When eating in excess, it is the number of calories we consume rather than protein that raises total body fat, researchers from the Pennington Biomedical Research Center, Baton Rouge, Louisiana, USA, reported in JAMA (Journal of the American Medical Association). In a study involving 25 healthy volunteers, they found that those on a low-protein diet gained less weight compared to others who were on a high-protein diet. However, calories alone impacted on the rise in body fat, and not protein.

The authors also report that it appears that protein contributes to alterations in energy consumption and lean body mass.

As background information on the article, the authors wrote:

“Obesity has become a major public health concern with more than 60 percent of adults in the United States categorized as overweight and more than 30 percent as obese.”

They added that it is still unclear what role diet composition may have on energy dissipation or overeating.

George A. Bray, M.D. and team set out to find out whether varying levels of protein in people’s diet might affect weight gain, body composition, and energy expenditure.

Their randomized controlled study included 25 adult Americans of both sexes, aged between 18 and 35. Their BMIs (body mass indexes) ranged from 19 to 30. The volunteers were admitted to the inpatient metabolic unit between June 2005 and October 2007.

They all initially consumed a weight-stabilizing diet for between thirteen and twenty-five days. They were then randomly selected to go on various different diets:

  • Low protein diet – 5% of the energy consumed came from protein
  • Normal protein diet – 15% of the energy consumed came from protein
  • High protein diet – 25% of the energy consumed came from protein

They stayed in the metabolic unit for 10 to 12 weeks. During the last 8 weeks they were overfed. “Overfed” was defined as consuming 40% more energy than during their weight stabilization period – an extra 954 calories each day.

During the ten to twelve week period they all gained weight. There was no noticeable difference in rates of weight gain between males and females.

The rates of weight gain were:

  • In the low protein diet group – 3.16kg (6.97lbs)
  • In the normal protein diet group – 6.05kg (13.3lbs)
  • In the high protein diet group – 6.51kg (14.4lbs)

The researchers explained:

“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.”

Below are the results in changes in lean body mass (body protein) during the overeating period:

  • In the low protein diet group – decreased by 0.70kg (1.5lbs)
  • In the normal protein diet group – decreased 2.87kg (6.3lbs)
  • In the high protein diet group – decreased by 3.18kg (7lbs)

They also found that resting energy expenditure rose considerably in the normal and high protein diets – 160 calories per day in the normal protein diet group, and 227 in the high protein diet group.

The authors wrote:

“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 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.”

Zhaoping Li, M.D., Ph.D., and David Heber, M.D., Ph.D., of the University of California, Los Angeles, explain that these findings provide useful data for general practitioners (primary care physicians) and policymakers regarding the impact of protein in body weight management.

They wrote:

“The results suggest that overeating low protein diets may increase fat deposition leading to loss of lean body mass despite lesser increases in body weight. Policy makers and primary care physicians need to understand the role of the Western diet in promoting overweight and obesity.

Because this diet increases the risks of overnutrition through fat deposition beyond that detected by body mass index, the method used to assess the current obesity epidemic and the magnitude of the obesity epidemic may have been underestimated.

Clinicians should consider assessing a patient’s overall fatness rather than simply measuring body weight or body mass index and concentrate on the potential complications of excess fat accumulation. The goals for obesity treatment should involve fat reduction rather than simply weight loss, along with a better understanding of nutrition science.”

Written by Christian Nordqvist