How To Adjust Your Workout Routine If You’re Intermittent Fasting

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by Gabrielle Kassel, CF-L1
By now you’ve most likely heard of intermittent fasting (IF), which involves regularly restricting your food intake for particular periods of time. Though intermittent fasting has gotten a lot of attention for weight loss (fewer hours of eating per week typically means fewer calories consumed), its benefits don’t end there.
“Emerging research suggests intermittent fasting may help improve your blood lipid profile, insulin sensitivity, and aging process,” says dietitian Lauren Harris-Pincus, R.D.N., author of The Protein-Packed Breakfast Club. Proponents of intermittent fasting also claim it boosts concentration, improves sleep quality, and supports clear skin.
Despite IF’s many proposed perks, though, it may not be all positives for athletes and avid exercisers. In fact, there are a few things super-active people should keep in mind when considering giving IF a go.

Exercising While Intermittent Fasting​

“Intermittent fasting and exercise certainly can be compatible,” says trainer and dietitian Charlotte Martin, R.D.N., C.P.T. However, it really depends on what type of exercise we’re talking about—and when in your IF routine you pencil in your workouts.
Read More: 8 Major Mistakes People Make When Intermittent Fasting
As a general rule of thumb, the lower the intensity of the exercise, the safer and easier it is to do while intermittent fasting. If you are healthy and relatively fit, you can absolutely go to a yoga class, for a short run, or a walk around the park while intermittent fasting, Martin says. And you can do so during either a feeding or a fasting period.
If you want to get stronger or enjoy strenuous exercise, though, the situation gets dicey. “It’s nearly impossible to do intermittent fasting while also trying to optimize your athlete performance,” says Martin.

The Issue With Intermittent Fasting And Strenuous Exercise​

While your body primarily uses fat and oxygen for low-intensity exercise, it relies on glycogen (stored carbohydrates) as its primary fuel source for higher-intensity exercise, explains sports dietitian Rachel Fine, R.D., C.S.S.D.
Thing is, glycogen depletes rapidly—so you won’t have much available to power a workout after a period of fasting. So, if you do something high-intensity, like sprint intervals or a lifting session, you force your body to either burn fat or pull protein out of your muscles for energy.
Read More: How To Pick The Perfect Post-Workout Snack (That Won’t Wreck Your Stomach)
Consistently pull protein from your muscles for energy and you’ll lose muscle mass. Not only does this impact your workout performance, but it also messes with your metabolism.
“The more lean muscle mass you have, the faster your metabolism is, and the more calories you burn per day,” explains Harris-Pincus. Work out fasted and you could inadvertently slow down your metabolism and make weight loss harder.
Plus, your workouts will also feel crummy. “Exercising without adequate fuel is like asking your car to leave the garage when the tank’s on empty. You won’t go anywhere!” Harris-Pincus says.
Even if you train at the end of a feeding period, when glycogen stores are higher, you may suffer. “You’ll have more energy during the workout, but if you can’t recover with adequate protein and carbs afterward, you’re not actually going to reap the benefits,” says Martin.

When Fasters Can Do Higher-Intensity Workouts​

If you’re determined to practice intermittent fasting and still log high-intensity workouts, the time to do it is right smack-dab in the middle of a feeding period.
“This is the most optimal time for performance and recovery,” says Martin. It allows you to hit the gym with fuel in the tank and properly replenish those glycogen stores and support your muscles with protein after.
Unfortunately, that may not be possible for folks who need to exercise either before or after work. If you’re in that boat, you’re best off training at the end of a fasting period, which for most people will be in the morning, says trainer and nutritionist Jose Vallejo, R.D., C.P.T. “This allows you to eat a protein-rich breakfast—ideally within 30 minutes of leaving the gym—to support muscle recovery,” he says.

If You Have To Choose Between Fasting And Exercise, Pick Exercise​

Popular as intermittent fasting might be, it does require that you time your high-intensity workouts very carefully—and it’s not the end-all be-all for weight loss.
Plus, “the benefits of being active go way beyond just weight loss,” says Martin. “Exercising also supports the overall health of your mind and body.”
If you’re struggling to adjust your workout routine to make intermittent fasting work, don’t sweat it. You’re better off tweaking your eating habits than giving up on fitness altogether.
“There are plenty of other eating programs out there that may be better suited to your fitness goals and lifestyle,” says Harris-Pincus. When in doubt, a nutritionist or sports dietitian can help you figure out what’s best for you.

Safety Considerations Of Exercising And Fasting​

Regardless of when you work out, “you’ll need to ease off your usual fitness schedule in the first few weeks of intermittent fasting,” says Martin. This gives your body time to adjust to the eating plan. It also positions you to better tune into your body when you exercise again.
When you do get moving, though, be wary of any warning signs. “Lightheadedness, pain, nausea, and feelings of generalized fatigue or weakness are your body’s way of telling you something is wrong,” says Harris-Pincus. If you encounter these red flags, don’t push through them. Instead, ease off and give your healthcare provider a call if the sensation continues.
“If you push through and injure yourself, you thwart your overall health and weight-loss goals,” says Harris-Pincus.