The people followed diets with low, normal or high levels of protein. Normal was defined as fifteen percent of energy from protein.

Dr. Bray says all of them gained weight.

GEORGE BRAY: "Fat storage was exactly the same with all three levels of protein. It was the calories that they ate that affected the body fat that they stored."

The study found that those who ate a higher percentage of protein gained more lean body mass. The opposite happened on a low protein diet.

GEORGE BRAY: "If your protein intake's low, you'll actually lose body mass even though you're eating an excess amount of calories."

Dr. Bray tells his patients to weigh themselves regularly so they can know quickly if they gain weight.

His study on protein, calories and weight gain appeared in the Journal of the American Medical Association.

Dr. Dariush Mozaffarian at the Harvard School of Public Health studies the relationship between diet and conditions like heart disease and diabetes. He says keeping a healthy weight is not just about eating less, but also about what you eat.

DARIUSH MOZAFFARIAN: "We don't need to go down a list of 'avoid this, avoid that' -- become, you know, food police. It's really actually mostly foods that should be increased. It's fruits, vegetables, whole grains, fish, vegetable oils and nuts."

He says increasing these six foods by about one serving each day would reduce obesity and help slow the epidemic of obesity-related diseases.