Consuming excess calories increases body fat, regardless of how many calories come from protein. High-protein diets do affect energy expenditure and storage of lean body mass, just not body fat storage.
To evaluate the effects of overconsumption of low-, normal-, and high-protein diets on weight gain, researchers conducted a single-blind, randomized controlled trial of 25 healthy, weight-stable adults in an inpatient metabolic unit in Baton Rouge, La. Patients were ages 18 to 35 with a body mass index between 19 and 30. The study was headed by George A. Bray, MD, MACP.
After consuming a weight-stabilizing diet for 13 to 25 days, participants were randomized to diets containing 5% of energy from protein (low protein), 15% (normal protein) or 25% (high protein). Only the kitchen staff who supervised participants while they were eating knew the assignments. There was no prescribed exercise, and alcohol and caffeine were restricted.
Patients were overfed during the last 8 weeks of their 10- to 12-week stay with the protein diets, which provided 40% more energy intake, or 954 kcal/d.
Body composition was measured by dual-energy X-ray absorptiometry biweekly, resting energy expenditure was measured weekly by ventilated hood, and total energy expenditure by doubly labeled water prior to the overeating and weight stabilization periods and at weeks 7 to 8. Results appeared in the Jan. 4 issue of the Journal of the American Medical Association.
Overeating produced significantly less weight gain in the low-protein diet group (3.16 kg; 95% confidence interval [CI], 1.88 to 4.44 kg) compared with the normal protein diet group (6.05 kg; 95% CI, 4.84 to 7.26 kg) or the high protein diet group (6.51 kg; 95% CI, 5.23 to 7.79 kg) (P=.002). Body fat increased similarly in all three protein diet groups and represented 50% to more than 90% of the excess stored calories.
Resting energy expenditure, total energy expenditure, and body protein did not increase during overfeeding with the low-protein diet. Resting energy expenditure increased significantly with the normal and high protein diets (normal protein diet: 160 kcal/d [95% CI, 102 to 218 kcal/d]; high protein diet: 227 kcal/d [95% CI, 165 to 289 kcal/d]), as did lean body mass (normal protein diet: 2.87 kg [95% CI, 2.11 to 3.62 kg]; high protein diet: 3.18 kg [95% CI, 2.37 to 3.98 kg]).
"[C]alories are more important than protein while consuming excess amounts of energy with respect to increases in body fat," the authors wrote.
While the authors noted that low-protein diets resulted in less weight gain than normal- or high-protein groups when extra calories were eaten, the limits of a low-protein approach were evident when considering changes in body composition and energy expenditure.
With the low-protein diet, more than 90% of the extra energy was stored as fat. Because there was no change in lean body mass, a 6.6% increase in total energy expenditure reflects the energy cost of storing fat.
An accompanying editorial noted that low-protein diets are usually higher in empty calories and hidden fats, compared to protein-rich diets that encourage building lean muscle mass instead of fat.
"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," the editorial said. "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."