But now, a new study finds that the amount of calories, not protein, still determines whether you gain weight when you eat more than you should.
Daniel Kuhn says he's always been able to keep his weight in check. Not a fan of fast food, he tries to eat a well-balanced diet and not overeat. But recently, Kuhn participated in a study where he ate very differently.
"I was eating a lot of real butter, for instance, real whip cream and things of that nature that I don't normally indulge in," he said.
In the study, reported in Wednesday's Journal of the American Medical Association, Dr. George Bray, from the Pennington Biomedical Research Center, and co-authors conducted a randomized controlled in-patient trial.
"This study was designed to examine the effects of differing levels of protein on total calorie intake, in a condition in which you are eating more calories than you needed to keep your weight stable," said Dr. George Bray, of the Pennington Biomedical Research Center.
In the study, 25 men and women ages 18 to 35 who were normal or slightly overweight ate low, normal or high protein diets for eight weeks. Closely monitored, participants were also overfed by almost 1,000 calories daily.
"Fat storage was exactly the same with all three levels of protein," Dr. Bray said. "That is, it was the calories that they ate that affected the body fat that they stored."
But the protein made a difference in another way.
"Protein did affect weight gain, primarily by influencing the amount of body protein that was added," he said.
Kuhn ate a low-protein diet, gaining between 12 and 15 pounds.
"The low-protein diet actually was associated with a loss of lean body mass," Dr. Bray said. "So protein has one set of effects, calories have another set of effects and they are not directly connected."
The higher-protein diet also stored more protein than the normal diet.
Researchers say calories remain the major determinant of whether you gain weight when you eat more calories than you should.
"Protein does influence what happens to your lean body muscle mass during the course of any dietary intervention," Dr. Bray said. "So there's an important value to eating protein, but it doesn't influence your storage of calories."