Keto Diet and Its Effect on Sports Performance

Nutrition plays a fundamental role in the performance of athletes, and the nutrient that receives the most attention is carbohydrates. Carbohydrates (stored as glycogen in tissues such as muscle, liver, and brain) are the primary energy source as exercise intensity increases. One of the main metabolic objectives sought by athletes is a lower use of glycogen, and greater oxidation of fats, generating greater glycogen savings. A ketogenic diet seems to show greater fat oxidation during exercise, but can this type of nutritional strategy increase sports performance?

It is important that you place yourself well in the context and be clear about the keto diet since this article will investigate whether the keto diet is favourable or not for a better performance of sports activities. Therefore, it is important to clarify the concept of the ketogenic diet.

What is Keto Diet?

The ketogenic diet has reached the sports world, and more and more athletes are trying to lose weight, especially in the form of fat. It is one of the most popular diets today, and while many people consider they are on a ketogenic diet, few are doing it well.

Although there is no single definition, and it creates a bit of confusion, we are going to talk about the ketogenic diet as those nutritional approaches in which the daily amount of carbohydrates does not exceed 50 g/day or, usually, 5% of the total calories.

The ketogenic diet is defined by its ability to raise ketone bodies in the blood in such a way that a physiological state called ketosis is reached. It is an eating plan that seeks to generate a state of ketosis in the body similar to that produced when fasting. In this diet, carbohydrates are restricted to a maximum of 50 grams per day while maintaining moderate protein consumption (1.2-1.5 g / kg/day) together with the predominance of the remaining caloric intake from fat (60-80%).

Only the brain already consumes 100 grams of glucose daily, so by providing that minimum amount through the diet, the body must use an alternative energy substrate to glucose. This substrate is the ketone body, which it obtains from fatty acids. Therefore, there is a shift in metabolism in favour of fats.

After about 4-5 days of the ketogenic diet, the body begins to produce ketone bodies: aceto-acetate, beta-hydroxybutyrate, and acetone through a process called ketogenesis. 

Ketogenesis (physiological ketosis) is a vital and highly conserved adaptive mechanism throughout evolution. Ketone bodies act as an energy substrate for the brain, which cannot use fatty acids from our fat stores during periods of carbohydrate (glucose) deprivation or very long fasts. In addition to being used by the brain, these ketone bodies can fuel the heart and skeletal muscle.

Thus, when hepatic glycogen stores are decreased, or the glucagon ratio is high (due to low glucose availability), ketogenesis is induced. In contrast, when the concentration of ketone bodies is low, its synthesis is reduced. Thus, the concentrations of these substances in the blood will vary depending on the different situations in which you find yourself.

Sports Performance

Keto Diet and Sports Performance

The body uses fats as energy fuel at moderate and low intensities, to which endurance sports such as road cycling, medium and long-distance triathlon, marathon, ultramarathon, etc., are performed.

At these intensities, the aerobic oxidative system uses a combination of glucose and fat. The ketogenic diet prevents the body from having glucose during exercise, so it has no choice but to use fatty acids. Then its oxidation is favoured; that is, more fat is burned.

That's why ultra-resistance athletes can follow a keto diet without problems since, at the intensity at which they work, both glucose and fatty acids can be used. Some of the advantages of the keto diet in sports performance are:

  • Increases the energy capacity provided by fat before reaching the anaerobic threshold, delaying muscle fatigue.
  • Increases the muscle's ability to generate energy from fat, being used before and for longer than hydrate compared to a diet rich in carbohydrates.
  • By improving the use of fat, glycogen savings are enhanced for sprints, slopes or intensity peaks, like high-intensity exercises.
  • Body composition and weight/power ratio are improved by reducing the percentage of fat while maintaining muscle.

Impact of Keto Diet on Sports Performance in the Light of Evidence

The results of different studies suggest that the keto diet has a potential benefit over other procedures used to reduce body weight that carries health risks and result in a significant detriment to the athlete's physical condition. 

During the keto period, free fatty acids are in a much higher concentration in the blood and are used to a much greater degree than when the mixed diet is followed (which indicates a correct follow-up of both protocols by athletes).

However, it does not mean that it is optimal since it has not been shown to provide advantages in sports performance. In addition, in most of the long-term sports disciplines, the intensity at which it competes is high, so the intake of carbohydrates will be essential.

A person preparing keto food

Exercising and its Effects While You're on a Keto Diet

As the intensity of exercise increases, more carbohydrates are used as an energy substrate and fewer fats since the anaerobic oxidative system predominates. In such a way that, at high intensities, at which most sports are performed, fats are practically not used, but glucose, so they depend on the reserves of hepatic and muscular glycogen.

The effect of the ketogenic diet is to reduce these reserves to a minimum by providing that minimum amount of carbohydrates, which will therefore impair high-intensity performance (Burke, 2015; Aragon et al., 2017).

Muscle and liver glycogen is a rapid source of energy, and the main substrate at high intensities, having limited reserves of about 100g in the liver, and 300-700g in skeletal muscle, which can lead to the depletion of these deposits a performance loss.

Ketogenic diets (<50 g of carbohydrates/day) have taken a major role as a method of saving glycogen during exercise since they have shown a reduction in carbohydrate oxidation and a 2-3 times increase in the oxidation of carbohydrates. Fats, seeking the objective of becoming large fat oxidizers, the reserves of this energy substrate being the most abundant.


Despite this, their effectiveness in improving performance remains controversial, as no improvements appear to be found after using them.

The possible decreases in performance observed in some studies may be due to a lower level of glycogen at the beginning of exercise, despite its lower use later.

Similarly, it is recommended to prolong these for a period greater than 4 weeks so that in this way, the metabolic adaptations they generate are achieved and avoid the decreases in performance that usually occur in these first weeks.

A review of 926 articles was carried out, of which a total of 17 publications were finally included in this review, evaluating the impact on the performance of a ketogenic diet compared to a standard diet.

The studies were carried out in healthy non-obese people, both men and women, with a higher or lower level of performance, and all being over 18 years of age.

To accept the ketogenic diet group, the carbohydrate intake should be less than 50 g / day, and the levels of β-hydroxybutyrate ≥ 0.5 mmol / L. The diet must have been consumed for ≥ 14 days, the mean duration of the dietary intervention being 58 ± 27 days.

Studies were excluded if they did not meet the above requirements, if they had a carbohydrate replacement before the competition, if one group took some form of supplement and the other did not, or if dietary intake was not reported.


Final Words

Nutrition plays a fundamental role in the performance of athletes, and the nutrient that receives the most attention from athletes is carbohydrates. Carbohydrates (stored as glycogen in tissues such as muscle, liver, and brain) are the main source of energy as exercise intensity increases, being practically the only fuel at intensities higher than the so-called "anaerobic threshold" (approximately> 80% maximum consumption oxygen). That is why it has traditionally been recommended to increase the intake of this nutrient in athletes in such a way that the depletion or depletion of glycogen stores (with the consequent fatigue) is delayed as much as possible.