Are low-carbohydrate and ketogenic diets good for aerobic exercise performance?

Despite the increasing interest in ketogenic or low-carbohydrate diets among endurance athletes, there’s little evidence that these dietary strategies provide a performance advantage compared to higher-carbohydrate diets.

Ketogenic and low-carbohydrate diets improve body composition and increase fat-burning capacity and may be useful for increasing low- to moderate-intensity exercise performance.^[1] However, even after a period of keto-adaptation (i.e., 3 weeks to 4 months on a ketogenic diet), high-intensity endurance exercise performance is impaired, exercise economy is reduced, and the ability to utilize glucose and glycogen as a fuel source may be diminished in some people.^[1]

There are claims that the benefits of a ketogenic diet for endurance performance may be due to a “glycogen sparing” effect. In other words, a higher fat-burning capacity during exercise should allow athletes to save their limited glycogen stores for high-intensity efforts, especially near the end of a competition. However, the higher postexercise glycogen levels that have been observed in athletes on a ketogenic diet might actually be due to a reduced ability to utilize glycogen, which is not necessarily a beneficial adaptation.^[2]

Furthermore, when it comes to energy provision, carbohydrates are a more efficient fuel source — providing more ATP per unit of oxygen consumed than an equivalent amount of fat. This may be why some studies find that short-term keto-adaptation reduces exercise economy in athletes.^[1]