You might have a friend who won't eat before noon and then stops again once 8 p.m. rolls around. Or maybe there's a colleague of yours who goes on a severe diet just two days a week. Just what are they doing?

It's called intermittent fasting. "Intermittent fasting means choosing a period of time that you're intentionally restricting food with the thought that you're gaining health benefits," says Paul Salter, MS, RD, CSCS, founder of Fit In Your Dress Brides. It's actually legitimate—and has research backing it up (more on that later).

How Do You Do Intermittent Fasting?

There are various ways to incorporate intermittent fasting: There's the classic 5:2 approach (five days of normal eating followed by two fasting days), fasting every other day or eating in a designated time window (say noon to 8 p.m.), says New York City–based Joy Dubost, PhD, RD.

Most people use IF and its varying approaches as a way to lose weight, something that was initially popularized by the 5:2 diet, Dubost says. On 5:2, you maintain a regular diet five days a week and then consume 400 to 600 calories on the two fasting days. The weight loss results are largely built on math. "While the mechanism still needs to be understood, on average over the course of the week or month you're ultimately cutting down on calories, which is driving weight loss," she says.

This is also what happens if you're doing alternate-day fasting, where you may not eat anything on fasting days. The weight loss happens for a similar reason: You're eating fewer calories.

Having a fasting window (where you only eat during, say, an eight-hour period each day) also limits the amount of time you can consume food; thus, people are more likely to eat less. Another benefit: to potentially reduce bingeing. If you find that you have a tough time with, say, nighttime bingeing, you can set a daily fasting window appropriately. "IF can help curtail calorie intake by setting red- and green-light times for eating," adds Salter. Many times, the eating window is set at eight hours long with a 16-hour fast. For instance, you eat from 10 a.m. to 6 p.m. and designate the evening and overnight hours as fasts. (If this sounds similar to the person who says they're not going to eat after 8 p.m., that's because it is. Stopping eating after a certain time at night until the following morning is also a fast.)

Does It Work?

Research shows that alternate-day fasting (where people ate zero calories every other day) was as effective for weight loss as cutting 400 calories every day, and the participants were able to keep the weight off for nearly six months (though it goes without saying that this can be incredibly tough to do in the real world). Another earlier study compared premenopausal women on a calorie-restricted diet (decreasing cals by 25 percent per day) versus two consecutive days of fasting (eating about 500 calories each day) and found they were equally as effective in weight loss (about 12 to 14 pounds over six months).

One study from the University of Illinois at Chicago looked at obese adults who restricted themselves to an eight-hour window of eating (between 10 a.m. and 6 p.m.). Compared to a control group, those who fasted lost about 3 percent more weight and ate about 340 fewer calories per day over a 12-week span. They also saw their blood pressure decrease but no change in other metabolic measures like fat or belly fat, cholesterol or fasting insulin.

Overall, here's what to keep in mind: "The results are promising, but more research definitely needs to be done. There are mixed results when it comes to metabolic benefits, like changes to lipids, the insulin response and glucose control," Dubost says, adding that you should speak with your doctor first before starting any drastic dietary changes, especially if you're on medication or have a condition like diabetes.

The Bottom Line

If the traditional approach (eating fewer calories every day without adhering to an eating schedule) sounds more appealing, intermittent fasting is not necessary to lose weight. But if you'd rather not keep track of calories, IF may be another way to drop some pounds. Even if an eight-hour window of allowed eating seems too restrictive for your lifestyle, you can start with a 12-hour fast. For instance, you'd finish dinner by 7 p.m. and then wouldn't have breakfast until 7 a.m.

"This gives your digestive system a break and helps your body use energy efficiently," Salter says. This will provide more leeway and allows you to have a more normal eating schedule. The benefit here is that you're not eating around the clock; the drawback is that it may not help you eliminate as many calories as a longer fast.

Whatever approach you choose, just know that to get the most out of it, you still need to eat healthy, nutritiously dense foods. This isn't the time to go wild with doughnuts and pizza. Fruits, vegetables, legumes, nuts, seeds and lean meats should make up the biggest part of your menu. But that's the catch with any diet, right? General healthy eating should always come first.


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