How Heat and Carbs Affect Your Performance

Mitochondria production plays a crucial role in athletic performance. A recent study explores the relation between heat, carb intake, and mitochondrial activity.

For most athletes, understanding the role of mitochondria is necessary. Mitochondria are the little power plants in each of your cells where energy is produced. That alone should give you an idea of how important they are to an athlete, let alone to life in general. Even more interesting to the athlete is that it is possible to get more mitochondria and have them work better. But it’s also possible to do the opposite, and many athletes might be getting it wrong.

In fact, a surprising number of factors influence your mitochondria. One well-known and well-established factor is exercise itself. More exercise, especially more cardio, boosts your mitochondria production. But other factors may influence the production of mitochondria as well. These include how you exercise (not just that you exercise, but also the intensity and frequency of your workouts), the nutrient levels in the foods you eat, like fats, carbs, and even some other nutrients like bioflavonoids, your carb intake during exercise, and even how hot or cold it is during your workout.

In a study this month in the Journal of the International Society of Sports Nutrition, researchers examined the role of some of these other factors – specifically, carb intake and heat. This is an interplay we often find in athletics – for example, while exercising on a hot day and drinking a sugary sports drink.

Environmental temperature plays an interesting role in mitochondrial development. The cold increases mitochondria and their activity of creating energy, especially from fat, but hot temperatures seem to suppress this activity and cause the body to favor the use of carbs as fuel. Similarly, consuming carbs while exercising causes he body to favor carbs as a fuel source. So in realistic scenarios, we need to know if combining these two factors still results in the favoring of carbohydrate for fuel. Our intuition might be yes, but it is possible that the energy demands of both heat and exercise stress may be substantial enough to alter these effects.

Interestingly, in this study, when the participants exercised and rested in 38 degree Celsius (approximately 100 degree Fahrenheit) temperatures, the rate of carbohydrate used as energy was the same, whether they ingested a carb drink or not. When they were not drinking carbs, the amount of fat used as fuel increased.

It is likely that the excess heat is responsible for the equal levels of carb burning. The added environmental stress created a condition in which both carbs and fat could be used as fuel, but the use of fat decreased when drinking carbs. This was probably due to carb suppression of a protein called UCP3, which is a part of energy production process in mitochondria.

The researchers warned against interpreting this as a reason not to consume carbs while exercising, since carbs do boost performance when exercising longer than one hour. However, it is possible that this performance boost does not hold true in the long term. Although more research is needed, this study suggests that periodizing an athlete’s diet by consuming carbs during competition but not training could give you the best of both worlds.


1. Charles L Dumke, et. al., “Skeletal muscle metabolic gene response to carbohydrate feeding during exercise in the heat,” Journal of the International Society of Sports Nutrition 2013, 10:40.

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