TUESDAY, Jan. 3 (HealthDay News) -- It's too many calories, not too much protein, that leads to unhealthy weight gain associated with overeating, new research suggests.

In a "do-not-try-this-at-home" study, 25 healthy participants followed diets containing different levels of protein -- plus nearly 1,000 extra calories -- for eight weeks. The study took place in an inpatient setting, where participants had just completed weight-stabilizing diets for 13 to 25 days.

Those who ate low-protein diets gained less weight than the other groups, but the quality of the weight gained was worse, as it came from an increase in body fat. In contrast, the high-protein diets led to changes in lean body mass and helped participants burn calories.

"Most people are overeating and for those people who are, they need to pay attention to what they are putting into their mouths," said study co-author Leanne Redman, an assistant professor of endocrinology at Pennington Biomedical Research Center in Baton Rouge, La. "If you overeat a high-fat, low-protein diet, you may gain weight at a lower rate, but you are gaining more fat and losing more muscle."

The findings appear in the Jan. 4 issue of the Journal of the American Medical Association.

The researchers looked at how the level of protein in the diets affected body composition, weight gain and energy expenditure under tightly controlled conditions using sophisticated measurement techniques. Participants were young adults aged 18 to 35.

The diets varied in the amount of calories derived from protein. The low-protein diet had 5 percent of calories from protein, the normal-protein diet had 15 percent of calories from protein and the high-protein diet had 25 percent of its calories as protein. All three diets included the same amount of carbohydrates, and fat made up the difference in the calories. All participants were overfed by about 954 calories a day.

Everyone gained weight during the overeating period. However, people in the low-protein diet lost 2.2 pounds in muscle mass, while those in the normal- or high-protein groups gained muscle mass during the overeating period. Muscle weighs more than fat, which is why they gained more weight. The excess calories turned to fat among participants who ate a low-protein diet.

The make-up of the weight -- lean muscle or fat -- may be even more important than the number on the scale or body mass index, said Dr. David Heber, director of the Center for Human Nutrition at the University of California, Los Angeles, and co-author of an editorial accompanying the new study. "Calories count," he said. He encourages a high-protein, low-fat diet that is rich in colorful fruits and vegetables. "We are talking about lean protein such as white-meat chicken, ocean fish, turkey, egg whites and certain protein powders. Protein is more satiating, and helps reduce appetite," he explained.

Connie Diekman, director of university nutrition at Washington University in St Louis, said: "This study provides support to the calories-count message as it relates to percent of body fat. I find the conclusion of this study especially helpful in encouraging people to be aware of the calories they consume and to avoid focusing on just where those calories come from."

More information

Learn more about protein at the American Diabetes Association.