New research published in the Journal of the American Medical Association (JAMA) investigated the metabolic and physiological differences encountered when over-consuming calories while following a low-protein, normal-protein, or high-protein diet. The researchers found that those who followed the low-protein diet experienced less relative weight gain, but a greater relative increase in body fat. It appeared that with greater protein consumption, greater weight gain occurred (predominantly in the form of lean body mass) and greater changes were experienced relative to daily resting energy expenditure. George A Bray, M.D. and colleagues of the Pennington Biomedical Research Center in Baton Rouge, LA conducted the study to determine whether variable quantities of dietary protein variably affect body composition, weight gain, or energy expenditure under tightly controlled conditions. Twenty-five healthy, weight-stable US adult volunteers (male and female) between the ages of 18-35, with a body mass index (BMI) of 19-30 were included in the randomized and controlled study. The participants were first given a weight-stabilizing diet for 13 to 25 days; and then randomized to receive a diet of relative caloric excess (40% more energy, equating to 954 kcals/day) containing either 5% of energy from protein (low protein), 15% (normal protein), or 25% (high protein) for the last 8 weeks of their 10- to 12-week stay in the inpatient metabolic unit. All of the participants in the study gained weight, and there were no differences by sex. As mentioned previously, the rate of weight gain in the low-protein group was significantly less than in the other groups (6.97 lbs. vs. 13.3 lbs. for the normal-protein diet group and 14.4 lbs. for the high-protein diet group). However, also directly relevant to body composition, lean body mass decreased during the over-feeding period in the low-protein group by 1.5 lbs. while the normal-diet and high-protein diet groups experienced gains of 6.3 lbs. and 7.0 lbs., respectively. Furthermore, resting energy expenditure increased significantly with the normal- and high-protein diets (160 kcals/day and 227 kcals/day, respectively). In summary, the participants on an isocaloric, low-protein diet experienced less absolute weight gain when compared with those on a normal- or elevated-protein content diet, but it appears that excess calories alone contributed to an increase in body fat. Protein contributed to increases in energy expenditure and lean body mass, but directly to an increase in fat. As stated by the study authors, “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.”