By Helen Shen Globe Correspondent

For millions of Americans on a diet, eating the right combination of foods could be key to keeping off unwanted pounds. Diets that limit processed carbohydrates such as breakfast cereals and bagels may enable longer-lasting weight loss compared with other diets with the same number of calories, according to a study published Tuesday in the Journal of the American Medical Association.

“It says that from a metabolic perspective all calories are not alike,” said Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital. “The quality of the calories going in affects the number of calories going out.”

Ludwig oversaw the study to examine how three popular diets might change the body’s metabolism and its ability to maintain weight loss.

“We know that many people can lose weight for a few weeks or months, but most people have difficulty maintaining that for a long time,” he said. As dieters cut their food intake, their bodies naturally burn energy more slowly, making more efficient use of limited calories. Slower metabolism can work with increased hunger to contribute to weight re-gain. The authors found that different diets slowed the burning of calories to varying degrees.

Twenty-one overweight and obese young adults participated in the research at Children’s and at Brigham and Women’s Hospital. For the first phase of the eight-month study, participants ate a typical American diet but with portion-size limits intended to lower their weight by about two pounds per week.