Does Intermittent Fasting Work—and Is It Healthy?

You've probably heard of people fasting for religious reasons (for example, during Ramadan) or to prep for a medical procedure. But nutritionists generally throw shade at the idea of skipping meals as a weight-loss technique, since doing so can slow your metabolism, send your body into starvation mode, and cause you to cling to every spare ounce of fat.

Still, the recent rise of diets featuring intermittent or alternate-day fasting may be more than just a gimmick. Some evidence even indicates that, done properly, the occasional fast could be key to dropping stubborn pounds.

Less on the Menu
To be clear, fasting for weight loss doesn't mean not eating at all. Instead, on two or more nonconsecutive days per week, you limit your total daily intake to about 500 to 700 calories. The rest of the time you can chow down sans rules.

Depriving yourself for only one day at a time can make such plans easier to stick to, says Mark P. Mattson, Ph.D., who has studied fasting in animals and humans at the Laboratory of Neurosciences at the National Institute on Aging, in Baltimore. "You're not as likely to say 'Screw it!' in the heat of the moment and overeat, because you only have to hold out until tomorrow to eat what you want," he says.

And studies show that when tomorrow rolls around, most people don't gorge themselves to compensate. Personal trainer John Romaniello, author of Man 2.0: Engineering the Alpha, says that many people find they're less hungry than usual when they fast occasionally, even though they're eating less overall. He suspects this is because fasting affects the production of the hormone ghrelin, which regulates hunger and body weight, although more research is still needed to prove that link.

There's most likely a psychological component as well. "I find that intermittent fasting makes many people more aware of the sensation of true physical hunger," says integrative physician Frank Lipman, M.D., author of Revive: Stop Feeling Spent and Start Living Again. "Dieters realize that what they've often perceived as hunger is really more habit or thirst."

Regardless of the reason, evidence suggests fasting works: Research published in the International Journal of Obesity found that overweight women who ate 650 calories per day twice a week and up to 2,500 calories on the other days lost about 14 pounds over six months. In comparison, those who ate about 1,500 calories daily lost around 12 pounds.

Is Hunger Hardwired?
Fasting, even for short periods, can force your body to burn fat for fuel because its usual sources (glucose in the blood and glycogen in the liver, both typically derived from food you've eaten recently) aren't available, explains Mattson.

Some critics say that flies in the face of long-held beliefs that we need to eat every three to four hours to keep blood sugar levels steady and avoid the crashes that lead to hunger and overeating. And while that may be true for diabetics and others who are sensitive to low blood sugar—if you get headachy, cranky, and generally "hangry" (hungry plus angry) when you don't eat for a few hours, this could be you--Mattson says that the average dieter can go without food for that long without any drastic consequences.

"Physiologically, we can easily handle the intermittent fasting approach because it goes back to our ancestors' eating patterns, when food wasn't continually available," he says. In other words, we're genetically programmed to go hungry for longer stretches than we currently do.

Craving Change
If you try intermittent or alternate-day fasting, research indicates it takes about two weeks for your body to adjust. Some people experience gastrointestinal problems, but those usually go away within the first week.

It's a good idea to plan your fasting days ahead of time. Krista Varady, Ph.D., an associate professor of kinesiology and nutrition at the University of Illinois at Chicago and author of The Every-Other-Day Diet, recommends satiating foods like healthy fats (think nuts, avocados, and fatty fish), whole grains, and fiber-rich foods.

Also, exercise can be a temporary appetite killer, and according to one study, overweight people who worked out while on a program of alternate-day fasting doubled their total fat loss. Just eat after exercising, when you most need refueling, and listen to your body (stop working out if you feel uncomfortable).

Fasting isn't for everyone. If you're an all-or-nothing eater, fasting can trigger that behavior in a negative way. But if you do it correctly and safely—i.e., only to drop pounds if you're overweight, not to maintain or go below a healthy weight—it can work.

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