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Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration
Loïs Mougin1, Maisie Horner1, Dylan Edwards1
1School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, United Kingdom.
Journal of Applied Physiology (Bethesda, Md. : 1985)
|October 25, 2025
Summary
Running in hot weather significantly reduces exogenous carbohydrate oxidation by about 20%, even when euhydration is maintained. This indicates heat stress impairs carbohydrate use, increasing reliance on endogenous energy stores.
Area of Science:
- Exercise Physiology
- Environmental Physiology
- Nutritional Science
Background:
- Exogenous carbohydrate oxidation is crucial for endurance performance.
- The impact of heat stress on exogenous carbohydrate metabolism, independent of dehydration, requires further investigation.
Purpose of the Study:
- To determine the effect of a hot environment versus a temperate environment on exogenous carbohydrate oxidation during steady-state running.
- To assess the contribution of endogenous carbohydrate sources under heat stress while maintaining euhydration.
Main Methods:
- Ten trained runners completed two 100-minute running trials at ~65% V̇o2peak in temperate (19°C) and hot (34°C) conditions.
- Participants maintained euhydration by consuming water to replace ~90% of body mass losses and ingested a 35% dextrose solution enriched with [U-13C] glucose (60 g/h).
- Expired breath and blood samples were analyzed to measure exogenous carbohydrate oxidation rates, while gastrointestinal temperature and heart rate were monitored.
Main Results:
- Exogenous carbohydrate oxidation rates were significantly lower by ~20% in the hot environment compared to the temperate environment (average: 0.43 vs. 0.54 g/min; peak: 0.67 vs. 0.81 g/min).
- Total carbohydrate oxidation did not differ significantly between conditions, suggesting a greater reliance on endogenous carbohydrate stores in the heat (2.10 vs. 1.86 g/min).
- Gastrointestinal temperature and heart rate were elevated in the hot condition, indicating increased physiological strain.
Conclusions:
- Heat stress alone, even when euhydration is maintained, significantly reduces exogenous carbohydrate oxidation during prolonged running.
- Athletes exercising in hot environments may need to adjust carbohydrate intake strategies to account for impaired exogenous fuel utilization.
- A greater reliance on endogenous carbohydrate stores occurs under heat stress, potentially impacting endurance capacity if not managed properly.
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