How does running in the heat change VO₂max and fuel utilization?
In a new study, researchers compared 10 trained runners and 10 inactive individuals as they performed incremental treadmill tests to exhaustion in two conditions:
- Temperate (18°C, 43% humidity)
- Hot (34°C, 42% humidity)
The researchers measured VO₂max to assess aerobic capacity, substrate oxidation (fat vs. carbohydrate use) to determine fuel utilization, core temperature and skin temperature to evaluate thermoregulation, and blood lactate levels to assess metabolic stress.
Key findings:
🔥VO₂max dropped by 13-15% in the heat: Both trained runners and inactive individuals showed significantly lower peak oxygen consumption in hot conditions.
🔥 Fat oxidation decreased and carbohydrate use increased: In hot conditions, the body switched from burning fat to burning carbohydrates faster. Carbohydrate oxidation was significantly higher in trained runners in the heat, but not in inactive individuals.
🔥 Blood lactate accumulation was higher in the heat: Lactate levels rose earlier and reached higher peak concentrations.
🔥 Trained runners experienced more heat stress than inactive individuals: Core temperatures were significantly higher in trained runners than in inactive individuals in the heat.
The Rolex Life 
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