When it comes to keeping off fat, protein sounds to some like a magic bullet.
For decades, people have been making the case that eating a lot more of it, as in the Atkins diet, or lots less of it, will change the body's metabolism, spurring weight loss.
But alas, it ain't so easy.
No matter how much or how little protein they ate, volunteers on a high-calorie diet all gained weight, concludes a study published in the latest issue of JAMA, the Journal of the American Medical Association.
The people on the low-protein diet did gain fewer pounds: about 6 pounds over eight weeks compared to about 13 pounds for those on a diet with a normal protein and 14 pounds for the high-protein plan.
Those results might make it look like the low-protein diet works. But, no. "That is not the message!" Dr. George Bray, chief of the division of clinical obesity and metabolism at the Pennington Biomedical Research Center in Baton Rouge, Lousiana, says emphatically.
So Shots asked Dr. Bray to explain why low-protein diets don't work. First, he says, the low-protein dieters stored more energy as fat, and lost lean body mass. So even though they gained fewer pounds, their body composition changed for the worse. By contrast, the people eating more protein gained lean body mass, burned more calories at rest, and packed away only half the excess calories as fat.
That increased resting metabolism in the protein-eaters may explain why people who eat protein-rich diets tend to do better at losing weight and keeping it off over the long term, an editorial in the same issue of JAMA points out.
Doctors should think about how to assess a patient's overall "fatness," rather than just weight or body mass index, the editorial concludes. And Bray agrees. "The scale weight that you get isn't a good measure of how your body is responding to diets or overeating," Bray says.
Ultimately, the study participants gained weight not because of the composition of their diet, but because they were eating too much. Bray says: "The secret is calories."
The study had just 25 participants, but they stayed in an inpatient unit for eight weeks, where their food intake and calories burned were precisely measured. They were fed an extra 1,000 calories a day compared to a typical American diet. A tightly controlled situation like this is the gold standard for nutrition studies.
Bray and his colleagues took on this project because earlier studies had suggested that people who ate low-protein or high-protein diets were less metabolically efficient, and extracted fewer calories out of their food. This study proves that's not true, Bray says. And there was no evidence that eating more than 12 to 15 percent of calories in protein slowed weight gain or improved body composition.