Each day during the 10-week study, they ate about 10 ounces of meat -- including one beef serving -- as well as three servings of low-fat milk or cheese and at least five servings of vegetables. The study was funded by beef producers and Kraft Foods, which makes dairy products, as well as the USDA and Illinois Council on Food and Agricultural Research.
Meanwhile, the high-carbohydrate group of women ate only half as much protein, getting 16% of their total calories in protein, 58% from carbohydrates, and 26% from fat. They averaged 0.36 grams of protein per pound of body weight. The National Academies' guidelines announced five months ago suggest that most Americans get between 10%-35% of calories from protein, 45%-65% from carbohydrates, 20%-35% from fat.
Though all the women lost about 16 pounds each, those in the higher-protein group lost more body fat and retained more lean muscle than those the high-carb group. "However, when we did a follow-up four months later, we found the higher-protein group continued to lose weight while the high-carb group had plateaued and lost no additional weight," says Layman.
The researchers also found that the higher-protein group had lower blood sugar levels, making them less prone to type 2 diabetes.
However, those eating the more typical high-carb diet had lower cholesterol levels. At the study's end, their "bad" LDL cholesterol levels dropped 16% and their total cholesterol was down 12% -- that's nearly twice as much as those getting more protein. Layman notes there was no change in triglyceride levels in the high-carb group, but the higher-protein eaters averaged a 22% decrease.
These mixed results left one noted nutritionist with mixed feelings.
"This is certainly interesting and stresses the need for us to reassess where our focus should be in determining exactly how much we should be consuming of protein, carbohydrates, and fat," says Alice H. Lichtenstein, DSc, professor of nutrition at Tufts University and vice chairwoman of the nutrition committee of the American Heart Association.
"However, what concerns me is that in this study, they provided all the food for four weeks so they could make sure the participants that were getting more protein got very lean meat. They also received intensive counseling for the next six weeks ... and they still didn't have as good cholesterol reductions as those eating what is a typical diet," she tells WebMD. "If you tell someone to eat seven servings of beef a week, I doubt they will be as careful in choosing the leanest cuts of beef, and as a result, they may consume even more saturated fat and dietary cholesterol. What's more, if I was counseling someone to eat more protein to get leucine, I would stress baked or broiled fish, nonfat dairy products, and legumes."