NEW YORK - When it comes to packing on body fat, the volume of calories you ingest seems to count more than whether those calories come from lots of protein, or very little.
Researchers found that people who ate high-calorie diets all gained about the same amount of fat. Those whose diets were low in protein gained less weight overall than people on high- and moderate-protein diets, but that's because the low-protein group also lost muscle.
"Huge swings in protein intake do not result in huge swings in body fat gain," said Dr. James Levine, who studies obesity at the Mayo Clinic in Rochester, Minnesota but wasn't involved in the new study. "It really is the calories that count."
Previous research has suggested that when people over-eat, the amount of weight they gain varies from person to person. Whether the make-up of individuals' diets might be affecting how their body stores the extra calories has remained unclear.
For the current study, researchers led by Dr. George Bray from the Pennington Biomedical Research Center in Baton Rouge, Louisiana, recruited 25 young, healthy volunteers to live in their lab and eat a prescribed diet for two to three months.
During the first couple of weeks, the researchers tinkered with participants' diets to determine exactly how many calories they needed to maintain their body weight.
Then, for eight weeks, they piled on about 1,000 extra calories to those daily diets. One-third of the participants were fed a standard diet with 15 per cent of their calories coming from protein, while the others ate low- or high-protein diets with either five or 25 per cent of calories from protein.
That worked out to volunteers eating an average of 47 grams (1.657 ounces), 139 grams (4.903 ounces) or 228 grams (8.042 ounces) of protein per day.
The diets led to weight gains for all participants, but not equally. The low-protein diet group put on about seven pounds per person, compared to 13 or 14 pounds in the normal- and high-protein groups.
But people in the low-protein group stored more than 90 per cent of their extra calories as fat and lost body protein (muscle mass), while other participants gained both fat and healthier lean muscle, researchers reported in the Journal of the American Medical Association. So the groups all gained a similar amount of total excess fat.
Donald Layman, a food science researcher at the University of Illinois in Urbana, said it's difficult to see how the findings apply to a general population that isn't being overfed such a protein-deficient diet, in the case of the low-protein group.
"It's an interesting scientific study, but from an obesity standpoint, I don't think it tells us anything," he told Reuters Health.
Levine said there are a couple of messages people outside a strict scientific study can take away from the findings - especially that weight gain or loss might not be the best way to track how healthy a person's diet is.
"The scale that you step on isn't necessarily a good guide to the kind of weight you're gaining," he told Reuters Health.
"People who had the low protein gained about half as much weight as those that had normal or high protein, but the weight was different in one major component: they lost body protein, which is not healthy," Bray said. "The scale can fool you into thinking that you're winning when you aren't."
Regardless of that number, he concluded, "If you over-eat extra calories, no matter what the composition of the diet is, you'll put down more fat."