Nutrition plays an essential role for an athletes’ performance and health. One of the nutrients athletes pay more attention to, are carbohydrates. Carbohydrates (stored as glycogen in our muscle, liver and brain tissues) are the main source of energy as exercise intensifies, being our main fuel at intensities above the so-called “anaerobic threshold” (approximately >80% maximum consumption oxygen).1 For this reason, it has traditionally been recommended to increase the intake of this nutrient in athletes, so that the depletion or reduction of glycogen in the tissue (with the consequent fatigue) is delayed as far as possible.
Considering that glycogen stores are not unlimited and can be depleted during exercise, various nutritional interventions have been proposed that seek to “reserve” those stores. Following this idea, ketogenic diets have gained great popularity in recent years. These diets consist of a low carbohydrate consumption (normally less than 50 g/day), increasing fat consumption and trying to maintain the optimal protein dose (>1.2 g/kg/day).
Ketogenic diets have been shown to increase fat consumption during exercise by reducing the energy contribution of carbohydrates, mainly when performing low intensity exercise.2 Consequently, these diets could be potentially effective in preventing glycogen depletion and thereby increasing performance (or reducing fatigue during performance). However, there’s some controversy in relation to the effects of the ketogenic diet on performance. A study published in the prestigious journal Metabolism, evaluated a group of endurance athletes who had followed a ketogenic diet for 12 weeks, compared to another group who followed a high-carbohydrate diet.3 The results showed that after following a ketogenic diet, fat consumption during a 100 km bicycle test was higher, and, in addition, these athletes achieved a higher performance in a critical power test performed after 100 km.3 However, several studies led by researcher Louise Burke have shown that the ketogenic diet can also have negative effects. In 2017, Louise Burke’s group showed that a ketogenic diet for 3 weeks in elite endurance athletes increased fat consumption during exercise, but also decreased their performance and effort at the intensities at which the athletes competed (specifically, in a 20 km race walk).4 The same research group has recently confirmed these results, showing that even when carbohydrate intake is increased the days before the competition to restore glycogen stores, the ketogenic diet (which was performed for 25 days) influenced negatively to their performance results compared to a high-carbohydrate diet.5 In fact, these authors have shown that 5-6 days of ketogenic diet is sufficient to increase fat intake during exercise, but also worsen the athletes performance at high intensity (even when carbohydrate intake was restored before competition).6
Given the existing controversy, a recent systematic review tried to summarize the evidence in relation to the effects of a ketogenic diet on performance, including 17 studies conducted to healthy people.7 Out of the 13 available variables for endurance performance, none were found to be beneficial for a ketogenic diet, and two were even negative. Additionally, of the 16 available variables for strength or power, in two there were benefits for the ketogenic diet, but, in the others, there were no benefits and in three there were even negative effects.
Consequently, most studies suggest that a ketogenic diet has no effects on performance, although there are also certain studies that have found negative effects (possibly due to a lower inability to use glycogen at high intensities). However, it is important to mention that a ketogenic diet can be potentially beneficial in certain disciplines in which the intensities are low, and glycolysis is not so much required, as it may be in ultra-resistance disciplines (e.g., ultra-trail, Ironman). Nevertheless, in order to confirm this hypothesis, further research has to be done. Also, a ketogenic diet can bring certain benefits in other aspects such as weight loss. For instance, a study carried out with Olympic weight lifters showed that they lost body weight (-3.2 kg) after three months following a ketogenic diet without affecting their performance, although 2.3 kg were due to a loss of muscle mass.8 Another study carried out with CrossFit athletes showed that they decreased fat mass by 12% after three months following a ketogenic diet, although they could observe a 1.4% loss of muscle mass in the legs and an 8% loss in muscle thickness of the vastus lateralis. Nonetheless, these changes in body composition had no effect on the performance measured for strength and endurance tests.9
The ketogenic diet has still not shown a substantial benefit on performance so far, and in fact, most studies conclude that it has no benefits or can even be harmful as it decreases performance at high intensity. Also, its benefits on body composition are still unclear. Having in mind these research papers, this type of diet may not be recommended for elite sports until there is more evidence to support it.
Pedro L. Valenzuela
- Mata F, Valenzuela PL, Gimenez J, et al. Carbohydrate Availability and Physical Performance: Physiological Overview and Practical Recommendations. Nutrients. 2019;11(5):1084. doi:10.3390/nu11051084
- 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. 2018; 81:25-34. doi: 10.1016/j.metabol.2017.11.016
- 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
- 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 reproducibleitle. PLoS One. 2020;(Ahead of Print):1-31. doi:10.1371/journal.pone.0234027
- Burke LM, Whitfield J, Heikura IA, et al. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. J Physiol. 2020; In press. doi:10.1113/JP280221
- Murphy NE, Carrigan CT, Margolis LM. High-Fat Ketogenic Diets and Physical Performance: A Systematic Review. Adv Nutr. 2020;(8):1-11. doi:10.1093/advances/nmaa101
- Greene D, Varley B, Hartwig T, Chapman P, Rigney M. A low-carbohydrate ketogenic diet reduces body mass without compromising performance in powerlifting and Olympic weightlifting athletes. J strength Cond Res. 2018;32(12):3373-3382.
- Kephart W, Pledge C, Roberson P, et al. The Three-Month Effects of a Ketogenic Diet on Body Composition, Blood Parameters, and Performance Metrics in CrossFit Trainees: A Pilot Study. Sports. 2018;6(1):1. doi:10.3390/sports6010001
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