If you’re hoping to drop a few pounds for an upcoming vacation, the old “calories in, calories out” method can be effective. Combine a meager diet with lots of exercise, and in the short term your body will plunder its fat stores in search of energy. You’ll lose weight—maybe even a lot of weight, depending on how much you were eating and exercising before you got going.

But veteran dieters know, and lots of research shows, that nearly all calorie-restriction plans ultimately fail. Most people gain their old weight back, and then some.

“The long-term success rate of obesity treatment is abysmal, which is why every year we have new diets and weight-loss treatments, along with a billion-dollar weight-loss industry,” says Dr. David Ludwig, an endocrinologist and professor of nutrition at Harvard School of Public Health.

In his new book Always Hungry?, Ludwig attacks the conventional wisdom that assumes eating less is the key to losing weight.

When you cut back calories, your body fights back in a number of ways, he says. First and foremost, you get hungry. “Hunger is very powerful and very primal,” he says (as anyone who’s ever felt “hangry” knows). “You can ignore it for a few days or weeks or months, or trick it by drinking lots of water or going for a walk, but it’s very hard to ignore it permanently.”

While you’re feeling famished, your metabolic rate also drops as your body attempts to conserve energy. This means you’ll naturally burn fewer calories, so you’ll have to cut even more of them from your diet in order to keep the scale on a downward trajectory, Ludwig explains.

Do this long enough, and eventually your brain will assume you’re starving (because to some extent, you are). At that point, it will trigger biological responses that tell your fat cells to start storing whatever calories they can get their hands on, which is exactly what you’re trying to prevent in the first place.

“In all these ways, traditional diets work against your body and your biology,” Ludwig says. “You need to work with your body for sustained weight loss.”

How do you do that? You need to eat more of the right types of foods, Ludwig says. In particular, he says healthy sources of dietary fat—like avocados, olive oil, full-fat dairy, nuts and nut butters, full-fat salad dressings and real dark chocolate—are your greatest weight-loss allies.

For two weeks, try to load up your diet with these foods while cutting refined carbs, including chips, pretzels, cookies, breads and other processed snacks, as well as starchy vegetables like potatoes. Hunger will subside, while your body will lose its motivation to store fat, Ludwig says. Non-starchy vegetables, fruit, fish, meat, beans, and other whole foods are all on the menu. “You can eat until you feel full,” Ludwig says.

Other experts agree the right sources of fat help promote weight loss. By increasing fullness and quelling hunger, “healthy fats make weight loss comfortable,” says Dr. Lydia Bazzano, a professor of nutrition at Tulane University. Her research into various diets shows the weight-loss benefits of replacing unhealthy carbohydrates with healthy fats.

After two weeks, you can add back in small amounts of starchy vegetables and whole grains—stuff like steel-cut oats, quinoa, buckwheat, and all those “old-world” whole grains, Ludwig says.

A pilot study based on this diet shows people lost up to 2 pounds a week—without the hunger or across-the-board restriction favored by conventional diets. Eventually, once your weight reaches a new, lower “set point,” you can add back in small amounts of processed carbs,” Ludwig says. Your diet should more or less resemble a Mediterranean-style eating plan—the kind linked to so many brain and body benefits.

It’s not radical or extreme. But if your goal is healthy, sustainable weight loss, this is the kind of diet the latest science supports.

You’re probably wondering, “What about exercise?!” For so many different reasons, regular exercise is essential for a healthy body and mind. But absent major diet changes, most research shows exercise alone won’t lead to more than a couple pounds of dropped weight.

Still, if you’re wondering which workouts will best support your diet-driven weight-loss goals, research suggests high-intensity physical activity is best. Think short bursts of all-out sprinting or cycling, rather than extended runs or rides. All exercise is good for you. But these bouts of super-intense activity have been linked to blood-sugar changes that support lower body weights.

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