The Ketogenic Diet for Athletes: Pros and Cons

The ketogenic diet has moved from epilepsy clinics into endurance sport and gyms, with claims that fat-adapted athletes tap into almost unlimited energy reserves. Some high-profile athletes swear by it, while sports scientists remain sceptical. Our editorial team reviewed the key studies to weigh up what keto offers athletes and what it may cost them.

What a ketogenic diet involves

A ketogenic diet drastically restricts carbohydrate, typically to below about 50 grams per day, while fat supplies most of the energy and protein is kept moderate. Under these conditions, the liver produces ketone bodies from fatty acids, which the brain and muscles can use for fuel instead of glucose.

The diet differs from a general low-carbohydrate approach because the aim is sustained nutritional ketosis, usually defined as blood ketone levels above about 0.5 millimoles per litre. Reaching and maintaining that state requires strict and consistent restriction.

Adaptation takes time. During the first week or two, many people experience fatigue, headaches, irritability and reduced exercise capacity, often called the "keto flu". Over several weeks, the body increases its ability to oxidise fat during exercise.

The appeal for endurance athletes is simple: even a lean athlete stores tens of thousands of kilocalories as fat, but only around 2,000 kilocalories as glycogen. Becoming better at burning fat could, in theory, reduce dependence on limited carbohydrate stores.

What the research shows for endurance

Early work by Phinney and colleagues in 1983 found that trained cyclists maintained submaximal endurance after four weeks on a ketogenic diet, while dramatically shifting towards fat oxidation. This study is often cited by keto advocates, though it involved only five participants and moderate intensity exercise.

Volek and colleagues later showed in 2016 that keto-adapted ultra-endurance runners could oxidise fat at remarkably high rates, far higher than those on conventional diets, while maintaining similar muscle glycogen. This demonstrated the metabolic flexibility possible with long-term adaptation.

The key question, however, is performance. Burke and colleagues, in a carefully controlled study of elite race walkers published in the Journal of Physiology in 2017, found that the ketogenic diet increased fat oxidation but impaired exercise economy, meaning athletes used more oxygen at a given pace. Those on the ketogenic diet did not improve their race performance after intensified training, unlike those eating carbohydrate.

A repeat study by the same group in 2020 confirmed these findings. The researchers concluded that the diet's effects on economy could be particularly costly at the higher intensities that decide many races.

Strength, power and team sports

For strength athletes, the picture is somewhat different. A study by Paoli and colleagues in 2012 found that elite artistic gymnasts maintained strength performance over 30 days on a ketogenic diet while losing body fat. Several other small studies in resistance-trained people have reported similar maintenance of strength.

However, muscle gain appears more difficult on a ketogenic diet, particularly when energy intake is not high enough. Insulin and carbohydrate availability support muscle protein synthesis and glycogen replenishment, and some studies show smaller increases in lean mass compared with higher-carbohydrate diets.

Team sports and combat sports involve repeated high-intensity efforts that depend heavily on glycogen. Restricting carbohydrate in these contexts risks reduced sprint performance and earlier fatigue in the later stages of matches or rounds.

For weight-category athletes, the ketogenic diet is sometimes used for rapid weight loss. Part of the early drop reflects glycogen and water loss rather than fat, which may come at the cost of performance on competition day.

Health considerations

A ketogenic diet is not simply a performance question. Restricting whole food groups such as fruit, starchy vegetables, wholegrains and pulses can lead to low intake of fibre, certain vitamins and minerals. Careful planning is required to avoid deficiencies.

Bone health has emerged as a concern. Heikura and colleagues found in 2020 that just three and a half weeks on a ketogenic diet impaired markers of bone formation and increased markers of bone breakdown in elite race walkers. This is particularly worrying for endurance athletes already at risk of stress fractures.

Low carbohydrate intake can also contribute to low energy availability, especially if appetite is suppressed. This may affect hormonal health, menstrual function and immunity, and is part of the picture described as relative energy deficiency in sport.

Blood cholesterol may rise in some people on a ketogenic diet, depending on the types of fat consumed. People with existing heart disease, diabetes on medication, kidney disease or a history of disordered eating should not start a ketogenic diet without medical supervision.

Who might benefit and practical alternatives

Some athletes in very long, low-intensity events, such as ultra-endurance races, may find fat adaptation reduces gastrointestinal problems associated with high carbohydrate intake during competition. Individual preference and tolerance matter here.

For most athletes, current sports nutrition guidelines, including the joint position statement from the American College of Sports Medicine, recommend periodising carbohydrate intake to match training demands rather than eliminating it. Common strategies include:

• Training some low-intensity sessions with lower carbohydrate availability to stimulate adaptation.
• Fuelling high-intensity sessions and competitions with adequate carbohydrate.
• Adjusting daily carbohydrate intake to the training load of each day.
• Ensuring sufficient protein and overall energy to support recovery and body composition goals.

This approach, sometimes called "fuelling for the work required", aims to capture the metabolic benefits of fat adaptation without sacrificing high-intensity performance.

Conclusions of the editorial team

The ketogenic diet reliably increases fat burning, but this does not automatically translate into better performance. In endurance athletes competing at high intensity, the evidence suggests it impairs economy and performance; in strength athletes, it may maintain strength but slow muscle gain.

Health concerns, particularly regarding bone and energy availability, mean that athletes considering keto should do so with professional support. For most, periodised carbohydrate intake is a more effective and sustainable strategy.

Related reading includes our articles on carbohydrate periodisation, on relative energy deficiency in sport, and on protein needs for strength training.

References

1. Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983;32(8):769–776.
2. Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016;65(3):100–110.
3. 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.
4. Burke LM, Sharma AP, Heikura IA, et al. Crisis of confidence averted: impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible. PLoS One. 2020;15(6):e0234027.
5. Paoli A, Grimaldi K, D'Agostino D, et al. Ketogenic diet does not affect strength performance in elite artistic gymnasts. J Int Soc Sports Nutr. 2012;9(1):34.
6. Heikura IA, Burke LM, Hawley JA, et al. A short-term ketogenic diet impairs markers of bone health in response to exercise. Front Endocrinol (Lausanne). 2020;10:880.
7. Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine joint position statement. Nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–568.