Calories, not protein, behind increase in body fat

Washington: When overeating, calories alone, and not protein may contribute to an increase in body fat, according to a new study.

The researchers also found that protein did contribute to changes in energy expenditure and lean body mass.

George A. Bray, M.D., of the Pennington Biomedical Research Center, Baton Rouge, La., and colleagues conducted a study to determine whether the level of dietary protein differentially affected body composition, weight gain, or energy expenditure under tightly controlled conditions.

The randomised controlled trial included 25 U.S. healthy, weight-stable male and female volunteers, ages 18 to 35 years, with a body mass index between 19 and 30.

After consuming a weight-stabilizing diet for 13 to 25 days, participants were randomised to receive diets containing 5 percent of energy from protein (low protein), 15 percent (normal protein), or 25 percent (high protein), which they were overfed during the last 8 weeks of their 10- to 12-week stay in the inpatient metabolic unit.

All participants in the study gained weight and there were no differences by sex. The rate of weight gain in the low protein diet group was significantly less than in the other 2 groups.

"Body fat increased similarly in all 3 protein diet groups and represented 50 percent to more than 90 percent of the excess stored calories. Resting energy expenditure, total energy expenditure, and body protein did not increase during overfeeding with the low protein diet," the researchers said.

Lean body mass (body protein) decreased during the overeating period by 1.5 lbs. in the low protein diet group compared with a gain of 6.3 lbs. in the normal protein diet group and 7 lbs. in the high protein diet group.

Resting energy expenditure (normal protein diet: 160 calories/day; high protein diet: 227 calories/day) increased significantly with the normal and high protein diets.

"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," they concluded.

The study appeared in the January 4 issue of JAMA.