Last updated: Feb 10, 2016
last diet
Getty Images
There's a reason so many of us struggle with losing weight (and keeping it off). Cutting-edge research is pointing toward a surprising new explanation—one that has little to do with lack of willpower. (Thank goodness.) In fact, the problem is that you've been doing what you were told to do—slash calories, cut fat.

Conventional wisdom holds that weight loss is nothing more than simple math. Take in fewer calories than you expend, and the pounds will fall off as predictably as leaves from an autumn tree. But thousands of failed diets have shown that the low-calorie approach doesn't work, says David Ludwig, MD, an endocrinologist at Boston Children's Hospital and a professor at Harvard Medical School. "When you cut calories, the body fights back, making you hungrier, among other things," he explains. "Weight is controlled by our biology more than our willpower."

RELATED: 16 Ways to Lose Weight Fast

What's more, despite everything you've heard for years, all calories aren't created equal. "Although a bottle of cola and a handful of nuts may have the same number of calories, they have dramatically different effects on metabolism," says Dr. Ludwig. (Bet you can guess which is worse.)

Three new books offer insights into the latest thinking on smart eating, and they're all penned by eminent weight-loss experts: Dr. Ludwig; Louis Aronne, MD, director of the Comprehensive Weight Control Center at Weill Cornell Medicine and NewYork-Presbyterian Hospital; and Mark Hyman, MD, director of The Cleveland Clinic Center for Functional Medicine. Health chatted with these groundbreaking diet crusaders to uncover what we all need to know to slim down and stay healthy for good.

Minimize simple carbs
"The 'calorie is a calorie' myth is perhaps the most misleading nutrition lie ever," says Dr. Hyman. Here's why: Sugary snacks and drinks and low-fat, highly processed starches raise blood sugar quickly, which triggers your pancreas to release a flood of insulin—the hormone Dr. Ludwig calls "the ultimate fat cell fertilizer" because it instructs your body to store calories as fat, causing fat cells to increase in number and size.

Once insulin ushers calories into your fat cells, it closes the door, restricting their ability to get out. With calories, aka fuel, trapped in your fat cells, there's too little glucose and too few lipids circulating in the bloodstream to power your brain and muscles. Your brain, sensing the fuel shortage, prompts you to feel hungry and slows down your metabolism—the worst possible combination for long-term weight control.

RELATED: 26 Weight-Loss Myths You Shouldn't Believe

"Overeating hasn't made our fat cells grow," says Dr. Ludwig, whose book is titled Always Hungry?. "Processed carbs and added sugar have programmed our fat cells to grow, and that makes us overeat." And it becomes a vicious cycle. Break it: "If you're going to have simple carbs, like bread with dinner, have them after you've eaten some protein and veggies first," says Dr. Aronne. "Our studies show that when you save them for later in the meal, they don't trigger as big a bump in blood sugar—or insulin."

Enlist your metabolism
Doctors have long known that when you lose weight, your metabolism slows down, says Dr. Aronne, author of The Change Your Biology Diet. "If you lose 10 percent of your body weight, the number of calories you burn during the day drops by 30 to 40 percent, because a smaller body requires fewer calories and your muscles become more efficient," he says. But research shows that what you eat when you're trying to shed pounds can determine how big a hit your metabolism takes.

In a 2012 study published in JAMA, Dr. Ludwig and his colleagues looked at 21 people between the ages of 18 and 40 who were overweight or obese. They had each participant lose about 10 to 15 percent of their body weight, then put them on three different maintenance diets—low-fat (with about 60 percent of daily calories coming from carbs); low-glycemic-index (with about 40 percent of daily intake from carbs that cause only moderate spikes in blood sugar, such as legumes and vegetables); and a very low-carb approach, with just 10 percent of daily calories from carbs. All three diets involved the same total number of calories. And every participant tried each diet for a month.

RELATED: The Best Weight Loss Foods of All Time

After each diet period, the researchers tested the folks' metabolic rates—and found that the low-carb diet completely prevented the metabolic slowdown often seen after weight loss. "People on the low-carb diet burned an average of 325 more calories a day—about the same number you'd burn during a moderately vigorous workout—than those on the low-fat diet, and those on the low-glycemic diet burned 150 more calories than those on the low-fat diet," he says. One theory for why that happens: Reducing processed carbs, and as a result insulin levels, allows fat cells to release calories back into the bloodstream, helping to readjust the body-weight set point naturally, speculates Dr. Ludwig. He posits that reducing carbs even moderately—with a focus on the quality of your carbs—would be beneficial for shedding weight as well.

Let go of your fear of fat
"Dietary fat has been unfairly demonized," says Dr. Aronne. "Olive oil, nuts and monounsaturated fats play an important role in a healthy diet, and these days the jury is even out on saturated fat." Dr. Ludwig agrees: "The fats in dairy appear to be healthier than those in red meat, and saturated fat is worse when you eat it in combination with processed carbs." Fat can actually be surprisingly helpful when you're trying to lose weight. Healthy fats can shut off craving centers in the brain and help you eat less sugar and refined carbs—"the primary cause of obesity and diabetes," says Dr. Hyman, whose book is titled Eat Fat, Get Thin.

One of the strongest studies vindicating fat was published in The New England Journal of Medicine several years ago. In it, researchers assigned 322 overweight people to either a low-fat diet, a moderate-fat Mediterranean diet or a low-carb, high-fat, high-protein diet. The trial lasted two years—a relative lifetime in the realm of diet studies. What they discovered: Those on the low-carb, high-fat diet not only lost the most weight but also had the most favorable changes in heart-disease-related factors, like levels of triglycerides and HDL cholesterol.

Don't cut too many calories
Sure, if you starve yourself, you'll slim down, so it seems like the strategy would be an instant success. But eventually everyone regains. Why? "Because when you drastically reduce the amount you eat, your body launches potent countermeasures designed to prevent additional weight loss," says Dr. Ludwig—and the more weight you lose, the more fiercely the body tries to gain it back.

RELATED: 57 Science-Backed Weight Loss Tips

For one thing, it shifts into conservation mode and simply burns fewer calories, notes Dr. Aronne. In addition, he says, levels of hunger- and satiety-related hormones change to increase your desire to eat, making you feel less satisfied with a reasonable amount of food and more obsessed with high-calorie, highly processed goodies. "It's sort of like your brain goes haywire," explains Dr. Aronne, "and you can no longer trust the messages it's sending about hunger and fullness."

But eating the right foods can help you minimize these biological defenses. The key: Consume a satisfying amount of protein, high-quality fat and fiber-rich, low-starch carbs from veggies, legumes, nuts, and seeds. "When you eat that kind of diet, insulin levels decrease and you reprogram your fat cells to release excess calories," says Dr. Ludwig. "So there's more glucose and lipids available as fuel—which means you're not battling hunger and your metabolism stays high."

To bolster these new dietary strategies, there's some tried-and-true advice you should absolutely abide by: Move more, sleep plenty, stress less—all of which can keep insulin levels, as well as hunger and satiety hormones, at optimal levels, says Dr. Ludwig. Diet guidance may evolve, but these three fundamentals have stood the test of time.

RELATED: 31 Quick-and-Easy Fat-Burning Recipes

A day of eating, reimagined
While each MD's diet approach varies in its specifics, the general strategy is the same: To keep your body from fighting your slimdown efforts, eat whole foods with adequate protein and plenty of healthy fat, and reduce sugar, refined carbs and processed foods. Here are a few recipes pulled from the pages of their books.

Breakfast: Southwest omelet made from 4 to 6 egg whites with onions, peppers, tomatoes and salsa, plus 2 slices of turkey bacon.

A.M. snack: 1 small container of plain Greek yogurt with ½ cup of blueberries.

From Dr. Aronne's The Change Your Biology Diet ($26,

Lunch: California kale Cobb salad with ½ bunch kale, ¼ avocado, 3 or 4 halved cherry tomatoes, ¼ can water-packed artichoke heart quarters, 1 slice of turkey bacon and 4 ounces of diced, cooked chicken.

Snack: ¼ cup of raw, organic nuts and seeds. For a greater nutritional punch, soak them in warm salt water overnight, rinse thoroughly, then dry in the oven at no more than 120 degrees.

From Dr. Hyman's Eat Fat, Get Thin ($28,

Dinner: 1/3 pound of white-fleshed fish or salmon broiled with garlic and lemon, plus ½ medium roasted sweet potato and 1 cup of chopped greens (such as chard or kale) sautéed in olive oil, garlic and a pinch of salt.

Dessert: ½ medium pear, apple, peach or apricot poached with cinnamon, cardamom and ground nutmeg.

From Dr. Ludwig's Always Hungry? ($28,