Ketogenic diet promises athletes "unlimited" energy supply from fats, stable appetite and easy fat loss. Critics point out that intensive work is largely dependent on carbohydrates. The editors weighed the arguments of both sides based on controlled studies.
What is ketosis and ketoadaptation
A ketogenic diet is a diet with a very low carbohydrate content (usually 20-50 g per day), moderate protein and high fat, often 70-80% of calories. Under such conditions, the liver begins to actively produce ketone bodies - beta-hydroxybutyrate and acetoacetate.
The state of nutritional ketosis is usually determined by the level of beta-hydroxybutyrate in the blood of approximately 0.5–3 mmol/l. It should not be confused with diabetic ketoacidosis, in which the concentration of ketones is much higher and is accompanied by dangerous acidification of the blood against the background of insulin deficiency.
Ketoadaptation is a process that lasts several weeks: muscles and brain gradually adapt to use fat and ketones as the main fuel. In early studies, Phinney et al. (1983) trained cyclists after four weeks of a ketogenic diet maintained endurance at submaximal intensity but significantly reduced carbohydrate utilization.
This is the main contradiction of the keto diet in sports: it improves the ability to burn fat, but at the same time reduces the availability and ability to use carbohydrates, which are needed for high intensity.
Keto and Endurance
The FASTER study (Volek et al., 2016) compared elite ultramarathoners and triathletes who followed a low-carbohydrate diet for a long time with athletes on a traditional diet. Keto-adapted athletes demonstrated very high rates of fat oxidation—approximately 1.5 g/min—while preserving glycogen stores.
However, high fat oxidation does not necessarily mean a better result. In a controlled study, Burke et al. (2017) elite sport walkers on a ketogenic diet with significant fat oxidation had poorer oxygen utilization (decreased movement efficiency) and did not improve 10 km performance after an intense training block, whereas the high-carbohydrate groups did.
The reason is biochemistry: the oxidation of fats requires more oxygen per unit of energy than the oxidation of carbohydrates. At high speeds, when oxygen consumption is close to maximum, this becomes a limitation.
For moderate-intensity ultradistance runners, keto-adaptation may theoretically have advantages (less dependence on track nutrition), but convincing evidence of improved performance in competition is not yet available.
Keto, strength and body composition
For strength sports, the situation is somewhat different, because short efforts depend more on phosphocreatine than on glycogen. The study of Paoli et al. (2012) on elite gymnasts showed that 30 days of ketogenic nutrition did not worsen strength indicators and contributed to a decrease in fat mass.
At the same time, many studies on strength athletes record a smaller increase in fat-free mass on a keto diet compared to diets with carbohydrates for the same calorie and protein content. Part of the "loss" is explained by a decrease in glycogen and water in the muscles, but the effect on hypertrophy has not been definitively clarified.
Regarding weight loss: a tightly controlled study by Hall et al. (2016) found no metabolic benefit of an isocaloric ketogenic diet for fat loss—fat loss was even slowed in the first weeks. So, keto helps you lose weight primarily through appetite control, not a specific metabolism.
| Parameter | Potential pluses | Potential cons |
|---|---|---|
| Stamina | High oxidation of fats, less dependence on nutrition | Worse economy at high speeds |
| Force | Short efforts are mostly preserved | Possible drop in volumetric work |
| Hypertrophy | â | Often a smaller increase in fat-free mass |
| Weight loss | Decreased appetite, simplicity of rules | No metabolic advantage at equal calories |
| Weight categories | Rapid weight loss due to water and glycogen | Risk of impaired performance |
Side effects and risks
The first 1-2 weeks of switching to a keto diet is often accompanied by the so-called "keto flu": fatigue, headache, irritability, dizziness, decreased performance. Much of the symptoms are related to the loss of water and sodium when insulin decreases.
- Constipation due to low fiber content in the diet.
- Increase in LDL cholesterol in some people, especially with a high content of saturated fat.
- Deficiency of micronutrients (magnesium, potassium, vitamin C, folate) with restriction of vegetables and fruits.
- Risk of low energy availability in athletes with high expenditure.
- Decreasing the quality of high-intensity training and sprints.
For people with diabetes, especially those taking insulin or SGLT2 inhibitors, the unsupervised keto diet can be dangerous because of the risk of hypoglycemia or ketoacidosis.
Female athletes and athletes with large training volumes should especially carefully monitor calorie intake: in combination with a limited appetite, keto easily leads to a chronic energy deficit with hormonal consequences.
Keto diet is contraindicated in certain metabolic and liver diseases, pancreatitis, and requires caution in kidney diseases and gallstone disease.
Who is keto suitable for?
A ketogenic diet may be an acceptable choice for people who find it easier to stick to a deficit on a low-carb diet, for amateurs with moderate workloads, and for ultramarathoners experimenting with keto-adaptation under the supervision of experts.
For athletes whose results depend on high intensity — sprinters, representatives of team sports, wrestlers, middle-distance runners — a traditional diet with a sufficient amount of carbohydrates remains the best choice.
Compromise approaches—carbohydrate periodization, separate low-glycogen workouts, moderately low-carbohydrate diets—allow you to get some of the adaptations without losing the quality of key sessions.
If an athlete decides to try keto, it is wise to do so in the off-season, monitor training results, body composition and lipid profile, and be prepared to switch back to carbs if performance declines.
Editorial conclusion
Keto diet significantly increases the ability to burn fat, but does not improve, and often worsens results at high intensity.
For strength in the short term, keto is mostly neutral, but for maximum muscle gain it is probably inferior to diets with carbohydrates. There is no metabolic advantage in fat burning at the same caloric intake.
This is an individual tool, not a universal strategy, and it should be used consciously, taking into account the type of sport and health.
We also advise you to read our articles "Carbohydrates and glycogen restoration", "Intermittent fasting and muscle mass" and "Fats in the athlete's diet: role in hormonal synthesis".
The material is exclusively informative and does not replace a doctor's consultation. Switching to a ketogenic diet in the presence of diabetes, diseases of the kidneys, liver, pancreas or heart is possible only under medical supervision.
References
- 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):2785â2807.
- Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016;65(3):100â110.
- 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):769â776.
- 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.
- Hall KD, Chen KY, Guo J, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Am J Clin Nutr. 2016;104(2):324â333.
- 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):543â568.
