You've tried everything—low fat, high protein—but the pounds won't budge. One possibility: You're eating all the wrong foods. We explore the science (and stunning results) of a low-glycemic diet.

When LeeAnn Henn turned 12, extra pounds almost magically appeared on her frame. "I was always a little chunky," she says, "but right around puberty I put on a lot more." When she left her hometown of Sauk Prairie, Wisconsin, to attend the Massachusetts Institute of Technology in Cambridge, her weight problems went from bad to worse. Though she tried to cut calories and fat on a number of different diets over the years, the numbers on the scale just kept going up. "I'd eat less, exercise more, get frustrated, then quit," recalls Henn, now 28. "I could never lose much, and over time, I just got heavier."

By 2004, 5'10" Henn had reached 285 pounds. Obese and desperate for a solution, she responded to an ad about a weight loss study being conducted by the Harvard-affiliated Children's Hospital Boston. "I thought, 'It's a respectable, local hospital, and I'm not getting anywhere myself,'" she says. Henn's choice may have been made out of convenience, but it could not have been better for her particular weight problem.

The study was being run by David Ludwig, MD, PhD, director of the Optimal Weight for Life (OWL) program at the hospital and an associate professor of pediatrics at Harvard Medical School. He's one of a handful of researchers trying to prove that all calories are not, in fact, equal; some of us are genetically programmed to pile on pounds much faster when we eat the wrong type of food, even foods we think of as healthy.

For several decades, the science on weight loss has been a one-size-fits-all message that can be boiled down to this: Losing pounds is a matter of calories in versus calories out; you have to burn more energy than you take in. This resulted in the low-fat craze, since fat is far more calorically dense than protein or carbohydrates. But as the percentage of fat calories in the American diet declined, the obesity epidemic took off. "When you look at low-fat-diet studies," says Ludwig, "at best they show a modest short-term weight loss, with near total regain by a year's time."

Some people have found success with a low-fat approach. But in recent years, dieters have turned to low-carb, high-protein plans, which did work better than low-fat ones—for about six months. "Ultimately, there was weight regain," says Ludwig. He decided it was time to study an entirely new approach to weight loss—one that didn't single out fat, carbs, or protein but, rather, emphasized the proper combination of these nutrients.

From the December 2007 issue of O, The Oprah Magazine

Subscribe & Save up to 78%
Discover more ways YOU can live your best life in every issue. O, The Oprah Magazine: Health