Calories are more important than protein with respect to increases in body fat when consuming excess amounts of energy, according to a new study.

Researchers at the Pennington Biomedical Research Center (PBRC; Baton Rouge, LA, USA), the National Institutes of Health (NIH; Bethesda, MD, USA), and other institutions conducted a randomized controlled trial to evaluate the effects of overconsumption of low, normal, and high protein diets on energy expenditure, and body composition. To do so, 25 healthy, weight-stable male and female volunteers--aged 18 to 35 years with a body mass index (BMI) between 19 and 30--were randomized to diets containing 5% of energy from protein (low protein), 15% (normal protein), or 25% (high protein), on which they were overfed by 40% more energy intake during the last 8 weeks of their stay in a metabolic unit. The main outcome measures were weight gain, body composition, resting energy expenditure, and total energy expenditure.

The results showed that overeating produced significantly less weight gain in the low protein diet group compared with the normal protein diet group or the high protein diet group. Body fat increased similarly in all three protein diet groups, and represented 50%-90% of the excess stored calories. Resting energy expenditure, total energy expenditure, and body protein did not increase during overfeeding with the low protein diet. In contrast, resting energy expenditure and body protein (lean body mass) increased significantly with the normal and high protein diets. The study was published in the January 4, 2011, issue of the Journal of the American Medical Association (JAMA).

“Fat storage was exactly the same with all three levels of protein; protein, on the other hand, had no effect on storage of fat, but it did affect weight gain,” said lead author George Bray, MD, of the PBRC. “Protein does influence what happens to your lean body muscle mass during the course of any dietary intervention, so there's an important value to eating protein, but it doesn't influence your storage of calories.”

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