A new study may help end the perpetual debate over how best to maintain or slim down to a healthful weight. Researchers found that, while there are many elements to a good diet, carefully limiting calorie intake is the most important part of controlling one's weight.

Sixty percent of US adults are overweight and more than 30 percent are obese.  But Americans are not the only ones struggling with their waistlines.  The World Health Organisation reports that obesity has reached epidemic proportions globally, with more than a billion adults overweight. 

Dr. Dariush Mozaffarian studies the relationship between diet and chronic diseases at the Harvard School of Public Health.  "We need a shift, a paradigm shift, in our focus on and attention to diet,"  Mozaffarian said.

But people trying to change their eating habits and reduce their weight must sort through a bewildering variety of diets. Some recommend cutting out carbohydrates. Others say cut out the fat, eat more protein, eat less protein. The messages can be confusing. 

Dr. George Bray, at the Pennington Biomedical Research Center in the southern state of Louisiana, wanted to find out if protein affects weight gain.  "This study was designed to examine the effects of differing levels of protein on total calorie intake," Bray said.

Twenty-five healthy adults participated in the study. Daniel Kuhn was one of them. He and the others were overfed by almost 1000 calories a day. 

"I was eating a lot of real butter, real whipped cream and things of that nature that I don't normally indulge in," Kuhn said.

All of the participants followed low, normal or high protein diets. The result:  all of them gained weight. 

"Fat storage was exactly the same with all three levels of protein. That is, it was the calories that they ate that affected the body fat that they stored," Bray said.

But those who ate a higher percentage of protein gained more lean body mass while those on a low protein diet experienced just the opposite.

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

Dr. Bray tells his patients to get on the scale regularly so they can catch added weight early. Dr. Mozaffarian says keeping weight in check is not just about eating less. 

"We don't need to go down a list of 'avoid this, avoid that,' and becoming the food police. It's mostly foods that should be increased. It’s fruits, vegetables, whole grains, fish, vegetable oils and nuts," Bray said.   

Dr. Mozaffarian says increasing these six foods by about a serving each day would reduce obesity - and help slow the epidemic of chronic diseases associated with it. 

Dr. Bray's study on protein, calories and weight gain was published in theJournal of the American Medical Association.