Racing or training in the heat has a way of making every normal endurance rule feel a little shakier. Paces that usually feel controlled suddenly feel too ambitious. And fueling (which is already part science and part personal experiment) gets even more complicated when your gut is bouncing, and your sweat rate is climbing.
Do carbohydrates still help endurance performance in the heat, or does heat stress change the equation?
A new systematic review1Salame, A., Brown, D., Oueijan, K., & McCullough, D. (2026). Carbohydrate supplementation for endurance exercise in the heat: a systematic review with practical recommendations. Journal of the International Society of Sports Nutrition, 23(1). https://doi.org/10.1080/15502783.2026.2669307 looked at carbohydrate supplementation during endurance exercise in hot environments. The researchers synthesized nine randomized crossover studies. In practice, these were all lab-based cycling studies performed in environmental chambers at 80–95 degrees Fahrenheit (27-35 degrees Celsius), with humidity ranging from 20% to 78%. That is important right away: this review is highly relevant to endurance athletes, but it is not directly a running review.
The carbohydrate strategies varied a lot. Some studies used glucose or maltodextrin; others used multiple transportable carbohydrates such as glucose, sucrose, fructose, or maltodextrin-fructose combinations; and one used sago. Intake rates ranged from a very low 14 grams per hour to an extremely high 140 grams per hour. Exercise trials lasted roughly 50 to 152 minutes, with performance measured through either time-to-exhaustion tests or time trials.
The results were mixed, which is probably the most honest answer here. Five studies found performance benefits from carbohydrate supplementation in the heat, while four found no clear effect. In the positive studies, carbohydrates improved time to exhaustion by 13.4% to 19.3% and improved time-trial performance by 3.3% to 12.7%. That sounds impressive, but the picture gets messier when you look closer. Some of the biggest benefits came from time-to-exhaustion tests, which are useful in the lab but less reliable than real-world time trials.
The bigger physiological question is whether carbohydrate intake in the heat actually changes fuel use. Heat stress can increase glycogen breakdown, which creates the theory that athletes might need more carbohydrate when racing in hot weather. But in this review, most studies did not find major differences in respiratory exchange ratio (a marker of relative fat and carbohydrate use during exercise) or carbohydrate oxidation between carbohydrate and placebo (no-carb) conditions. Taking in carbs did not consistently appear to reduce the body’s reliance on stored carbohydrate. That does not mean fueling is useless; it means heat may create a more complex metabolic environment in which hydration, thermoregulation, gut function, and central fatigue all compete with the simple “more carbs equals more performance” logic.
The gut may be the hidden variable. Only two of the nine studies actually measured gastrointestinal symptoms, which is a major limitation because heat is notorious for making the gut more fragile. One study found more nausea, fullness, and stomach upset with a 12% glucose drink providing about 100 grams per hour compared with a 6% drink and a placebo. Another found greater fullness with a 16% maltodextrin-fructose drink at 90 grams per hour. The review also noted that one study using a very concentrated 14% carbohydrate drink, equivalent to roughly 140 grams per hour, resulted in worse performance on average, possibly related to gut distress, even though GI symptoms were not measured.
Hydration and body temperature did not seem to be strongly affected by carbohydrate intake overall. Most studies found no meaningful differences in hydration markers, core temperature, or perceived exertion between carbohydrate and placebo.
What this means for runners
I would not take this review as a reason to fuel less in the heat, but rather as a reason to fuel smarter. Carbohydrates can still help, especially for longer races and harder efforts, but hot conditions make the gut less forgiving, so forcing huge carb targets without practice may backfire. The practical hierarchy should probably be: start well-fueled, prioritize hydration and sodium based on sweat losses, use carbohydrate amounts you have practiced in similar conditions, and be cautious with very concentrated drinks or aggressive fueling rates when it is hot and humid. Since all included studies were cycling-based, runners should be especially careful when extrapolating the higher intake ranges, as running tends to cause more gut jostling and GI distress.
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