If you’ve decided to try out the ketogenic (“keto” for short) diet, you probably hope to reap some of the myriad health benefits it claims to bring. The keto diet is designed to put the body in a state of ketosis. When your body is in ketosis, it is burning fat as a fuel source, as opposed to running on carbohydrates. The touted advantages of keto include weight loss, boosted energy, and fewer blood sugar swings. One question mark around the diet, however, is whether or not it does your exercise any favors.
“We have very little evidence that says the ketogenic diet is better for anything exercise-wise,” says Andy Galpin, PhD, a professor of kinesiology at California State University in Fullerton. Though for some types of exercise, it might affect performance more than others.
Here’s what you should know.
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1. It May Be Tougher to Boost Performance When It Comes to High-Intensity Exercise While on the Keto Diet
The majority of high-intensity exercise — activity that requires short, intense bursts of energy — is powered by the carbohydrates you eat. The human body stores carbohydrates in the muscle cells as muscle glycogen. That muscle glycogen becomes an immediate fuel source for your body when you do any sort of activity that periodically requires intense movement, including strength training, sprinting, and other sports.
If you’re on the keto diet, your body is burning fat for energy rather than carbohydrates (if you’re doing keto right). As a fuel source, that fat isn’t as easily burned as carbohydrates, so the process is overall less efficient, Galpin explains.
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The result is that the keto diet tends to limit performance during high-intensity workouts, while lower-intensity workouts may be less affected.
One small study from February 2017 that followed 42 healthy adults after they spent six weeks on the ketogenic diet found that the participants’ performance in endurance capacity and peak power dropped.
Galpin says he wouldn’t recommend the diet as a means to improve performance. “It’s not going to be any better for physical performance than the standard diet,” he says.
2. Keto May Boost Fat Burn
While keto may not be best for things that require short bursts of energy such as weightlifting or Spinning, it does seem to work well in burning more fat in people who like to do steady state aerobic exercise, like distance running or cycling at a steady pace.
A 2017 study of elite competitive walkers showed that the athletes who were on ketogenic diets were able to significantly increase fat burning in their bodies during training compared with walkers on a more standard diet that included carbohydrates. But despite burning more fat during training while on the keto diet, those athletes also experienced worse athletic performance than the athletes on the more standard diet. The race walkers on the keto diet complained that they felt it took greater exertion to complete the same workouts as the athletes on a more standard diet, and they were less likely to complete the assigned workouts compared with the athletes on the more standard diet.
3. You May Feel Like Your Gas Pedal Doesn’t Work as Well, Especially at First
As your body becomes “keto-adapted,” it begins to train itself to burn fat for energy instead of carbohydrates. Initially, that process may leave you feeling less energized than usual when it comes to working out.
Some research findings suggest that athletes have been successful in increasing their ability to use fat as a fuel source, according to Nitin K. Sethi, MD, a clinical associate professor of neurology at Weill Cornell Medicine in New York City, who began studying the effects of the keto diet on human performance because one of its original purposes was to treat epilepsy.
Dr. Sethi cites a small July 2017 study: “While athletes on a high-fat diet experienced reduced energy initially, the athletes experienced a return of higher levels later on, especially during exercise.”
Sethi cautions that the athletes in the study had more difficulties undertaking higher-intensity forms of exercise.
4. Your Body May Burn Calories Quicker on Keto
A 2018 study found that when overweight adults replaced carbohydrates in their diet with fat for a period of five months, their bodies were able to burn approximately 250 more calories per day than people who ate high-carbohydrate and low-fat diets. Keep in mind though that gram per gram, fat is more calorie-dense than carbohydrates: Each gram of fat has 9 calories, while a gram of protein or carbohydrate has 4 calories, according to the U.S. Department of Agriculture.
That difference in caloric burn over the course of a day can add up when it comes to weight loss. Remember, to lose weight the body needs to burn more calories than it takes in. “Calories are not the only thing that’s relevant, but they still matter,” Galpin says.
5. Keto Is Better for Maintaining Muscle Mass Than Adding Muscle
Maintaining and growing muscle mass helps slow down the aging process and protect against the risk of age-related bone loss. Muscle mass helps you to burn more calories each day, even when you aren’t exercising.
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If you are in the gym to keep the muscle mass you have, a ketogenic diet could work for you. But if you are looking to add muscle mass, the lower amounts of calories and protein you take in each day while you are on keto could make it tougher for you. “The average person is going to have a much harder time adding muscle on keto,” Galpin says.
Part of that difficulty would arise if you’re trying to consume fewer calories than you burn, meaning if you’re on the diet for weight loss (and that would be the case if you’re on any diet). But it’s also more difficult to build muscle on keto because your body has fewer carbohydrates available, which it would otherwise store in your muscles and use as part of the muscle growth process. It’s not impossible, but it’s a much less efficient way to build muscle, Galpin says.
6. Just Because You Are Burning Fat, That Doesn’t Mean You Are Losing Fat
While the ketogenic diet can be a good way to train your body to use fat as a fuel source, it doesn’t mean that when you exercise, the body will use up all that fat. You still need to be burning more calories overall than you’re consuming to actually lose fat (and lose weight).
“Burning fat doesn’t always mean fat loss,” Galpin says. When you are on keto, you burn more fat, but you also store more fat, because more of your calories are coming from fats than they otherwise would, he explains. “You still have to have a calorie deficit to see weight loss results.