Submitted by Amy Sercel MS RD CD
Edited by Marcia Bristow MS RDN CSSD CD
Carbohydrates are the preferred source of energy for your brain and muscles. When you eat carbohydrates in fruits, vegetables, grains, beans, dairy products, or sweets, they are converted into glucose and either used to provide immediate fuel for your muscles and brain, or stored as glycogen to be used for fuel later. Conventional recommendations suggest that everyone get 45-65% of their calories from carbohydrates to ensure there is enough fuel to support daily activities and exercise. For the average person who needs 2000 calories per day, this might mean eating around 250 grams of carbohydrates; for an athlete with much higher calorie needs than the average person, this could mean eating closer to 400 grams of carbohydrates per day or more. The ketogenic diet, however, has people questioning whether carbohydrates are really the ideal fuel for athletes.
On the ketogenic diet, people get 75-80% of their calories from fat and 12-20% of their calories from protein, and eat less than 20 grams of carbohydrates per day. It was originally designed as a therapeutic diet prescribed to children with epilepsy. When your brain doesn’t have glucose to use for energy, your body converts fat into compounds called ketone bodies, which the brain can use for fuel. People have begun following the ketogenic diet with the idea that it will force the body to burn fat for energy instead of carbohydrates. Since the body stores more fat than glycogen, this would allow the athlete to perform for longer without needing to eat more carbohydrates before running out of fuel.1
There are very few scientific studies looking at the impact of the ketogenic diet on athletic performance. From these studies, it appears that people following the ketogenic diet will experience adaptations that allow them to burn about twice as much fat during exercise than those following a typical high-carbohydrate diet.1–3 It’s not clear whether this actually results in any performance benefit. Additionally, some studies contain design flaws that prevent readers from drawing any firm conclusions.
For example, one study placed 20 male endurance athletes into either a high-carbohydrate diet (HCD) group or a low-carbohydrate ketogenic diet (LCKD) group based on their typical dietary intake. The researchers found that people in the LCKD group burned more fat during exercise, increased their peak power during a sprint, and lost significantly more weight than those in the HCD group. When the data is looked at more closely, though, it becomes apparent that the participants in the LCKD group started out with a significantly higher body fat mass than those in the HCD group. At the same time, the men in the HCD group were only provided with general guidelines about following a high-carbohydrate diet, but the LCKD group received sample meal plans and shopping lists to help them follow the ketogenic diet.3 Having more information about the diet they needed to follow and starting with a higher body fat mass makes it more likely that the men lost weight simply as a result of following a more regimented diet, and makes it impossible to claim that any improvements in physical performance are due to diet alone.
Another commonly-cited study looked at endurance runners and triathletes who consumed a ketogenic diet for almost two years and found that they burned almost twice as much fat during activity than athletes eating a typical diet. The study didn’t measure the impact of the ketogenic diet on the athletes’ physical performance.2 There is a risk of research bias in this study, however, because it was funded by Quest Nutrition and the Atkins Foundation. Quest Nutrition makes low-carbohydrate sports nutrition products, and the Atkins Foundation is run by Dr. Robert Atkins, the same individual who created the low-carbohydrate Atkins diet.
One study does examine the impact of the ketogenic diet on both fat burning and exercise performance. Like the others, the researchers found that subjects following a low-carbohydrate diet did burn more fat during exercise. However, the participants on the ketogenic diet needed to take in more oxygen while they exercised because the body uses more oxygen to turn stored fat into energy than it does to turn glycogen into energy. As a result, the athletes’ overall physical performance was worsened by the ketogenic diet.1
Taken together, these studies show the need for more, well-designed research on the impact of the ketogenic diet on athletic performance in addition to its influence on the body’s ability to burn fat. Until that research is available, it is best to continue following the evidence-based recommendation to use carbohydrates to fuel your workouts. Fuel up on whole grains, low-fat dairy, and fruit before a workout, bring snacks to keep you energized during exercise, and make sure to eat plenty of carbohydrates and protein to help recover from your activity.
- Burke LM, Ross ML, Garvican-Lewis LA, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017;595(9):2785-2807. doi:10.1113/JP273230.
- Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016;65(3):100-110. doi:10.1016/j.metabol.2015.10.028.
- McSwiney FT, Wardrop B, Hyde PN, Lafountain RA, Volek JS, Doyle L. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. Metabolism. doi:10.1016/j.metabol.2017.10.010.