Want to reap the benefits of a keto diet? Before you adopt the high-fat lifestyle, know this: It has major implications for your fitness routine. Whether you’re a barbell junkie or an ultra-marathoner, here’s what you need to know about training while in fat-burning mode.

Keto: A Metabolic Shift

The reason following a keto diet can have such a significant impact on your weight and well-being? It causes a fundamental shift in your body’s metabolism (a.k.a. how it uses food to produce energy).

Under normal circumstances, our bodies use carbohydrates as their primary source of energy, says sports and performance nutritionist Lauren Manganiello, M.S., R.D., C.P.T. However, when we restrict carbohydrates, our bodies eventually turn to fats for energy.

The unicorn-like state in which we break down fat into usable energy compounds called ketones: ‘ketosis.’

This significant metabolic shift has been credited with helping us shed body fat and feel more energized and focused. It also significantly impacts how our body responds to exercise.

Whether keto enhances or sabotages your fitness, though, depends on what kind of workouts you’re into.

The Keto Diet, Weight Training, And HIIT

If you’re a regular CrossFit-er or weight room devotee, keto can have a notable, not-so-great impact on your workout routine.

Weightlifting and HIIT workouts (like sprints) are forms of anaerobic exercise, meaning our body powers them more or less without using oxygen. Instead, it relies on sugar, either in the form of glucose (which circulates in our blood) or glycogen (which is stored in our muscles and liver).

Why? The body can create energy from sugar faster than it can from other sources—and activities like lifting heavy weights and sprinting require a hefty, quick supply of energy.

Since anaerobic exercise relies so heavily on sugar, it’s not exactly compatible with the virtually sugar-free keto diet. “If you’re following a keto diet, you don’t have many carbs in your system,” says Manganiello. The result: Your performance takes a hit.

One 2018 Journal of Sports Medicine and Physical Fitness study, for example, found that men and women who followed a keto diet experienced declines in performance during short, intense anaerobic exercise.

Similarly, a small 2017 Journal of the International Society of Sports Nutrition study found that when five athletes went keto for 10 weeks, their HIIT performance tanked. (They did, though, shed fat and improve their body composition.)

Ultimately, whether you’re trying to hit a new squat PR or sprint fast while keto, you’ll probably feel like you’re running on empty, because, well, you are.

There’s just no getting around it: The body needs sugar to power these types of workouts.

The Fix: Carb Cycling

If you want to reap the benefits of a lower-carb, higher-fat diet and still feel good in the gym, Manganiello recommends carb cycling.

Instead of following the same typical keto macros every day, carb cycling involves adjusting how many carbs you eat based on your workout for the day.

So, while you might eat full-blown keto-style on low-intensity or rest days, you’ll up your carb intake on more intense training days, she says. This way, you provide your body with the carbs it needs to fuel your workouts, but no more.

Related: I Tried Carb Cycling For A Month—Here’s What Happened

Of course, how many carbs you need to power a quality lift or HIIT session depends on your size and training level.

Start by upping your carb intake to between 100 and 150 grams, which, though not keto, is still considered low-carb. From there, gauge how you feel and perform, and adjust as needed. (If performance is top priority, you’ll likely need to up the carbs significantly more.)

The Keto Diet And Endurance Exercise

While keto might be a performance-killer for weight-room regulars and HIIT class lovers, it can be a useful tool for endurance athletes.

Unlike weightlifting and HIIT, lower-intensity endurance exercise doesn’t heavily rely on carbs for fuel, explains Manganiello.

Instead, this type of exercise—called aerobic exercise—primarily relies on oxygen (and some fats).

That’s why keto can be a good move for cardio buffs. Not only do you have more fats readily available to support aerobic exercise, but your body is better-adapted to utilizing those fats.

In fact, one 2018 Metabolism study found that 20 endurance-trained males who followed keto for 12 weeks saw greater improvements in fat loss and performance than athletes who followed a higher-carbohydrate diet. Unsurprisingly, the keto athletes oxidized (‘burned’) more fat than the carb-eaters.

Just keep in mind that adapting to a keto diet takes time, so it’ll likely take a few weeks to feel groovy during training.

The Bottom Line

Popular as keto may be, it’s still an extreme shift from the average American diet—even a healthy one. Manganiello suggests working with a dietitian, making the switch slowly, and listening to your body throughout the process.

Related: How To Eat Carbs And Still Lose Weight

While you can shed fat and improve your body composition on keto, remember that you’ll be better equipped to handle endurance-focused workouts than high-intensity training while fueling with fat.

If a certain breed of fitness is top priority for you, tweak your nutrition as necessary to power your performance.

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