An increasing number of runners are trying the ketogenic diet—a low-carbohydrate, high-fat diet that causes the body to produce ketones to substitute for the carbs it’s no longer getting from food. Originally intended to treat epilepsy, the ketogenic diet went on to become a popular way to lose weight and is now being used by runners and other athletes to increase endurance.

The typical ketogenic runner aims to get about 70 percent of his or her daily calories from fat and limits carbohydrate consumption to 10-15 percent of total calories. If that sounds extreme—well, it is.

But proponents of the ketogenic diet for runners insist that it greatly increases the ability of the muscles to burn fat during exercise and thereby boosts endurance.

Science tells a different story, however. Studies have shown that, although the ketogenic diet does improve fat-burning capacity, this adaptation does not translate to better performance.

In fact, performance tends to decrease on a ketogenic diet, especially at higher intensities of exercise.

 

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This was shown most recently in a Polish study involving competitive mountain bikers. Eight athletes were placed on each of two diets for four weeks in random order. One diet was ketogenic (70 percent fat, 15 percent carbohydrate) and the other was balanced (50 percent carbohydrate, 30 percent fat).

The ketogenic diet resulted in a significant increase in the athletes’ capacity to burn fat during moderate-intensity exercise. But performance tests revealed that this adaptation came at a steep cost.

On the balanced diet, the subjects generated 257 watts at lactate threshold intensity and 362 watts during a 15-minute maximal effort.

 

On the low-carb diet, these numbers dropped to 246 watts and 350 watts. According to the study’s authors, the cause of the decline on the ketogenic diet was impairment of the muscles’ ability to burn carbs, which is critical to performance at higher intensities.  

Reduced performance is not the only downside of a ketogenic diet for runners. Another common consequence is unfavorable changes in blood lipids.

Not everyone sees their cholesterol level spike on a 70 percent fat diet, but many people do, so you’re rolling the dice by even trying it.

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Another potential pitfall is vitamin and mineral deficiencies. This happens because eliminating carbohydrate-rich foods from the diet also does away with the micronutrients—among them thiamin, folate, and potassium—that are abundant in high-carb foods.

Other negative effects of a ketogenic diet for runners are psychological and social. In order to consistently get 70 percent of your daily calories from fat, you have to eat large amounts of a very short list of foods (eggs, dairy, fatty meats, oils) and go out of your way to avoid perfectly healthy foods such as fruit and whole grains that are staples in most cuisines.

This makes it very hard to relax and enjoy a nice family meal or a dinner out with friends. And for some people the problem goes even further. Men and women who are attracted to extreme diets like the ketogenic diet tend to be susceptible to disordered eating, a predisposition that is then brought out by the diet.

Is it any wonder that, in my worldwide research on the diets of the healthiest and fittest humans—professional endurance athletes—I have not found a single one that follows a ketogenic diet?