Overeating protein will not contribute to an increase in body fat, however calories alone will, according to a study released Tuesday. 

Protein also contributed to changes in energy expenditure and lean body mass, according to researchers.

Scientists from Pennington Biomedical Research Center examined whether the level of dietary protein affected body composition, weight gain, or energy expenditure differently under tightly controlled conditions.

There were 25 participants in the study, and all of them were healthy and had a stable weight.  They were each admitted to an inpatient metabolic unit and were each given one of the three diets that consisted of different levels of protein.  A diet containing 5 percent of energy derived from protein, 15 percent and 25 percent.  All participants were overfed approximately 40 percent more calories than normal.

All participants gained weight, and the group with the low protein diet had the lowest weight gain corresponding with most weight gain by participants from the group with the highest protein diet.  However, body fat increase seemed to be evenly distributed among all three groups who had diets differentiated in protein levels but not calories.

"In summary, weight gain when eating a low protein diet (5 percent of energy from protein) was blunted compared with weight gain when eating a normal protein diet (15 percent of energy from protein) with the same number of extra calories. Calories alone, however, contributed to the increase in body fat. In contrast, protein contributed to the changes in energy expenditure and lean body mass, but not to the increase in body fat," the researchers wrote.

"The key finding of this study is that calories are more important than protein while consuming excess amounts of energy with respect to increases in body fat," researchers added.