The Dos (and Don’ts) of Intermittent Fasting and Workouts


Intermittent fasting is a trendy diet these days, and with research showing it can do everything from speed weight loss and boost immunity to regulate hormones and prevent disease, it may just be here to stay.

One reason proponents love it is because it’s straightforward: Simply reduce the window in which you eat during the day and watch the pounds melt off. Most people start with a 12-hour eating window and 12 hours of fasting. Stricter plans involve eating during an 8-hour window and fasting for 16 hours. Then, eat pretty much whatever you want during your eating window within reason (no carb counting or calorie restriction!) and you’ll see results.

Even better, say fans of intermittent fasting and medical experts: Exercising before you break your fast can help you burn more fat and boost your endurance. There are a few important points to keep in mind if you want to see the best results. Here are the dos and don’ts of intermittent fasting workouts:

Here’s how it works, says Shanshan Chen, PhD, assistant professor in the department of nutrition and basic sciences at Bastyr University in California: After about 8 hours of fasting, your body has used up its glycogen stores — the carbohydrates we usually burn for energy and which get stored as fat if we don’t. “If you exercise when your glycogen is gone, the body tries to find energy from the next best source — which ideally is fat,” says Chen.

Here’s the catch, says Chen: If you work out too hard in a fasted state — say, you do high-intensity interval training or go for a hard run instead of a moderate one — your body may require more energy than it’ll be able to draw from your fat cells alone. Which means it will burn muscle for energy. “Low-intensity exercise gives your body the best shot at burning fat,” she says.

If you’re doing a fasted workout and start to feel lightheaded, dizzy or like you don’t have enough energy to power you through even a low-intensity workout, stop immediately, says Chen. “Keep in mind that while fasted workouts might work for some people, it may not be the same for you,” she says. “Test the water, keep track of how you feel and see what works best for you.”

Some intermittent fasting plans call for up to 48 hours of fasting. If that’s what you’re doing, you’ll almost certainly not have enough energy to make it through even remotely strenuous exercise, says Chen.

To boost how long you can work out, you might assume a steady stream of calories is best. Yet Chen says if you’re used to eating three meals and two snacks a day during a 14-hour window, your body comes to expect those calories — and when that doesn’t happen, you have an energy crash. “After 1–2 months of intermittent fasting, your body will gradually adjust to a new pattern of eating,” says Chen, “and you won’t feel as hungry or have the same crashes. The more stable energy level translates to better endurance when you’re working out.”

“These people need more energy and nutrients than others,” says Chen. It’s also a good idea to check with your doctor before trying fasted workouts if you have a chronic condition, such as diabetes. “For those with diabetes, we want to make sure your sugar levels are well managed — not fluctuating,” says Chen.

After you’ve depleted your stores of glycogen (and maybe burned some excess fat), you’ll be craving nutrition. “Ideally, you should break your fast with protein, vegetables and a complex source of carbohydrate,” says Thanu Jeyapalan, a certified strength and conditioning specialist and clinic director at Yorkville Sports Medicine clinic. “For me, that generally means a chicken breast, quinoa or brown rice and high-fiber veggies,” he says.

Jeyapalan adds that it’s important to let your body get used to the shorter time window in which you’re eating before adding exercise to the equation. “Give yourself at least a week of intermittent fasting before adding exercise to the mix to give yourself the best shot at success,” he says.

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