Despite the claims of various diet gurus, excess calorie consumption alone and not the amount of protein in an individual’s diet contributes to the accumulation of unwanted fat, according to results of a small randomized trial published in JAMA today.
Some diet plans are purported to allow individuals to eat as much as they want and still lose weight by increasing or decreasing the proportion of protein consumed. But whether such strategies really work has been debated by clinicians. To probe the effects of high- and low-protein diets on weight gain, George A. Bray, MD, of the Pennington Biomedical Research Center in Baton Rouge, La, and colleagues conducted a 10- to 12-week randomized trial of 25 healthy men and women who had a stable weight. Participants stayed in an inpatient center and were fed a weight-stabilizing diet for 13 to 25 days. They were then randomized to receive a diet containing 5% protein, 15% protein (normal level), or 25% protein, all of which provided 40% more calories than the participants needed to maintain their weight.
The researchers found that the low-protein group gained less weight (3.15 kg on average) compared with the normal protein group (who gained an average of 6 kg) and the high-protein group (who gained an average of 6.5 kg). But body fat increased similarly in all 3 groups, while lean body mass increased in the normal-protein and high-protein groups (by between 2 and 4 kg).
Dr Bray discussed the results with news@JAMA via e-mail.
news@JAMA: Why did you decide to conduct this clinical trial?
Dr Bray: Earlier studies in human beings suggested that diets containing either high or low [levels of] protein are less “metabolically efficient” than diets with normal protein levels. This concept is appealing from an evolutionary perspective because the ability to waste “excess” calories when eating an unbalanced diet would ensure an adequate supply of nutrients while at the same time avoiding risks to survival as a result of excess weight gain. This study was designed to determine whether the level of dietary protein did indeed affect weight gain or energy expenditure under tightly controlled conditions.
news@JAMA: What do the results tell us about the effect of protein intake on weight gain?
Dr Bray: This randomized trial clearly shows that the amount of protein in the diet does not influence the amount of body fat that is formed when overeating. However, it did affect weight gain through changes in the amount of body protein that was made or lost. Protein also affected metabolic rate.
news@JAMA: What are the practical implications of these findings for patients trying to lose weight or for the physicians trying to counsel them?
Dr Bray: The first practical implication is an old one: calories count. We showed very clearly that the increase in body fat was due to the increased intake of calories and that the amount of protein in the diet did not change it. To avoid that slow weight gain that many adults experience in their middle years, people need to watch their weight and increase activity, decrease food intake, or both; changing the diet alone will not do it. The second implication is that diet composition may deceive you if you only look at the scale, since the people eating the low-protein diet gained less weight but not less fat. The scale can’t tell fat from nonfat weight.