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When it comes to dieting for weight loss or performance, cheating is normal—condoned, even. Pro cyclist Phil Gaimon says cheat days—or the one time per week when you can eat what you want, when you want, without any guilt—can help you stay on track during the off season. Pro bodybuilders King Kamali and Troy Alves are their secret weapon. And in his book , Tim Ferriss, champion of the ketogenic diet, preaches that for one day a week, you should eat whatever you want, so long as you’ve started with a healthy breakfast.

Research shows that losing weight slows your metabolism and that cutting calories too quickly stalls weight-loss progress. The thinking goes that a cheat day, and the rush of calories it brings, will get your metabolism cranking at full speed again. However, there’s no evidence supporting this claim, and lots of experts are skeptical about the purported benefits. For one thing, it’s easy to go overboard. “One of the problems with a restrictive diet is that when people go off it, there can be a tendency toward binge eating,” says Dr. Adrienne Youdim, who spent a decade as the medical director of the Center of Weight Loss at Cedars Sinai Medical Center and now works at the Lasky Clinic in Beverly Hills. “In one day of bingeing, you could take in 8,000 calories or more, which can easily compensate for the deficit you worked to create during the week.”

Sherry Pagoto, a professor of medicine at the University of Massachusetts Division of Preventive and Behavioral Medicine, has spent a large portion of her career researching why so many Americans fail at diets. One very clear reason: our brains adore sugar. that when rats are granted free access to sugar and then have it taken away, they end up with neuroadaptations, or changes to specific parts of their brains. “Not coincidentally, [the parts of the brains affected in the rats] are the same regions implicated in drug addiction,” Pagoto adds. Giving yourself a single hit of sugar is a bit like giving a recovering addict a hit of their substance of choice. You’re not just going to cheat for that one meal—you’re going to want to cheat for every meal.

But advocates for cheat days say they boost mental fortitude. Psychologists call this theory ego depletion—the idea that the human brain is capable of making only so many good decisions in a row. For example, if you give people a choice of cookies or radishes, and then ask them to do a hard test (as the researchers ), those who take the radishes are more likely to give up first on a test because they were mentally depleted from their first task.

Granted, even that seminal research has been contested. In July, researchers from Curtin University in Australia that refuted nearly 20 years of ego-depletion experiments. When they tried to replicate the experiments, they found between an initial hard decision causing quick failure on a secondary task. In other words, taking a cheat day on Saturday may not actually strengthen your willpower for the coming week.

The bottom line: do what works best for you. There may be people who need a break from diet drudgery to recharge their resolve, Pagoto says. But you can do other things to strengthen willpower. For one, you can get outside. “Exercise activates the prefrontal cortex, which is the same area that works on self-control,” she says. Also, try to get more rest: sleep deprivation can make it harder to make good choices. Finally, Pagoto recommends creating smart-choice architecture, which means surrounding yourself with healthy foods and keeping them within arm’s reach. “We call eating cookies a failure of willpower, but having them nearby is a failure of design,” she says.

If you do decide to cheat, here’s how to do it intelligently.

How to Cheat

1. Opt for a cheat meal, not a day. The damage you can do in a day of binge eating is enough to negate an entire week of careful eating, according to Youdim.
2. Aim for about 600 calories per cheat meal. But know this amount may have to be adjusted down as you lose weight. The 200-pound version of you has a faster metabolism than the 160-pound version.
3. Avoid the Pixy Stix. “Where your blood sugar goes, your insulin follows, and insulin triggers the body to increase fat storage,” says Holly Lofton, assistant professor of medicine and surgery, and the director of the weight management program at New York University's Langone Medical Center. Eat a balanced cheat meal—pizza, for example, should be topped with cheese, veggies, and meat to keep your blood sugar steady.
4. Structure your cheating. Don’t open a family-size bag of chips and go to town, warns Youdim. Pour those chips in a reasonably sized bowl—when they’re gone, the cheat meal is over.
5. Or sub in an extra rest day instead. If you feel like you need a break, an extra rest day can help. “You’re nourishing your body while resting,” says Youdim. “A binge is not nourishing.”
6. Rethink your overall diet plan. “If your diet is such misery that you need to cheat on it regularly, that's probably not a good sign,” says Yoni Freedhoff, an Ottawa-based obesity doctor and author of . “Healthy living is like a marriage—it’s a hugely important relationship. Sure, you’ll get into fights with it, but if to cope you need to cheat on it every week, maybe you need to move on.”