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Exercise and intermittent fasting: Experts weigh in on whether the combination is safe

Intermittent fasting ranks right up there with keto and Whole30 among trendy, often misunderstood, dietary practices. Several of the fitness experts (and influencers, admittedly) that I follow abide by intermittent fasting, and many even claim to work out during their fasting periods. As a non-faster who feels sluggish and hangry without my pre-gym protein bar, I’ve often wondered how people pull off intermittent fasting and exercise — and whether it’s even safe to do so.

Before we dive into that loaded inquiry, some background: Intermittent fasting involves alternating between periods of eating and fasting. You can do it in a number of different ways. Under the 16:8 regimen — arguably the most popular — you limit yourself to eating during a roughly eight-hour period and fast for around 16 hours, which can basically look like nixing breakfast and foregoing food after dinner, per Healthline. You can also simply stop eating after a certain hour, or go up to 24 hours without food, for instance, by fasting every other day, or one or two days a week.

Intermittent fasting proponents tout its potential to promote weight loss, as well as prevent diabetes, heart disease, and other chronic conditions. Several studies have looked at intermittent fasting in animals, although the research on humans is much more limited. But generally speaking, if you’re healthy, it doesn’t pose any major risks, Alex McDonald, a family physician specializing in sports medicine and internal medicine in San Bernardino, California, tells Mic.

For the most part, intermittent fasting is relatively safe, and exercising is one of the best things you can do for both your physical and your mental health. But is it a good idea to combine the two?

As with any intensive diet, though, you might want to consult with your family physician before you start. McDonald doesn’t recommend intermittent fasting if you have diabetes, kidney problems, or certain metabolic syndromes. In a Men’s Health story I wrote recently, experts also advise against the practice if you’re pregnant, breastfeeding, younger than 18, underweight, or at risk for developing an eating disorder.

For the most part, then, intermittent fasting is relatively safe, and as McDonald points out, “exercising is one of the best things you can do for both your physical and your mental health.” But is it a good idea to combine the two? According to the experts I spoke to, it depends on which intermittent fasting regimen you’re following, as well as what type of exercise you’re doing.

If you’re like most of the population and work out mainly to lose weight, feel good, or for some other health-related reason, and you adhere an intermittent fasting regimen with a less than 24-hour fasting window, you’ll probably be fine exercising, even during a fasting period, says Katherine Beals, an associate clinical professor of nutrition and integrative physiology at the University of Utah.

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As long you keep it to moderate intensity — say, a brisk walk or a leisurely bike ride, per the American Heart Association — for an hour or less, within roughly four hours of the last time you ate, you won’t deplete the glycogen stores (chains of a form of sugar called glucose found largely in your muscles and liver) that your body breaks down for fuel. The same applies if you follow the 16:8 regimen and work out in the morning before eating, which is what many people do anyway, even if they don’t call it intermittent fasting.

But McDonald advises against prolonged, high-intensity exercise — activities such as sprinting or very heavy weightlifting with few reps and minimal rest, for more than an hour to an hour and a half — during fasting periods. Doing so may use up your low blood glucose reserves and require your body to break down its glycogen stores into glucose, which it probably won't be able to do fast enough to fuel your workout. The end result of all of this could be an extremely challenging workout and a potentially fatiguing aftermath.

He recommends a relatively low-intensity strength and cardio combination during fasting windows. “The key is to start very slow and gradually build up," he says. If you feel lightheaded, dizzy, or very weak, he suggests eating something immediately and sticking to exercising only during feeding periods.

On the other hand, if your fasting periods last a full 24 hours (for instance, if you fast every other day), it’s probably best to limit your workouts, even if they're moderate-intensity, strictly to feeding days, Beals says. For starters, exercising during your fasting days will make you feel miserable. You’ll likely experience symptoms of low blood glucose levels, or hypoglycemia, which can include dizziness, fatigue, and confusion. And since your immune system runs on glucose, you may also be more prone to getting sick.

If you make it a habit to work out during your fasting periods, you risk not only losing muscle mass, but also damaging your vital organs, over time.

What’s especially concerning, though, is that exercising even at a moderate intensity requires some glucose, but if you've fasted for 24 hours, you've depleted your liver glycogen stores, meaning your body now needs to make glucose — and it’ll use amino acids, the building blocks of protein, to do so. How will your body get those amino acids? By breaking down the proteins that make up your blood, muscle tissues, and vital organs, Beals says. If you make it a habit to work out during your fasting periods, you risk not only losing muscle mass, but also damaging your vital organs, over time.

And if you’re an athlete — that is, training to improve performance, regardless of the sport you’re competing in — “fasting, intermittent or otherwise, is not going to be a good idea,” Beals says, since pairing rigorous athletic training with fasting carries the same risks.

If your fasting periods last for 24 hours and you really want to stay active during fasting days, Beals suggests something mild, like a short walk, or gentle stretching or core work.

I’m good with my current dietary pattern, including my strategy of pounding a protein bar before I hit the gym, but in case I ever do decide to take up intermittent fasting, I’ll know how to do it safely.