Calories, Not Protein, Lead to Increase in Body Fat

Weight gain is caused by overconsumption of calories, not the ratio of protein to carbohydrates in one’s diet, researchers from Pennington Biomedical Research Center in Louisiana have found. Further, low-protein diets can actually lead to loss of lean muscle mass, which can result in poor long-term weight control.

The researchers, led by George A. Bray, MD, set up a randomized controlled trial involving 25 healthy adults aged 18 to 35 with a body mass index (BMI) between 19 and 30.

After following a weight-stabilizing diet for 13 to 25 days, participants were overfed for eight weeks according to one of three diets: a low-protein diet (5% of energy from protein), a normal-protein diet (15%), or a high-protein diet (25%). The protein diets contained roughly 40% more energy (954 extra calories per day) than the initial weight-stabilization diet.

By the end of the 10- to 12-week trial, those following the low-protein diet had gained an average of 6.97 lbs, those following the normal-protein diet had gained on average 13.3 lbs, and those following the high-protein diet had gained on average 14.4 lbs.

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

Lean body mass, however, decreased in those following the low-protein diet by an average of 1.5 lbs, which does not bode well for anyone attempting long-term weight loss. “The study demonstrates how low-protein foods with hidden sugars or fats may be contributing to the obesity epidemic,” Zhaoping Li, MD, PhD, and David Heber, MD, PhD, both of the Center for Human Nutrition at UCLA’s David Geffen School of Medicine, wrote in an accompanying editorial. “Sugars such as sucrose, fructose, and high fructose corn syrup are converted efficiently to fat with calorie excess.”

By comparison, the normal- and high-protein diets resulted in average gains of 6.3 lbs and 7 lbs, respectively, in lean body mass. Resting energy expenditure also increased in the normal- (160 calories per day on average) and high-protein diets (227 calories per day on average).

"Weight gain when eating a low protein diet … was blunted compared with weight gain when eating a normal protein diet … with the same number of extra calories. Calories alone, however, contributed to the increase in body fat,” the researchers write, according to a press release. “In contrast, protein contributed to the changes in energy expenditure and lean body mass, but not to the increase in body fat.”

"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 add.

The study was published in the January 4 issue of JAMA.