Calories are the culprits that cause weight gain when people overeat, not the amount of protein in the diet, according to a new study published in the Journal of the American Medical Association.

Researchers led by Dr. George Bray of the Pennington Biomedical Research Center in Baton Rouge, La., fed 25 healthy adults about 1,000 extra calories a day during an eight-week period under very carefully controlled laboratory conditions.  The only dietary elements that varied among participants were fat and protein levels: Some ate a low-protein diet (6 percent protein), others a normal-protein diet (15 percent ), and a third group a high-protein diet (26 percent). The diets for all three groups consisted of about 40 percent carbohydrates, considered a moderate dietary amount.

Participants who ate a low-protein diet gained significantly less weight than the other groups, but all three groups gained a similar amount of body fat.

"The hypothesis was that the low-protein and high-protein diets might affect fat gain, but they didn't.  Fat gain isn't modulated to any significant degree by protein intake," he added.

"This study simulates in a controlled environment what is occurring on a daily basis in the U.S. - overeating," said Dr. John Morton, director of bariatric surgery & surgical quality at the Stanford School of Medicine in Stanford, Calif., who was not involved in the research. "Ultimately, what matters most is how many calories are ingested when it comes to weight gain."

Other experts not connected to the study say the research emphasizes the importance of protein in the diet.

Although participants in the low-protein group gained less weight, they also lost more muscle mass, which experts say could be detrimental to their overall health.

"Five percent [protein] is too low and is not good, even if one loses weight, as dietary protein is used to build and repair tissue.  Low protein is a form of malnutrition," said Carla Wolper, research faculty at the New York Obesity Research Center at St. Luke's Hospital.

But that doesn't mean people should gorge themselves on protein.  The study's normal and high protein groups gained muscle mass, but also gained body fat.

"What the public should take away here is that total caloric intake matters when it comes to weight gain," said Lona Sandon, a spokesperson for the American Dietetic Association.  "Choosing more of your calories from protein may help increase lean muscle mass, but people must keep calories in balance to avoid body fat gain."

"Eating different levels of protein will not protect against the excess consumption of calories," said Thomas Badger, director of the Arkansas Children's Nutrition Center in Little Rock.  People still need to consume a certain amount of protein, which the United States Department of Agriculture says is about 15 percent of caloric intake.

Losing lean body mass, said dietitian Elizabeth Ward, can cause people to become overweight over time because it lowers the rate of metabolism.

"While the low-protein group didn't gain as much weight as the other two groups, losing lean muscle tissue plus gaining any fat is a double-whammy when it comes to long-term weight maintenance," she said.

But Dr. Eric Westman, associate professor of medicine at Duke University School of Medicine in Durham, N.C., and co-author of "The New Atkins for a New You," said the carbohydrate content of the participants' diets could have caused the gains in fat, not the protein.

"Carbohydrates were not manipulated in this study, so you can't say they didn't contribute to the gains," he said.

In an accompanying editorial, Drs. Zhaoping Li and David Heber of the Center for Human Nutrition at UCLA's David Geffen School of Medicine wrote about the dangers of low-protein diets.

"The study demonstrates how low-protein foods with hidden sugars or fats may be contributing to the obesity epidemic," they wrote.  "Sugars such as sucrose, fructose and high fructose corn syrup are converted efficiently to fat with calorie excess."