But that's where the trouble begins. It's hard enough to figure out what to eat. Eating less of it is even harder. Researchers had hoped to get study participants to eat 750 calories fewer than they expended each day - an objective that proved impossible. Dieters adhered to the initial plan for the first several weeks, but by the six-month mark, they were consuming only 225 calories less than they expended - about a third of the goal - according to a calculation based on overall weight loss. "It's very difficult to reduce your calories enough to really sustain a lot of weight loss," Loria says.

One failure of most diet plans is that people get hungry and quit, says Sacks, who acknowledges that the sudden reduction of 750 calories in his study was perhaps too steep. "I think what that teaches us is that maybe it's better to make a more gradual change in intake," says Sacks. "That's what I recommend to my patients, let's try to pick a gradual or realistic reduction in calories that's not going to make you really hungry a lot and that you can sustain day after day."

But eating less, however simple it sounds, is hardly a one-man job. Some nutrition experts argue that the balance of responsibility needs to fall more heavily on society at large. Martjin Katan, a professor of nutrition and health at Amsterdam's VU University, wrote an accompanying editorial that analyzed the merits of the diet study. He suggests that focusing on individual diet plans of any kind may be misguided, and that only community-wide change will truly be able to stem the tide of obesity. He points to a small town in France that tapped all of its residents to solve the problem - building more outdoor sports facilities and creating walking routes, hosting cooking classes and even intervening with at-risk families. After five years, obesity among children was down to 8.8%, less than half the rate of neighboring towns. That success, he writes, "suggests that we may need a new approach to preventing and to treating obesity and that it must be a total-environment approach."

It's a useful lesson for American adults, two-thirds of whom are overweight or obese. Long-term weight loss has proved frustratingly elusive for many obese individuals, but study after study has shown the benefit of community and peer support for helping people take off weight - and keep it off. In this study, the participants who took advantage of group and individual counseling offered as part of the diets had far greater success than those who chose to go it alone. Over the course of two years, participants who went to at least two-thirds of the counseling sessions dropped about 22 lbs., 13 lbs. over the average of the entire study population. "Losing weight and sustaining it for two years is difficult," Sacks says. "To help people do that, they need some level of support to keep their motivation and focus."

But the bottom line, according to most obesity experts, is to set realistic goals. Expect what is achievable: a 250-lb. person isn't likely to slim down to supermodel proportions in her lifetime, but she may be able to lose 10 or 20 lbs. A moderate 5% or 10% reduction in body weight can significantly improve health, by lowering cholesterol and the risk of heart disease, stroke and diabetes. For many doctors who work with obese patients, the goal is not thinness, but well-being - and, ultimately for the patient, self-acceptance.

As for the secret to losing weight? There is none. "It's basic physiology," Loria says. "Eat fewer calories than you expend."

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Editor:Yang Jie