In a seven-month experiment, 21 overweight men and women had their diets strictly controlled down to each last morsel. Scientists found that a traditional low-fat diet seemed to make the metabolism far slower than a high-protein one during the most difficult part of weight loss, which entails keeping the fat off.

The study supports a growing group of scientists who argue that what people eat may be just as important as how much they eat.

"From a metabolic perspective, all calories are not alike," study senior author Dr. David Ludwig says. The director of the New Balance Foundation Obesity Prevention Center at Children's Hospital Boston says that "the quality of the calories going in affects the quantity of the calories going out."

Only one in six overweight and obese adults say they have ever held onto a loss of 10 percent body weight or greater for even a year, the team noted in its report.

Weight loss is typically accompanied by a slowdown in the body's metabolism. To test whether different foods might influence that, Ludwig and his colleagues recruited overweight and obese adults ages 18 to 40. From 2006 to 2010, they marched the volunteers through several controlled feeding studies.

The 13 men and eight women followed a 12-week weight-loss regimen which enabled them to shed 10 percent to 15 percent of their body weight followed by a four-week weight-stabilization phase.

Each subject was fed three different diets for four weeks at a time: a traditional low-fat diet (60 percent carbohydrates, 20 percent fat and 20 percent protein), a low glycemic index diet (with 40 percent carbs, 40 percent fat and 20 percent protein) and a very low-carbohydrate diet a la Atkins (with 10 percent carbohydrates, 60 percent fat and 30 percent protein).

At the beginning and at the end of each four-week stint, the subjects were hospitalized for three days to undergo a battery of tests. Scientists measured their resting energy expenditure using indirect calorimetry, which assesses gases in the breath to calculate calories burned.

For all three diets, the rate of calories burned at rest was lower than before weight loss. Over the course of a day, the subjects burned more than 300 additional calories on average when on the very low-carbohydrate diet compared with the low-fat diet.

"That's roughly equal to an hour of moderate physical activity - without lifting a finger," Ludwig said.

Subjects burned 200 additional calories on the low glycemic index diet than on the low-fat diet. A low glycemic index diet is rich in whole grains, fruits and vegetables.

Weight-loss experts not involved in the research praised it, while acknowledging its limitations.

"It's a small study, so I'd want to see it repeated. But I have no reason to doubt the result," said Susan Roberts, a professor of nutrition and psychiatry at Tufts University in Boston.

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