Low-fat, low-carb, high-protein — there's a diet plan of every flavor. And if you're one of the millions of Americans who struggle with weight, you've probably tried them all, likely with little success. That wouldn't surprise Dr. Frank Sacks, a professor at the Harvard School of Public Health and lead author of a new study published in the February 26 issue of the New England Journal of Medicine, whose findings confirm what a growing body of weight-loss evidence has already suggested: one diet is no better than the next when it comes to weight loss. It doesn't matter where your calories come from, as long as you're eating less.

    "We have a really simple and practical message for people — it's not so much the type of diet you eat," says Sacks. "It's how much you put in your mouth."

    In the analysis of 811 obese patients from Massachusetts and Louisiana, participants were randomly assigned to one of four heart-healthy diets: low fat or high fat, with either average or high levels of protein. All four regimens also included high amounts of whole grains, fruits and vegetables, and substituted saturated fat, found in foods such as butter and meat, with unsaturated fat, found in vegetable oil and nuts. The participants were encouraged to exercise 90 minutes a week.

    On average, the study participants lost about 13 lbs. after six months of dieting, or about 7% of their starting weight, regardless of which diet plan they followed. At the year mark, the dieters had regained some of the lost weight, and after two years, average weight loss was about 9 lbs. Only about 15% of participants were able to lose 10% of their body weight or more. Across the board, however, patients lowered their risk of diabetes and reduced blood levels of bad cholesterol (LDL) while increasing good cholesterol (HDL) and overall heart health.

    Catherine Loria, one of the study's co-authors and a nutritional epidemiologist with the National Heart, Lung and Blood Institute, which funded the study, was encouraged by the findings. "People do have to choose heart-healthy foods," she says, but "I think the beauty of the study is that they have a lot of flexibility in terms of the dietary approach."

    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."