The Keto Diet & Endurance Sports: What You Need to Know

keto diet

The ketogenic (“keto”) diet is a relatively new eating trend that has sparked quite a bit of discussion among both the general public and athletes—particularly endurance and ultra-endurance athletes. Proponents claim a number of potential benefits and effects on performance.

What does the evidence have to say about the diet’s benefits and effects on athletic performance?

Comparing Keto to a “Typical” Athletic Diet

As illustrated below, a keto diet is characterized by fat as the primary fuel source, with limited carbohydrate intake. The intent of the diet is to increase lipid oxidation as a fuel substrate. In a keto diet, carbohydrates make up around 5 to 10 percent of macronutrient intake. Protein makes up around 15 percent of intake, and fats make up the remaining 60 to 80 percent of calorie intake.

keto diet breakdown

This is in sharp contrast to a more typical diet, in which carbohydrates make up about 60 percent of calories, with fats and protein making up the remaining 40 percent.

With this in mind, it’s worth looking at how the body uses fuel for energy based on the type of activity being performed:1

  • High intensity, short duration exercise primarily utilizes creatine phosphate in the first four seconds followed by muscle glycogen for up to sixty seconds.
  • Endurance exercise that lasts for more than two minutes uses muscle/liver glycogen, which begins to transition to fat metabolism, and finally to amino acids.
  • Low-intensity exercise primarily burns fat.
  • At relatively high intensities, in which most athletes perform during endurance training and competition, carbohydrates are the primary typical energy source.

The Diet Should Match the Need

No diet is “one size fits all” and appropriate for every athletic pursuit. Your athletes’ diets should be based on their own individual and sport-specific needs, particularly in regard to the type and duration of the activity being performed. For instance, ingesting carbohydrates during exercise may delay fatigue and improve oxygen-delivery rates and performance.2

The keto diet has some potential drawbacks that can make it a poor choice for endurance athletes. For one thing, ketoadaptation, the ability of the body to efficiently use ketones as fuel, can take up to four weeks.3 Research also shows endurance events that last up to three hours require carbohydrate-based foods and fat-based foods can cause a downregulation of carbohydrate metabolism.4

Despite changes in fat utilization, there is a clear lack of effect on performance. In fact, it may even impair performance during high-intensity exercise.5 Further research found that while a keto diet did decrease fat mass, it did not increase muscle mass with resistance training.6

The Bottom Line

The keto diet has continued to gain in popularity due to its proposed ability to decrease fat mass by increasing the body’s ability to oxidize fats and use them as fuel substrates. While the diet has been shown to improve fat oxidation and decrease body mass/fat mass, several limitations do exist, including potentially impaired performance during high-intensity exercise and a lack of increase in lean body mass in athletes. It appears that “carbohydrate tailoring” and individual planning may be helpful for endurance athletes.

It’s also worth keeping in mind that at this time, there are very few studies assessing the long-term effects of the keto diet on overall health and wellness.

  1. Brooks, G. A., Fahey, T. D., & White, T. P. (1996). Human Bioenergetics and Its Application, 2nd Ed. Mountain View, CA: Mayfield Publishing.
  2. Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Supple 1): 25–33.
  3. Christoph, L. & Miele, E. (2017). How can endurance athletes perform well with a very low carbohydrate diet? (part 2). AMAA Journal, 30(1).
  4. Hawley, J. A. & Leckey, J. J. (2015). Carbohydrate dependence during prolonged, intense endurance exercise. Sports Medicine, 45(Suppl 1): S5–12.
  5. Escobar, K. A., Morales, J., & Vandusseldorp, T. A. (2016). The effect of a moderately low and high carbohydrate intake on Crossfit performance. International Journal of Exercise Science, 9(3): 460–470.
  6. Vargas, S., Romance, R., Petro, J. L., Bonilla, D. A., Galancho, I., Espinar, S., & Krieder, R. B. et al. (2018). Efficacy of ketogenic diet on body composition during resistance training in trained men: a randomized controlled trial. Journal of the International Society of Sports Nutrition, 15(1): 31.