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Weight gain from eating more protein: more lean body mass, not more fat

People who don't exercise and who eat more calories than they burn get fatter. In the short term, the amount of body fat they build up won't go down if they start getting more of their energy from protein, but a protein-rich diet will help them build up more muscle mass, write researchers from Pennington Biomedical Research Center in JAMA.

George Bray's study is causing a commotion in the web world. Will Brink [] and Colin Champ [] have blogged about it, and the abstract of the study has shown up on just about all forums on fitness, bodybuilding, strength training, weight loss and the paleodiet.

The researchers took 25 women aged between 18 and 35, with a BMI between 19 and 30, and determined how many calories a day they needed to keep their weight stable. Then they gave the women 954 calories a day more than they burned for eight weeks, so the women gained weight.

The researchers divided the women into three groups. The low protein diet group were given 5 percent protein; the normal protein diet group got 15 percent protein in their diet; and the high protein group 25 percent.

At the end of the eight weeks it looked as though the low protein diet group had done best. They had 'only' put on 3.6 kg. The normal protein diet subjects had put on 6.1 kg and the high protein diet subjects had put on 6.5 kg. But when the researchers looked at the changes in body composition, the picture changed.

The women in all three groups had gained 3.5 kg body fat. But when it came to lean body mass, the women in the low-protein diet group had lost 0.7 kg; the women in the normal protein diet group had gained 3.2 kg and the women in the high protein diet group had gained 4.0 kg.

The amount of energy that the women in the low protein diet group burned remained stable; in the other two groups the amount rose.

The study shows that the motto 'a calorie is a calorie' is not the case. Of course the total number of calories determined how much fat the subjects gained. But if you look at the protein intake you don't see this relationship. A higher protein intake resulted in more lean body mass build up, and did not correlate with the growth in fat mass.

The subjects in the experiment did no exercise. If they had done weight training, the results may have been even better. And if the study had gone on for longer the researchers may have seen that the fat mass of the high protein diet group actually decreased.

This is the first time that researchers have looked at subjects who have put on weight and looked at the effect of different concentrations of protein.

JAMA. 2012;307(1):47-55.

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