How Sweet It Is (to Lose Weight)

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.

If you want to learn about healthy ways to snack, whether a certain food is low or high glycemic, or what type of pasta to buy, check out these sources:

GlycemicIndex.com
This Web site is managed by the Human Nutrition Unit in the School of Molecular and Microbial Biosciences at the University of Sydney. You can use the free database to find the GI value of any food they've tested.

Ending the Food Fight (Houghton Mifflin)
David Ludwig's new book is designed for families, but anyone can appreciate his mix-and-match meal planners (in which you select low-glycemic ingredients to create a meal), the low-to-moderate-glycemic shopping list, and easy-to-make recipes.

The New Glucose Revolution Pocket Guide to the Top 100 Low-GI Foods (Marlowe)
Also includes neat little facts like this: Overcooked pasta has a high GI, while al dente has a low GI. The reason is that the less gelatinized, or swollen, the starch, the slower the rate of digestion.

In a small study that Ludwig and colleagues conducted several years ago, he fed teens a meal that dramatically raised blood sugar levels. (Instant oatmeal, bagels, and a number of breakfast cereals—many of the foods we believe to be good diet choices—provoke sharp spikes followed by sudden crashes in blood sugar.) Overall, the volunteers in the high-glycemic (glycemia refers to the presence of glucose—sugar—in the bloodstream) group described themselves as "very hungry" throughout the day and ended up eating 600 to 700 more calories than those who ate foods that didn't cause wild blood sugar swings, like vegetable omelets, low-fat cheese, apples, and grapefruit. In subsequent studies, Ludwig found that keeping rodents on a high-glycemic diet prompted most—but not all—of them to overeat and gain weight. What set the weight gainers apart was a high insulin response. One of insulin's jobs is to direct excess blood sugar to the liver and muscles, where it can be stored for later use. Because this hormone also helps signal when we've had enough to eat, a sudden surge of insulin followed by a sharp dip could trigger the sensation of hunger and the need to eat.

"We decided to return to humans to test our hypothesis that people vary in their reaction to a blood sugar spike," Ludwig says, and that variation could explain their weight gain and loss. In a study published earlier this year—the one that LeeAnn Henn joined—Ludwig and his colleagues recruited 73 obese adults between the ages of 18 and 35 to take part in an 18-month trial. First, the researchers measured the participants' insulin response. Then volunteers were asked to follow one of two eating approaches: a low-glycemic diet that included certain fruits and vegetables, whole grains such as barley, and other carbs that help keep blood sugar levels even (see The 5 Commandments of Low-Glycemic Eating), or a low-fat diet in which carbohydrate foods were not as restricted but fat was limited to 20 percent of total intake. There were no other restrictions—the volunteers could eat until they were satisfied.

Ludwig's theory held up: By the end of the 18 months, people whose bodies reacted most strongly to a blood sugar spike—they secreted the most insulin—lost an average of 13 pounds on the low-glycemic diet but only about 3 pounds on the low-fat plan. People who secreted less lost an average of three pounds no matter which diet they followed. "These findings provide an explanation for why some people do so poorly on standard low-fat and low-calorie diets," Ludwig explains. "Variability in weight loss isn't just because some people are more motivated."

LeeAnn Henn knows this is true. As she found out later, her body overreacts to blood sugar spikes. She was lucky enough to be assigned to the low-glycemic group, and she lost 75 pounds over the 18 months of the study. Better yet, she has maintained the loss since she finished the study over a year ago. "It's surprising how easy it is to eat this way," says Henn. "I'm evangelical about the plan to my friends." Henn's markers for heart disease and diabetes also improved, as did, on average, those of the entire low-glycemic group, regardless of their insulin patterns: Good (HDL) cholesterol rose, and levels of the potentially harmful blood fats called triglycerides decreased. However, bad (LDL) cholesterol dropped lower among the low-fat dieters.

Because a normal range of insulin response has yet to be established, it's too early to get the kind of blood test used in the study. (For the purposes of his research, Ludwig simply drew a line through the middle of his volunteers' scores to separate high and low responders.) But a mirror might help you tell if the low-glycemic diet is right for you. "High-insulin secretors tend to be apples, with more fat around the middle," maintains Ludwig. "Low-insulin secretors tend to be pears."

But the diet may be worth a try for anyone who's losing a battle with the bathroom scale. "Most overweight people have insulin issues," says Walter Willett, MD, chair of the department of nutrition at the Harvard School of Public Health. He, too, believes that for many people a low-glycemic diet is a more effective way to lose weight than limiting fat. The Mediterranean-based eating plan he espouses in his book Eat, Drink, and Be Healthy happens to be low glycemic. "The majority of people are metabolically better off with a diet higher in healthy fats," Willett says. "We have a built-in craving for fat, and low-fat diets are generally not satisfying, which is why they often don't work for the majority of people."

In the next few years, we may enter a new era of weight management when diets can be customized based on biological factors. But there's no reason you should wait to try a low-glycemic plan; it's essentially healthy, thanks to the focus on eating good fats and fiber and fewer simple sugars. "Obviously, hunger is one of the key downfalls of dieting," says Ludwig. "If weight loss can be achieved by eating till you're satisfied, that's an attractive approach for anyone."