Intermittent Fasting? Here's How To Exercise Safely & Effectively

The idea of working out while intermittent fasting (IF) might seem pretty daunting.

If you've ever gone for an extended period without eating, you know that it can make you feel weak and fatigued at times—not to mention extremely hangry.

But many people tout the benefits of fasted cardio. So, how can you safely and productively combine your IF plan with your workout plan?     

The short answer is yes. The longer answer is that it depends on how you feel. "It's important to listen to your body," says Vincent Pedre, M.D., a functional medicine physician who often recommends IF to his patients.

As with most things, it all comes down to making a plan that works well for you. There are several different versions of IF, including the 16:8 plan (where people eat all their meals in an eight-hour window, then fast for 16) or the 5:2 plan (where people eat 500 to 600 calories for two days per week, then eat normally for the other five).

And depending on your preferred workout type and the time of day you like to exercise, you'll need to adjust your fast accordingly to get the nutrients your body needs.

"If you feel too weak to work out from fasting, then you should take care of your nutrition and work out later," Pedre explains.

When you roll out of bed in the morning and head to your 7 a.m. spin class, you likely haven't eaten anything since dinner the night before.

Doing cardio on an empty stomach like this may actually aid in your weight loss goals. 

A 2016 study in the Journal of Nutrition and Metabolism found that working out in a fasted state can increase fat oxidation1, which means that instead of relying on the carbs from your last meal for energy, your body is burning its fat stores.

A 2017 study in the Obesity journal found that regimented fasting periods are more effective than overall caloric restriction at helping with weight loss.

Other studies have been less conclusive about fasting—a 2014 study in the Journal of the International Society of Sports Nutrition found no difference in body composition changes between people who worked out fasted and those who ate before exercising. 

If you feel great working out on an empty stomach—more power to you! Keep doing what works for you.

But be aware that if you feel weak or lightheaded during your workout, it might be time to rethink your routine. 

"It is true that you may lose more fat working out fasted; however, you may also lose more muscle," says Jaime Schehr, N.D., R.D. "If the body's glycogen stores (aka energy stores) are depleted, the body can break down protein for fuel—the opposite of what most people are looking for." 

Not every workout is the same when it comes to timing it with your intermittent fasting plan.

When done correctly, fasted cardio can be a great addition to your exercise regimen. And depending on which type of workout you do, you may or may not need a meal directly afterward.

"I generally tell my clients that if it was a high-intensity interval training cardio where there was maybe some strength-based component to it, we want that closer to when they would break their fast," Schehr explains to mbg. "Versus if it's more of a steady-state cardio, then that doesn't necessarily have to be as close to breaking the fast."

If you're going for a slow and steady morning run, you might be OK waiting several hours after your workout to start eating. But if that makes you feel weak and dizzy, try consuming a meal directly after you finish working out. 

And with any intense workout on this new diet, it's important to ease into it.

In order to build muscle mass, it's important to fuel your body with protein and complex carbohydrates, either before working out or directly afterward.

"If somebody is working to increase mass and strength, then I would say that they need to have that workout either right before they break [their] fast or in their eating window, not at the end of their eating window when they then can't recover after that," Schehr suggests. 

Doing your strength training during your eating window will ensure your muscles have enough fuel to do the work without breaking down. 

Abby Cannon, J.D., R.D., CDN, explains it really comes down to what makes you feel best. "We kind of have to balance, what are your workout goals? How do you want to feel during your workout, and how do you actually feel during your workout?" she says. "If you are finding that you're totally depleted and therefore you can't effectively work out, something's not right. Having a little bit of food before might serve you so that you can actually hit those weightlifting goals or the HIIT workout goal that you have."

On days when you're feeling lower in energy, or during the period when you're first adjusting to intermittent fasting, low-intensity workouts can be easier on your body, either in a fasted state or during your eating window. 

Schehr says that if you're looking for an easy workout to fit in during the fasted window of your day, these can be great options. "

Ease into it. There is no perfect plan that works for everyone, so try building up your workouts slowly as your body adjusts to this new way of eating. "If you're used to eating beforehand, then you do an hourlong workout in the morning, that's going to be a lot," Cannon says. "But if you do like a 15-minute [or] 20-minute workout, you might be able to get away with it in transition."

Think about what time of day you prefer to work out. If you're someone who only works out pre-8 a.m., you may need to adjust your eating hours so you can consume a meal directly after a cardio workout. If you're a fan of afternoon workouts, that's a sweet spot for weight training. Low-intensity workouts can be done at any time of day.

Be flexible with your eating window. Your friend who does IF may swear by an eating window of 12 p.m. to 8 p.m., but if you love your morning runs, this won't work as well for you. You may need to try switching to a 9 a.m. to 5 p.m. eating window so you can fit in a post-workout protein shake. 

Hydrate. Just because you're going an extended period without eating does not mean you should skimp on water! "For somebody who's intermittent fasting, something that's really crucial is to make sure that if they're going to do fasted cardio, they are hydrated," Schehr says. Try to consume at least 72 ounces of water per day—more if you're sweating a lot. 

Use electrolytes. Low-calorie options like coconut water or natural sports drinks can help ensure your body gets electrolyte replenishment without breaking your fast. 

Vary your workouts. It's helpful for your body to do a mix of strength training and cardio to build muscle while also blasting fat. This can help you on your IF schedule, too. On days when you can fit in a morning workout, focus on cardio, and on days when you need to hit the gym in the evening, strength training is your BFF. When you're feeling super depleted, skip your workout or try yoga or Pilates instead. 

If your fasting plan involves extreme caloric restriction on certain days, Schehr recommends only a light, low-intensity workout (or no workout at all) on those lower-calorie days.

"I would not put anything that is going to increase caloric expenditure on a day when you're trying to conserve," she says. 

The reason for this? You don't want to deplete your body further by burning more calories than necessary.

Again, play around a bit and see what works for you. And remember—intermittent fasting means eating on a schedule, but it does not mean eating less overall.

"Fasting does not mean you don't eat," Schehr emphasizes. It means that you eat your calories and your proper nutrition in a windowed period of time. She explains that the misconception of "skipping meals" can be detrimental to the overall goal of IF. "What we really want to do here is make sure that we're optimizing the diet in that feeding window," she says. "And then you're going to get the best results."