A new JAMA study published online on Wednesday suggests that when people consistently eat more calories than their bodies can burn each day, it appears they gain body fat and lose lean muscle if their diet is low in protein. Experts commenting in the same issue of the journal say this means in tackling obesity we need to focus on people’s overall fatness and not just body mass index or body weight.

Lead author George A. Bray, of the Pennington Biomedical Research Center in Baton Rouge, Louisiana, USA, and colleagues recruited 25 healthy individuals who agreed to spend time in a controlled setting and be randomized to overconsume diets containing different levels of protein.

They found that:

  • Participants on the low-protein diet gained less weight compared to counterparts on the normal and high-protein diets,
  • Calories alone, and not protein, appeared to account for an increase in body fat, however
  • Protein did account for changes in energy expenditure and lean body mass.

In their introduction, Bray and colleagues explain that in order to tackle the growing obesity crisis where nearly two in three American adults are overweight and nearly one in three is obese, we need more information from research, and they set out to do this study because the “role of diet composition in response to overeating and energy dissipation in humans is unclear”.

The 25 volunteers who took part in the study were healthy, American men and women aged from 18 to 35, with a body mass index ranging from 19 to 30 (normal to overweight/nearly obese). They were recruited to an inpatient metabolic unit where they agreed to stay and be kept under “controlled conditions”. The first recruit joined the study in June 2005 and the last in October 2007 and they stayed in the unit for about 10 to 12 weeks.

At first, the participants followed a “weight-stabilizing” diet for between 13 and 25 days. After that, the researchers randomly allocated them to one of three diets: low protein (where 5% of the energy in their diet came from protein), normal protein (15% of energy from protein), and high protein (25% of energy from protein).

They overfed on these diets for about 8 weeks of their stay in the inpatient metabolic unit, that is, they consumed around 40% more energy than they did on the weight-stabilizing diet, or about 954 extra calories a day.

The results showed that:

  • All the volunteers put on weight and there was no difference between the men and the women.
  • The participants on the low protein diet put on less weight than their counterparts on the normal and high protein diets (average 6.07 lbs or 3.16 kg compared to 13.3 lbs, 6.05 kg and 14.4 lbs, 6.51 kg respectively).
  • Body fat increased in all three groups.
  • The increase in body fat represented between 50% and more than 90% of the excess stored calories.
  • In the low protein diet group, resting energy expenditure, total energy expenditure and body protein did not increase during the overfeeding phase.
  • However, lean body mass (body protein) decreased in the low protein diet group during the overfeeding phase, whereas it increased in the normal and high protein diet groups.
  • The low protein diet group lost 1.5 lbs, 0.70 kg of lean body mass on average during the overfeeding phase, compared to a gain of 6.3 lbs, 2.87 kg in the normal protein diet group and 7 lbs, 3.18 kg in the high protein diet group.
  • And, unlike the low protein diet group where there was no change, resting energy expenditure increased significantly in the normal and high protein diet groups (up by 160 and 227 calories per day respectively).

The authors write:

“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.”

They say the key finding of the study is that when you look at what contributes to increase in body fat when healthy people consume more calories than they burn, then calories appear to be more important than protein.

In an accompanying editorial, Zhaoping Li and David Heber, of the University of California, Los Angeles, write that these findings inform “primary care physicians and policy makers about the benefits of protein in weight management.”

They note the results suggest overeating on a low protein diet may result in gaining less overall body weight, but it appears to increase body fat and decrease lean body mass.

Because of its apparent effect on body fat, a Western diet (like the low protein one of the study) may raise the health risks of overeating beyond that which we can detect through just using BMI. This means we could be underestimating the method used to assess the obesity epidemic , they add.

“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,” they write.

Written by Catharine Paddock PhD