The ketogenic diet is the latest trendy diet with people vehemently defending its claims ranging from increased endurance capacity to prevention of fatigue during exercise. Most of these enthusiastic claims have been based purely on personal testimonies. Call me old fashioned, but anecdotal evidence is not enough to change how we address sports nutrition for athletes. New scientific studies have emerged with the ketogenic diet’s rebirth onto the scene as a diet for performance and weight loss.

The ketogenic diet (keto for short) is a low carbohydrate, adequate protein and high fat diet. The original ketogenic diet was designed to have daily calories coming from 5% carbohydrates, 15% protein and 80% fat. The ketogenic diet actually has roots in fasting. In the early 1900’s, fasting was researched as a treatment for epilepsy. By removing carbohydrates from the diet and replacing them with high amounts of fat, doctors realized that the body went into a ketogenic state using ketone bodies as a fuel sources instead of glucose- just like in times of starvation. The ketogenic diet was then used by the Mayo Clinic as a diet to reduce the occurrences of seizures in pediatric epilepsy patients. The use ketogenic diet for seizure control is well documented and used to have it’s own chapter in medical textbooks, but it has taken a back seat to pharmaceuticals as the first line of treatment for epilepsy.

The ketogenic diet has changed slightly from its original protocol. Daily calories are usually made up of less than 10% carbohydrates, greater than 70% fat, and 15-20% protein. Keto is followed in a variety of ways because some people can stay in a ketogenic state with a higher amount of carbohydrate intake, but most of the time you will see recommendations ranging from 20-40 grams or less of carbohydrates a day. Keto is considered an adequate protein or moderate protein diet. High protein intake will knock an individual out of ketosis. In order to truly follow a ketogenic diet, it is recommend to test blood ketones. Some individuals use ketone urine testing strips, but the readings are not as accurate as testing blood ketone levels. Many people like to throw around the term “modified keto.” That’s an inaccurate way of saying they follow a low carbohydrate diet. To truly follow the ketogenic diet, you have to be in a state of ketosis. It is important to note that jumping in and out of ketosis can be harmful to the body. If you choose to follow the keto diet, take it seriously and test your ketones. To reap the possible benefits of the ketogenic diet, you will need to be in ketosis for at least a week. Although some researchers say it can take up to 3 weeks for the full adaption process.

Keto and Endurance

Low carb high fat diets, usually with less than 25% of daily calories coming from carbohydrates and 60% from fat, have been researched since the 80’s as a possible performance enhancer for endurance athletes. It is clear that after an adaption phase, the body can utilize fat at an increased capacity to support the muscles during exercise. However, whether this adaption enhances athletic performance is unclear.

Endurance based athletes enhance their capacity for fat oxidation during exercise simply through training. Reducing the body’s reliance on limited glycogen stores by up-regulating fat oxidation is the goal of the use of the ketogenic diet for endurance athletes. This would allow athletes to not have to be so reliant on carbohydrates for energy during training and competition. Increasing fat intake right before or during bouts of physical activity to increase free fatty acids for use by the muscles has been proven unsuccessful. So the interest was then directed to a chronic fat adaption and its possible performance benefits.

Endurance capacity has been been tested in both athletes and untrained individuals. In conditioned athletes an increase or no difference in endurance capacity at low intensity was noted. In athletes with who exercise at 70% VO2max or greater, negative impact on performance occurred. However, in untrained individuals there are contradictory results. Some studies show no change and others show an increase in endurance capacity, but researchers hypothesize this is due to the decrease in body mass and fat mass coupled with the introduction of a training program in untrained individuals. In regards to VO2max threshold, some studies showed a reduction while others showed improvement or no change. The time to exhaustion for endurance activity both increased and decreased in studies with conditioned athletes.

This very inconclusive evidence leads me to believe the question should still remain open, does the ketogenic diet benefit the performance of endurance athletes? I personally think this answer will vary greatly based on person to person. Even researchers in the studies noted that certain athletes would adapt amazingly, while others just were negatively affected by the metabolic changes. I think the jury is still out on endurance athletes, and this might be a trial and error experiment for an endurance athlete to see how they personally respond.

Keto and Strength and Power

Overall, an energy adequate ketogenic diet with adequate protein doesn’t appear to lead to muscle breakdown in low to moderate intensity exercise. Energy adequate means that you are eating enough calories for your resting metabolic rate and activity level, no more or no less. Some studies in high intensity sports did report a slight decrease in muscle mass with a decrease in body fat even when adequate calories were provided. Numerous studies report that power output for athletes decreased while on a ketogenic diet. While a small few, report no difference in power output as long as adequate protein intake was maintained.

The ketogenic diet appears to blunt insulin-like growth factor which plays a role in muscle growth and development. Insulin-like growth factor is considered to be the primary mediator in the anabolic response to when the muscles are put under mechanical stress like resistance training. Insulin-like growth factor mediates muscle cell growth and the formation of new muscle fibers. One study also reported a decrease in testosterone while participants were on a ketogenic diet. Based on the current data, it is very difficult to increase muscle mass while on the ketogenic diet. One researcher boldly stated it’s nearly impossible to build muscle mass on the ketogenic diet.

Keto and Weight Loss

It is fairly well documented that the ketogenic diet can result in weight loss. However, the question remains- is keto superior to other weight loss approaches? Currently, the data on weight loss diets has demonstrated that low carb, low fat and overall decreased caloric diets yield the same results at the one year mark. Initially, the low carbohydrate diets see a greater reduction in weight. This is due to the fact that each gram of glycogen is stored with 3 grams of water. People sometimes report feeling ‘deflated’ or ‘depleted’ on low carb diets. This is because the carbohydrate stored as glycogen in the muscles has been depleted along with the water. The actual mechanisms of how the ketogenic diet is effective for weight loss is still debated to a certain extent. The possible reasons for its effectiveness are appetite reduction due to protein intake, increase fat oxidation, reduction in reactive oxygen species and cytokines that cause an inflammatory response, and the thermic effect of digestion of proteins called gluconeogenesis. Gluconeogenesis is the energy intense process of converting protein to glucose for use by the body. One studied noted that there appears to be a decrease in the thyroid hormone free triiodothyronine (T3) while on the ketogenic diet, which regulates many body functions including metabolism. Low T3 levels would result in weight gain. The exact metabolic processes and mechanisms of how keto works for weight loss also need some more research.

The ketogenic diet may be a viable tool for sports that require athletes to compete in weight classes. Athletes that need to be in weight categories may sometimes use diuretics, water restriction, extreme calorie restriction, induced excessive sweating and other harmful methods for weight loss. These different methods can lead to electrolyte imbalance, water imbalance and have detrimental effects on performance. By manipulating carbohydrates, athletes can achieve weight loss without severe calorie restriction or other harmful practices. There needs to be some more research on the appropriate timing of keto prior to a competition to mitigate any strength and power loss. One researcher suggested that the ketogenic diet for two weeks prior to competition would allow enough time for the body to adapt and for the negative symptoms, sometimes referred to as ‘the keto flu,’ to subside. No one want to go into an athletic event with brain fog, headaches, GI distress and irritability.

Conclusions with Limitations

Based on the research available, there is no strong evidence that the ketogenic diet can enhance athletic performance. Current studies suggests that keto may have a neutral effect on people who train or compete at low to moderate intensity levels, but that it can negatively impact athletes who train or compete at high intensity levels. Some studies suggest it may help untrained individuals, but there is contradicting evidence. The endurance performance side of it is a wash with studies reporting positive results, negative results and no change at all. It may be useful for endurance performance, but if increasing muscle strength and size is also a goal the use of keto might be not be appropriate for an athlete. One thing does stand out as a clear finding across all the studies- protein should be sufficient for the athletes demands or there will be negative effects on performance and muscle mass. One suggested approach is 1.2-1.7 grams of protein per kilogram of bodyweight for athletes on the ketogenic diet to preserve muscle mass. With the limitation of food choices on the ketogenic diet, supplementation of vitamins and minerals may also be necessary.

Unfortunately, nutrition for athletic performance isn’t at the front of the line for receiving research dollars. The scientific evidence on the ketogenic diet for athletic performance is fairly sparse. My major concern is that studies are very short term with small sample sizes. It’s hard to draw absolute conclusions from such little evidence. The use of the ketogenic diet for performance is still based largely on theory and anecdotal evidence. There is a need for additional research to have a bigger base of solids facts because testimonials and emotions aren’t going to cut it. A group of European researchers said it best, “the use of the ketogenic diet for athletic performance is still a twilight zone- the middle ground between light and shadow, between science and superstition.”

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