Some say the secret to losing weight is forgoing greasy, fatty foods like French fries; others swear that shunning carbs in favor of all-protein grub is key. Many popular weight loss plans recommend that dieters consume specific ratios of fat, protein and carbohydrates. (The Zone diet, for instance, prescribes 40 percent carbs, preferably complex carbs like veggies and whole grains, 30 percent protein and 30 percent fat). But a study published today in The New England Journal of Medicine suggests that the smartest way to lose weight is to eat heart healthy foods (think: Mediterranean diet—lots of veggies and fish, limited amounts of red meat) and reduce your caloric intake.

"Reduced calorie, heart-healthy diets can help you lose weight, regardless of the proportions of fat, protein and carbohydrates," says study co-author Catherine Loria, a nutritional epidemiologist at the National Heart, Lung and Blood Institute in Bethesda, Md.

The researchers, led by Frank Sacks, a professor of cardiovascular disease prevention at the Harvard School of Public Health in Boston, focused their study on 811 overweight and obese adults ages 30 to 70 in Boston and Baton Rouge, La. ("Overweight" includes those with a body mass index (BMI) between 25 and 29.9; people are considered obese if they have a BMI over 30. The BMI is a standard index used to gauge body fat based on a person's height and weight.)

The study subjects were divided into four groups, each assigned to a special diet. One group ate a "low-fat, average-protein" diet (20 percent fat, 15 percent protein, 65 percent carbs); a second consumed a "low-fat, high-protein" diet (20 percent fat, 25 percent protein, 55 percent carbs); a third followed a "high-fat, average-protein" diet (40 percent fat, 15 percent protein, 45 percent carbs); and the remaining group ate a "high-fat, high-protein" diet (40 percent fat, 25 percent protein, 35 percent carbs). All four regimens were heart-healthy (low in saturated fat and cholesterol) and included 20 grams (0.7 ounce) of daily dietary fiber. For each study participant, the researchers calculated personalized daily consumption levels ranging from 1,200 to 2,400 calories per day.

When the researchers measured the body weight of the participants at various points over two years, they found that all four groups were shedding roughly the same number of pounds over time.

"No matter which way you look at it, there were no [statistically significant] differences between any of the groups," Loria says. At six months, the average total weight loss for all of the groups was approximately 14 pounds (6.5 kilograms); by the end of two years that number had dipped to about nine pounds (four kilograms). "A lot of times in these weight loss studies, people tend to regain," notes Loria, adding that she will now study strategies that help people keep lost pounds off.

"This study dispels the long-held idea that a low-fat diet has an advantage over other diets," says Christopher Gardner, a nutrition scientist at Stanford University School of Medicine, who was not involved in this research. The only downside of this or any weight loss trial for that matter, he notes, is that people do not always stick to the diets assigned to them. (The study authors acknowledge that many participants failed to meet their target fat–protein–carb ratios, even though they were given regular counseling and feedback from nutritionists throughout the two-year period).

None of the individuals in the diet groups reduced their average carb consumption to less than 43 percent, including those given a 35 percent target, Gardner says, noting that tighter restriction of carbs may have led to greater weight loss.

"I would like to see the results of a study testing a diet that was about 30 to 35 percent carb, about 40 to 45 percent fat [and] about 25 percent protein," he says.

Another weakness of this study, according to Susan Roberts, a nutrition professor at Tufts University's Friedman School of Nutrition Science and Policy, is that it did not recognize the importance of fiber, which she refers to as "the up-and-coming weight loss factor." All four of the study diets called for 20 grams of daily fiber, which falls short of the daily intake recommended by the U.S. Institute of Medicine of approximately 35 grams (1.2 ounces) for men and 25 grams (0.9 ounce) for women.

Roberts says that studies she has led show that the more fiber one eats daily (up to about 50 grams, or 1.8 ounces), the more weight he or she is likely to lose. She speculates that is because fiber—which is found in veggies, fruit and whole grains—creates the sensation of fullness after eating by activating stretch receptors, nerve cells that are probably part of the group of signals sending the "I am full" message to the brain, in the digestive tract; it also slows digestion, extending the time that nutrients are dribbling into the blood stream from the intestine, which may lead to feelings of satiety. She notes that the average daily fiber intake in the U.S. is around 13 to 15 grams (0.5 ounce).

So what's the best way to slim down? "Focus on high-fiber, minimally processed, plant-based foods," Gardner says, "which translates into lots of vegetables, beans, nuts and seeds, whole grains, and fruits."