Everything You Need to Know About Working Out On the Keto Diet

This low-carb, high-fat diet is popular among athletes, but there are some important things experts want you to know before you start mixing keto and exercise.

By now, you’ve probably heard about the ketogenic diet—you know, the one that allows you to eat *all* the healthy fats (and almost totally nixes carbs). Traditionally used to treat patients with epilepsy and other serious health issues, the keto diet has made its way into the mainstream and is especially popular with the fitness crowd. While it’s true that it may have some performance benefits, experts say there’s some very important info you need to know if you’re thinking about working out on keto.

You might not feel so great at first.

And, naturally, that might affect your workouts. “You may feel like you are in a fog for the first few days,” says Ramsey Bergeron, C.P.T., a seven-time Ironman, keto athlete, and owner of Bergeron Personal Training in Scottsdale, Arizona. “Your brain’s primary fuel source is glucose (from carbs), so as it switches over to ketone bodies created by breaking down fats in the liver, it will take some adjusting.” Luckily, the mental fog will typically pass after a few days, but Bergeron recommends skipping workouts that require quick reactions to stay safe, like riding your bike on roads with cars or doing a long, challenging outdoor hike.

The first few weeks on keto are not a good time to try a new workout.

“Keep doing what you are doing,” advises Bergeron. This is mainly because of the first point—most people don’t feel so great at first on keto. When extreme, this initial icky period can be dubbed the “keto flu” thanks to its flu-like grogginess and stomach upsets, which generally pass within a few days to a couple of weeks. Still, it’s probably not the best time to try out a new class or go for a PR. “I always recommend that my clients limit the variables when they do something different,” says Bergeron. “If you change too many things at once, you won’t know what worked and what didn’t.”

It’s super important that you don’t undereat before working out on keto.

“Make sure you’re giving your body enough energy and you’re not cutting calories too strictly,” says Lisa Booth, R.D.N., dietitian and health coach at 8fit. This is especially key because people on keto are likely to undereat, she says. “When you restrict an entire food group (in this case, carbs), you often naturally cut calories, but a keto diet also has an appetite-suppressing effect, so you might think you’re not hungry even if you don’t give your body enough energy.” When you reduce calories too much and combine that with working out, you will not only feel crappy but it can also affect your performance and results.

You could burn more fat during cardio.

This is one of the main reasons people swear by keto for weight loss. “When in ketosis, you aren’t using glycogen as your energy source,” says Booth. “Glycogen is a substance deposited in muscles and tissues as a reserve of carbohydrates. Instead, you’re using fat and ketone bodies. If you are following aerobic exercises such as running or biking, a keto diet can help increase fat oxidation, spare glycogen, produce less lactate and use less oxygen.” In other words, that could translate into more fat burned during aerobic exercise. “However, it probably won’t enhance performance,” he adds.

You really need to eat enough fat.

Otherwise, you’ll miss out on all the benefits, and your performance could suffer. “If you don’t eat enough fats on a keto diet, you are essentially doing an Atkins diet: high protein, low carb, AND low fat,” says Bergeron. “This can leave you extremely hungry, can actually lower your muscle mass, and is almost impossible to maintain.” There’s a reason why most low-carb diets get a bad rap. Without enough fat to compensate for the carbs you’re missing, you’re likely to feel tired and miss out on actually going into ketosis. That’s why it’s super important that the majority of your calories come from healthy fat sources like grass-fed meats, fish, avocado, and coconut oil, says Bergeron.

Working out on keto could help you reach your body composition goals.

“Studies have shown that ketogenic diets coupled with moderate-intensity exercise can positively affect one’s body composition,” says Chelsea Axe, D.C., C.S.C.S., fitness expert at DrAxe.com. “They have shown that ketogenic diets enhance the body’s ability to burn fat, both at rest and during low- to moderate-exercise intensities, so your weight-loss efforts may be maximized while training in these zones.” A 2011 study published in the Journal of Endocrinology found that a ketogenic diet increased hepatic growth hormone (HGH), which can improve strength and youthfulness. Though the study was done in rats and thus can’t be translated directly into human results, this is definitely a promising finding when talking about keto and exericse.

You might need to rethink your favorite HIIT workouts.

“Studies have shown that diets high in a specific macronutrient like fat promote an increased ability to utilize that macronutrient as fuel,” says Axe. “However, during high-intensity exercise, the body shifts to use glycogen as fuel regardless of your macronutrient ratio intake.” As you’ll remember from earlier, glycogen stores are fueled by carbs, which means if you’re not eating many of them, higher-intensity exercise performance can be compromised. “Instead, moderate intensity exercise is ideal for optimizing the body’s fat-burning potential,” says Axe. Because of this, athletes and exercisers who are doing intense workouts like CrossFit or HIIT are better off doing keto in their off-season or when they’re less focused on performance and more focused on body composition improvements.

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Listening to your body is crucial when mixing keto and exercise.

This is especially true in the first couple weeks you’re on a keto diet, but also during your whole experience. “If you often feel tired, dizzy, or exhausted, your body might not be working well on a very low-carb diet,” says Booth. “Your health and well-being should be the most important. Add some more carbs and see how you feel. If this makes you feel better, the keto diet might not be the right choice for you.”

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