Overall Fatness, Not Just BMI, Weight, for Measuring Obesity?

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 volunteeers 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.