Hot-weather running is tough for many reasons, the main one being that the body simply has to work harder. However, we’re also beginning to learn that temperature (and humidity) can alter the way the body utilizes carbohydrates.
A new meta-analysis pooled data from 51 studies (encompassing over 500 participants) to determine how heat and dehydration impact carbohydrate metabolism during endurance exercise.
Researchers reviewed controlled trials where trained athletes exercised in hot (≥28°C/82℉) versus temperate conditions, and in hydrated versus dehydrated states. They examined three key outcomes: respiratory exchange ratio (RER, an indicator of substrate utilization), whole-body carbohydrate oxidation, and muscle glycogen utilization.
Compared to cooler conditions, exercising in the heat consistently increased carb use. Carbohydrate oxidation was higher, and muscle glycogen depletion was markedly higher. Simply put, your body leans harder on carbs instead of fat when it’s hot.
Going into or becoming dehydrated also increased carbohydrate oxidation and glycogen use, but the effect was less consistent—strong in hot conditions, weaker or absent in temperate ones. That means dehydration in the heat amplified the shift toward carb burning, likely due to reduced plasma volume, higher core temperatures, and elevated stress hormones like epinephrine. The mechanisms for all of this are well understood—heat diverts blood flow toward the skin for cooling, leaving less for working muscles. That reduced oxygen availability nudges metabolism toward faster but less efficient carb use. Dehydration worsens this by further cutting blood volume and muscle water content, which can accelerate glycogen breakdown.
What this means for runners
If you’re racing or training in the heat, expect glycogen to drain faster—even if you hydrate well. Plan on increasing your carb intake slightly if you can tolerate it. Managing heat stress is just as important as managing fuel, and getting both right could be the key to running better without a bonk.
 Effect of hot vs. temperate conditions on glycogen use, carbohydrate oxidation, and RER (dots to the right of the vertical dashed line indicate higher values/greater fuel utilization during hot-weather conditions). |
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