When it comes to weight loss, cutting total calories probably matters more than how those calories are carved up.

That’s the message of a new study that randomly assigned more than 800 overweight adults to one of four different diets: low-fat, average-protein; low-fat, high protein; high-fat, average protein; or high-fat, high-protein. After two years, all had lost roughly the same amount of weight, about seven pounds.

By design, none of the diets was easily recognizable, but they were similar to the plans espoused by bestselling books including the low-fat, high carbohydrate plan championed by Dr. Dean Ornish, and the high-fat, high-protein plan pushed by Dr. Robert Atkins. There was also a lower-fat, higher protein diet that a bit like "The South Beach Diet," and a plan that more closely resembled a balanced approach of "The Zone."

“The results, in a way, surprised us,” says Frank M. Sacks, MD, a professor of medicine at Harvard University and lead author on the study. “We thought that the higher protein diet would be more satiating and lead to better weight loss long run, but we didn’t find that at all, says Dr. Sacks.  “We found that protein content really didn’t matter.”

While the protein, fat or carbohydrate content of the diets did not predict who would lose the most weight, commitment did. Dieters who attended more group counseling sessions with a dietician lost more weight – about half a pound for every session they attended. Those who stuck most closely to their assigned diet also got better results.

“The result is actually a real, positive, optimistic result for people,” Dr. Sacks says. “As long as it’s a healthy diet, and they feel comfortable with it and can stick with it, then that’s what counts.”

The study, which was published in the Feb. 26, 2009 issue of The New England Journal of Medicine, is one of the largest and longest studies to measure the effect of macronutrient balance on weight loss.

Though each of the diets was designed to contain different percentages of fat, carbohydrate and protein, all of them were heart healthy. All four groups were counseled to keep their saturated fat under 8 percent of their daily calories, eat at least 20 grams of fiber each day and keep cholesterol under150 milligrams per 1,000 calories. Additionally, dieters were advised to stick with carbohydrate-rich foods with a low glycemic index, a measure of how a food impacts blood sugar.

All participants were additionally asked to try to reduce their daily calorie intake by 750 calories (but not to go lower than 1,200 calories a day) and to do at least 90 minutes of moderate exercise, like brisk walking, each week.

Those assigned to the low-fat, average protein group were asked to get 20 percent of their daily calories from fat, 15 percent from protein and 65 percent from carbohydrates. In the low-fat, high protein group, dieters had a goal of 20 percent fat, 25 percent protein and 55 percent carbohydrates. Those in the high-fat, average protein group were asked to eat 40 percent fat, 15 percent protein and 45 percent carbohydrates; and in the high-fat, high-protein group, the diet was designed to be 40 percent fat, 25 percent protein and 35 percent carbohydrates.

But not everyone met those goals. Fat intake, for example was supposed to differ by 20 percent between groups; but based on what participants reported that they ate in daily food diaries, fat intake actually differed by only 8 percent between groups. Protein was supposed to differ by 10 percent, but actually differed by only 4.4 percent.  Carbohydrates ranged only 14.4 percent between groups instead of the goal of a 30 percent difference.

Critics of the study point out that because so many people had such a hard time meeting the dietary goals that it would be a mistake to infer from the results that all diets work equally well.

“It’s not really a study of the effectiveness of the diets,” says Donald K. Layman, PhD, professor of nutrition at the University of Illinois. “It’s a study of how well people can follow a given diet.”

While Layman, who researches the effect of diet composition on weight loss but was not involved in The New England Journal paper, says he supports the main message of the study, that calorie reduction is the key to weight loss, he points out that research from his own lab and others has demonstrated that high-protein plans help dieters keep more muscle and lose more fat than high-carbohydrate, low-fat plans.

“This study doesn’t address body composition,” Layman says. “All weight losses are not equal. If you lose too much lean mass, your metabolic rate goes down and you can’t sustain the weight loss, so that was another weakness of the study.”

In fact, most participants could not sustain their initial drop.

At six months, willpower and weight loss seemed to peak. Members of all groups lost an average of about 13 pounds, or roughly seven percent of their body weight. But most could not maintain their progress, and over time, they regained about half the weight they had lost.

Asked about the relatively modest end result of the study, Dr. Sacks says it mirrored his own efforts to take off 13 pounds over two years. Weight loss, he says, “is not an easy thing.”