Overeating on a diet low in protein caused less weight gain than overeating on a normal- or high-protein diet – but most of the body mass gained was fat, rather than the leaner muscle mass created by high-protein overeating, according to a report published in the Jan. 4 issue of JAMA.
The distinction is important, because accumulation of excess fat is associated with obesity-related medical conditions, while accumulation of muscle mass is beneficial, noted Dr. George A. Bray of Pennington Biomedical Research Center, Baton Rouge, La., and his associates (JAMA 2012;307:47-55).
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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."
Dr. Bray and his colleagues examined whether the level of dietary protein affected weight gain and body composition. Their randomized trial included 25 healthy, weight-stable adults (BMI 19-30 kg/m2) who lived under tightly controlled conditions in a metabolic unit for 10-12 weeks. During the first 2-4 weeks of the study – a weight stabilization period – the 16 men and 9 women (aged 18-35 years) consumed an isocaloric diet with 15% of energy from protein, 25% from fat, and 60% from carbohydrates.
During the final 8 weeks of the trial, subjects were deliberately overfed 40% more energy than was needed for weight maintenance, an excess of approximately 1,000 calories each day. During that overeating period, they were randomly assigned to diets containing 5% of energy from protein (the low-protein diet), 15% of energy from protein (the normal-protein diet), or 25% of energy from protein (the high-protein diet).
Body composition was measured by dual x-ray absorptiometry. The study subjects included 7 non-Hispanic whites, 16 blacks, and 2 Asians.
All the study subjects gained weight during the overeating period, and there were no significant differences in weight gain by subjects’ race or sex. That suggests that the data are generalizable to all races and both sexes.
The weight gain in the low-protein group was 3.16 kg, significantly less than the weight gain in the other two groups – 6.05 kg in the normal-protein group and 6.51 kg in the high-protein group.
In the low-protein group, more than 90% of the extra energy was stored as fat. In contrast, in the normal- and high-protein groups, only about 50% of the extra energy was stored as fat, while the remaining 50% was lean body mass.
The findings imply that a low-protein diet is metabolically different from a normal- or high-protein diet, the investigators added.
The study was supported in part by the U.S. Department of Agriculture and Louisiana State University, New Orleans. Dr. Bray reported ties to Abbott Laboratories, Takeda Global Research Institute, Medifast, Herbalife, and Global Direction in Medicine. One of his associates reported ties to Bristol-Myers Squibb, Eli Lilly, Elcelyx, Merck, Philips, the International Life Sciences Institute, Catapult Health, and Domain & Associates.