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Ketogenic diets are not just for losing weight. Many endurance athletes also turn to these very low-carb, high-fat diets to boost their performance.

But athletes involved in high-intensity, short-duration sports might see drops in performance while on a ketogenic diet, suggests new research.

Researchers from Saint Louis University tested the anaerobic exercise performance of 16 men and women following either a low-carbohydrate ketogenic diet or a high-carbohydrate diet for four days.

People on the ketogenic diet performed more poorly at anaerobic exercise tasks than those eating more carbs.

Depending on the task, their performance was 4 to 15 percent lower than the high-carbohydrate group.

The study was published last month in the Journal of Sports Medicine and Physical Fitness.

Study author Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University, said that the results could make a big difference to athletes involved in sports that depend on short-burst anaerobic activities.

This includes sprint-type activities that occur in soccer and basketball and also short, intense activities like the 100-meter sprint and the triple jump.

Weiss added that the study “probably also applies to many aerobic activities, as other studies have demonstrated that high-intensity aerobic exercise performance may be compromised by low-carb diets — including keto.”

In light of these results, he advised athletes to avoid these diets unless they have “compelling reasons for following a low-carb diet.”

While this is a small study and people were on the two diets for only a few days, a 2017 review of previous research found similar early onset fatigue during short-duration activities while on a ketogenic diet.

Weiss said that additional studies will provide more insight into the pros and cons of ketogenic diets for athletic performance. For now, he suggests that athletes “err on the cautious side.”

Carbs or low carbs for energy

Endurance athletes such as marathon runners and long-distance cyclists might fare better on a ketogenic diet than players who use short bursts of energy.

Dr. Clifton Page, an assistant professor of orthopaedics and family medicine at the University of Miami Miller School of Medicine, said “ketogenic diets appear to be beneficial for endurance athletes after a period of adaptation.”

Page said it can take several months on a ketogenic diet for the body to switch from using carbohydrates as its main energy source to using fats — the “adaptation period.”

To support the body as it makes the switch, ketogenic diets are very high in fats.

Zach Bitter, an ultra-marathoner and holder of the 100-mile American record and 12-hour world record, said “fat is always the primary macronutrient in my diet. It can reach as high as 70 percent when I am recovering from a big race or workout.”

But this doesn’t mean ketogenic diets are high-protein diets.

In fact, eating too much protein can interfere with the production of ketones. These ketones are byproducts from the breakdown of fats and can be used as an alternative fuel source for the body when there isn’t much glucose.

Also, continuing to carb-load — such as with energy drinks and gels — can inhibit the body’s switch to using ketones for energy.

“Metabolically, a high-carbohydrate diet locks an athlete into a dependence on glucose as the dominant fuel for exercise,” said Jeff Volek, PhD, professor of human sciences and a registered dietitian at The Ohio State University and a leading researcher of carb-restricted diets.

The body stores some glucose for later as glycogen. But Volek said the body has only enough glycogen to last about one day, or for just a few hours of hard exercise.

So athletes on a high-carb diet need a steady intake of carbohydrates “to prevent this small carb fuel tank from running dry,” he said.

Bitter said that since he switched to a ketogenic diet, he has been able to cut his in-race fueling by over 50 percent.

He is quick to point out, though, that he doesn’t “demonize carbs.” But he tends to favor “low-glycemic sources of carbs in my meals when I do have them during peak training.”

One diet fits all athletes?

For endurance athletes, long-term use of ketogenic diets may boost not only performance, but also overall health.

“Keto-adaptation has enabled endurance athletes to set course and national records,” said Volek. “And a growing number of military personnel are using ketosis to improve physical and cognitive performance and manage obesity, metabolic health, oxygen toxicity symptoms, and post-traumatic stress disorder.”

Research — including a recent study by Volek — found that ketogenic diets may reduce body fat, type 2 diabetes, and metabolic syndrome. The last one is a group of conditions that includes high blood sugar and abnormal cholesterol levels.

“More than half of adults have prediabetes or diabetes in the U.S., including athletes,” said Volek. “A well-formulated ketogenic diet reverses the insulin-resistance phenotype more potently than any drug or lifestyle therapy.”

Bitter said he was attracted to a ketogenic diet not for the performance boost, but because during his training he would “wake up multiple times a night, experience big energy shifts throughout the day, and would get noticeable swelling in my legs and ankles after big workouts and races.”

The ketogenic diet helped with these symptoms.

Not everyone thinks ketogenic diets are for every athlete.

“In many cases, you can still perform well at your chosen sport on very few carbs. But you are unlikely to perform at as high a level as you’re accustomed to, and you’re certainly not likely to perform your best,” said Mike Israetel, PhD, head science consultant at Renaissance Periodization.

He added that your recovery after exercise will also be “significantly hampered, which will of course interfere with both performance and rates of improvement from training.”

Page warned that research shows that “without long-term adaption to the ketogenic diet, an athlete could experience adverse effects including reduced muscle glycogen, hypoglycemia, and impaired athletic performance.”

If you do opt for a ketogenic diet, it’s important to follow a plan designed by a nutritionist — or even work directly with someone experienced with these diets.

While a lot of research focuses on the benefits of ketogenic diets for competitive athletes, weekend warriors and others may also benefit.

“Recreational athletes tend to see more consistent benefits from adopting a ketogenic diet,” said Volek. “In part because, on average, they have a greater emphasis on weight loss, metabolic and health benefits.”