Some were fed a diet that was five percent protein, some ate 15 percent protein -- considered a normal level -- and others ate 25 percent protein, or a high amount.

The researchers' aim was to uncover how different levels of protein might affect overall weight gain, body fat and energy expenditure.

They found that people on the low-protein diet gained less weight overall, but that more of their extra energy was stored as fat than people on the mid-level and high-protein diets.

Low-protein eaters gained about half as much as the others -- putting on an average of 3.16 kilograms (seven pounds) during the study compared to 6.05 kg in the normal protein group and 6.51 kg in the high-protein group.

But a lot of that extra weight was in the form of lean body mass, which people on the mid- and high-level protein diets gained while those on the low-protein regime lost.

Ninety percent of the extra energy consumed by people on the low-protein diet was stored as fat, compared to about 50 percent in the other two groups.

"The key finding of this study is that calories are more important than protein while consuming excess amounts of energy with respect to increases in body fat," said the research, led by George Bray of the Pennington Biomedical Research Center in Baton Rouge, Louisiana.