Researchers at the Harvard School of Public Health and the Pennington Biomedical Research Center put four popular diets, high carb, high fat, low-fat and high protein, to the test to see which of the regimens resulted in more weight-loss success. The study is published this week in The New England Journal of Medicine.

Moreover some previous studies have found that low carbohydrate diets like Atkins work better than a traditional low-fat diet. But the new research found that the key to losing weight boiled down to a basic rule — calories in, calories out.

Researchers randomly assigned 811 overweight adults to one of four diets, each of which contained different levels of fat, protein and carbohydrates. "The hidden secret is it doesn't matter if you focus on low-fat or low-carb," said Dr. Elizabeth Nabel, director of the National Heart, Lung and Blood Institute, which funded the research.

This is the finding of the largest-ever controlled study of weight-loss methods. More than 800 overweight adults in Boston and Baton Rouge, La., were assigned to one of four diets that reduced calories through different combinations of fat, carbohydrates and protein. Each plan cut about 750 calories from a participant’s normal diet, but no one ate fewer than 1,200 calories a day.

After two years, every diet group had lost — and regained — about the same amount of weight regardless of what diet had been assigned. Participants lost an average of 13 pounds at six months and had maintained about 9 pounds of weight loss and a two-inch drop in waist size after two years.