in plasma volume (HOT: -9.0 ± 6.8%; TEMP: -10.3 ± 4.4%; P = 0.621). No differences in GI symptoms, including stomach bloatedness, were observed between conditions (P > 0.05).

CONCLUSION: Even with adequate hydration (within ± 1% body mass loss), running in a hot environment reduces exogenous carbohydrate oxidation likely due to impaired muscle glucose uptake, decreased intestinal absorption and slower gastric emptying. This led to a compensatory increase in endogenous carbohydrate oxidation to maintain similar total oxidation rates.