Keto diets and ketone supplements have become popular among endurance runners for their supposed ability to delay fatigue, but new research shows they do not provide a clear advantage. A review of studies, published in Sports Health: A Multidisciplinary Approach, found no significant impact on aerobic performance from following a ketogenic diet or using ketone supplements.
During endurance activities like long-distance running, the body uses oxygen to produce energy, relying primarily on carbohydrates. As aerobic capacity improves, the heart, lungs, and muscles can perform more efficiently, allowing for longer periods of exertion without fatigue. The reviewed findings align with a warning by the International Society of Sports Nutrition that keto diets may lead to various negative symptoms for ultra-marathon runners, such as fatigue, nausea, headaches, and gastrointestinal issues.
“A keto diet doesn’t improve endurance performance and could even be detrimental,” said Andy DuBois, head coach at Mile 27 Endurance Coaching in Australia. He advises against it for his clients, including Hong Kong ultrarunners Jeff Campbell and Ryan Whelan. Instead, DuBois recommends a high-carbohydrate diet for better results.
A ketogenic diet is typically composed of at least 60% fat and less than 10% carbohydrates. Ketone supplements aim to increase ketone levels in the body, promoting fat oxidation as a fuel source. In theory, this shift allows athletes to tap into fat stores during long-distance events. However, nutritionist Vicky Sham, based in Hong Kong, explains that most athletes train or race at intensities too high to be fueled effectively by fat. A low-carb diet may also deplete muscle glycogen, leading to muscle breakdown instead of fat oxidation—a problematic state for endurance runners.
Limited data exists on the effects of keto diets on female endurance athletes. Only one out of 133 athletes included in the reviewed studies was female. Sham suggests that female runners may see even fewer benefits from keto diets due to the hormone estrogen, which already promotes fat oxidation.
For endurance athletes, Sham and DuBois recommend prioritizing adequate carbohydrate intake for training and sufficient protein for recovery. Protein helps in muscle repair and growth, especially crucial after intense workouts. Sham also advises focusing on whole foods, gut-healthy fermented foods for immunity, and plenty of fresh vegetables and fruits for their antioxidant properties.
Ultimately, healthy runners can achieve similar fat-burning benefits from endurance exercises alone, without restrictive keto diets. DuBois recommends his clients practice consuming 40 to 60 grams of carbohydrates per hour during training and gradually increasing intake as they train their gut for race day.
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