Everything you need to know before you try a fasting diet to lose weight

Now it's the core of an increasingly popular (and increasingly research-backed) weight-loss approach. Intermittent fasting—periodically eating very little—is not only not bad for you, it may lower blood glucose levels and insulin resistance and reduce inflammation and cardiovascular risk.

Why? How? Theories abound, but some experts believe fasting puts your cells under mild stress, just as exercise taxes your muscles and heart, ultimately strengthening them and making them more resistant to disease.

And that's not all, says Courtney Peterson, Ph.D., an assistant professor in the department of nutrition sciences at the University of Alabama at Birmingham. "Studies suggest you keep more muscle and lose more fat than on other diets, even if you lose the same number of pounds." That's because after about 12 hours of fasting, you run out of stored energy from carbs and start burning stored fat.

But a troubling flaw has popped up in this system. (You knew there was a "but" coming, right?) In a recent study, people on an alternate-day fasting plan for six months lost about 6 percent of their body weight—the same as those on a conventional low-cal diet—but 38 percent of fasters dropped out, nearly 10 percent more than in the other diet group. A similar problem has surfaced in other trials.

Why try a plan with a high dropout rate and hours of hunger? Beyond the health benefits, some people actually like it—and find it the easiest way to control their weight. Mark Mattson, Ph.D., a neuroscientist at the National Institute on Aging, has studied intermittent fasting since the 1990s and himself been on a plan for years. "Once you get used to it, it's not a big deal," he says. "You adapt." Other fans? Reportedly, trendsters from Beyonce to Silicon Valley techies; and Jimmy Kimmel has said he lost 25 pounds fasting too. So... should you experiment?

THE SPECIFICS

To be clear: The new fasting is not about deprivation, but about divvying up your calories differently than the three-square-meals-plus-snacks pattern—which some scientists say is a mismatch with the way we evolved to eat, when food was sporadic.

Whether a regimen calls for two fasting days a week or eating your meals in a smaller "window" of time in the day, all plans share a near-freedom from calorie counting, a big plus for weary food diarists. Once you have planned your fasting-period menu—say, a 500-calorie day of chicken and veggies—you're set. And in your nonfasting periods, you eat normal, healthy meals (even that steak!) without worrying about every bite.

The key, of course, is to not go overboard on these "normal" days. Researchers have found, though, that fasters ate at most 10 to 15 percent more on their nonfast days, so overall they took in fewer calories.

THE HUNGER FACTOR

Yes, you'll be hungry at times—but it's not necessarily overwhelming or constant. "Hunger doesn't seem to get worse as the day goes on, and some of our studies report increased fullness and satisfaction," says Kristin Hoddy, Ph.D., R.D.N., a dietitian in private practice who has researched fasting. "Some subjects remarked that they'd get distracted and 'forget' they were hungry."

Feelings over hunger may also become more tolerable over time. Studies of alternate-day fasting have shown that people rate their hunger at 8 on a scale of 1 to 10 for their first few fast days, but after two weeks, that number drops to 3. Mattson puts the window at three to four weeks, after which, he says, "you're not hungry on fasting days."

THE BEST CANDIDATES

There is no long-term fasting research yet, but the benefits are promising and the risks low: You can always just quit. A limited-time fast might bump you off a plateau or out of a rut, says Keri Glassman, R.D., who advised our fasters during their diets, though she says that for some, fasting, even short-term, may be too rigid. That hints at the larger takeaway: Perhaps more than for traditional diets, these plans won't work for everyone.

Hoddy suggests trying a plan for two weeks and keeping a record of how you feel and what you eat. You can switch to another one if it's making you grumpy, or you can modify it; when Hoddy fasted, she split one meal into two smaller ones on fast days so she could have dinner with her husband. Or follow a plan partway, says Peterson. "Any form of fasting helps burn fat, and extending your overnight fast a little—say, eating dinner earlier—is an overall health benefit."