Heat stress impairs exogenous carbohydrate oxidation during running ⚠️

Irrespective of hydration status.

Tom Coughlin

Nov 03, 2025

Study Details

This new study investigated the effects of running in different temperatures…

🌡️ Temperate (19°C)

🥵 Hot (34°C)

…on exogenous carbohydrate oxidation whilst maintaining hydration status (“euhydration”) 🔍

10 trained runners completed sessions in both temperatures 🏃‍♂️

Each session consisted of…

⏰ 100 min of steady state running at ~65% VO2peak

💧 Water was provided every 20 min to replace ~90% of body mass losses

🥤 60 g/hr carbohydrate (bolus every 20 min)

Here is what they found ⬇️

Key Findings

During exercise in the heat…

📉 Average (40-100 min) carbohydrate oxidation was 20% lower

📉 Peak carbohydrate oxidation was 18% lower

…than in temperate conditions.

Interestingly, total carbohydrate oxidation was not significantly different between trials 🧐

…resulting in a greater contribution from endogenous sources in hot vs temperate conditions 🔥

Gastrointestinal temperature and heart rate were also greater in hot conditions 🫀

Conclusion

Even with adequate hydration, running in a hot environment reduces exogenous carbohydrate oxidation 🚨

Athletes may require greater emphasis on pre-exercise fuelling strategies ahead of running in hot environments 🍝

Reference

https://pubmed.ncbi.nlm.nih.gov/41138215/

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