The Study

The authors studied the effects of a ketogenic diet (KD) on physical performances and metabolism during training in off-road cyclists. The effects of the KD on body mass and body composition of these athletes were also assessed. The subjects were eight males around 28 years old who have been training in off-road cycling for at least five years.

The Method

 The selected athletes were in their period of preparation training before the competition season. They were monitored at rest and during a 105-minute exercise protocol. For each individual, the respective lactate threshold, corresponding to the intensity where the production of lactate in the body starts to increase exponentially, was calculated the day before the 105-minute protocol. During the exercise protocol, the first 90 minutes were done by the athlete at 85% of lactate threshold, corresponding to moderate intensity exercise, while the 15 last minutes were done at 115% of lactate threshold, corresponding to the maximal intensity for this particular individual.

This study was divided in two phases and each subject was in turn monitored after 4 weeks of a standard diet (50% carbohydrates, 30% fats and 20% protein) and after 4 weeks of a KD (70% fat, 15% protein and 15% carbohydrates). It is important to note that such endurance athletes already present some adaptations in their energy metabolism when compared with other people in order to be able to exercise longer before experiencing fatigue. Consequently, the conclusions of the present study are relevant for endurance athletes, but may be not for the general population.


Eating according to the KD for 4 weeks changed the body mass and the body composition of the athletes, particularly by decreasing their weight and modifying their lipid and lipoprotein profiles. The authors suggest that this type of diet might be beneficial in sports requiring body mass control, such as the ones with weight class divisions. Furthermore, the indicators of post-exercise muscle damages were lower after the KD, probably because of the high uptake of good fat, such as omega-3. It is interesting to note that this difference was even more pronounced after the 15-minute maximal intensity exercise.

Many indicators suggest an increase in lipid metabolism following the KD. Furthermore, the consumption of oxygen increased during the exercise, probably to help maintain the same energy production than with a normal diet. During the first 90 minutes of the exercise, corresponding to the moderate intensity, the metabolism of the athletes used more fatty acids to produce energy when fed with a KD. However, during the last 15 minutes corresponding to the maximal intensity, the metabolism shifted back to the burning of glucose and the lactate level subsequently increased. This metabolism shift was observed with both diets, but more pronounced with the KD. However, the total concentration of lactate remained lower throughout the exercise when fed with KD in comparison to the normal diet.


The authors see benefits of a ketogenic diet by changes in body mass and body composition, as well as in the lipid and lipoprotein profile. “This may be of significance, not only in aerobic endurance sport disciplines, but also in sports that include weight class divisions and require body mass control and management”.  They surmise that the most likely reason for such changes included the predominance of polyunsaturated fatty acids with a significant intake of Ω-3 fatty acids that may be relate to reduced post exercise muscle damage, which was observed by lower rest and exercise plasma CK and LDH activity in this research project. These differences were especially visible after the maximal effort phase.

Finally, the authors suggest that KD may be favorable for aerobic endurance athletes, during the preparatory season, when a high volume and low to moderate intensity of training loads predominate in the training process. “High volume training on a ketogenic diet increases fat metabolism during exercise, reduces body mass and fat content and decreases post exercise muscle damage.”  However, it is noted that ketogenic diets decrease the ability to perform high intensity work, due to decreased glycogen muscle stores and the lower activity of glycolytic enzymes.


 Zajac A, Poprzecki S, et al (2014) Nutrients, 6, 2493-2508