New research shows obese women who exercised regularly and ate a reduced-calorie diet high in protein lost more fat and less muscle than those who ate a similar diet high in carbohydrates. Both diets contained the same number of total calories and percentage of calories from fat.
"Both diets work because, when you restrict calories, you lose weight. But the people on the higher-protein diet lost more weight," says researcher Donald Layman, PhD, professor of food science and human nutrition at the University of Illinois, in a news release.
Researchers say women on the high-protein diet also lost more weight around the abdominal area.
"There's an additive, interactive effect when a protein-rich diet is combined with exercise. The two work together to correct body composition; dieters lose more weight, and they lose fat, not muscle," says Layman.
Protein May Keep Muscle, Burn Fat
In the study, researchers compared the effects of a high-protein, low-carbohydrate diet against a high-carbohydrate, low-protein diet combined with exercise in 48 obese women.
Both diets contained 1,700 calories, 30% of calories from fat, and about 17 grams of fiber.
But women on the high-protein diet substituted high-protein foods, such as meats, dairy, eggs, and nuts, for foods high in carbohydrates, such as breads, rice, cereal, pasta, and potatoes, to get about 30% of their total calories from protein.
Women on the high-carbohydrate diet, in comparison, ate about half that amount of protein and got about 60% of their daily calories from carbohydrates.
Both diets fall within the acceptable nutrient levels prescribed by the Institute of Medicine, according to the researchers.
Exercise for the low-intensity group emphasized voluntary lifestyle recommendations of a minimum of 30 minutes of walking five days/week.
After four months, the results showed that both groups of dieters lost weight, and those who exercised more lost less muscle tissue and more fat.
High-protein dieters in the high-exercise group lost an average of 22 pounds and less than a pound of lean muscle. High-carbohydrate dieters in the high-exercise group lost an average of 15 pounds but lost more than 2 pounds of muscle.
But the real key to losing weight while maintaining muscle appears to be exercise.
The high-protein, low-exercise dieters lost an average of 19 pounds but lost over 4 pounds of muscle. The high-carbohydrate, low exercisers lost 17 pounds, but nearly 6 pounds of that came from muscle.
Nearly 100% of the weight lost in the high-protein exercise group was fat, while 25%-30% of the weight lost in the high-carbohydrate exercise group was muscle, says Layman.
Amino Acid Behind Fat-Burning Effect
Layman says the weight loss advantage of a high-protein diet may be its high level of the amino acid leucine. Leucine works with insulin to stimulate protein synthesis in muscle, he says.
"The diet works because the extra protein reduces muscle loss while the low-carbohydrate component gives you low insulin, allowing you to burn fat," says Layman. "Some people refer to this as the metabolic advantage of a protein-rich diet."
He says the average American diet contains about four or five grams of leucine and to get the metabolic effects seen in this study you need 9 or 10 grams.
The study, which appears in the August issue of the Journal of Nutrition, was funded by the Illinois Council on Food and Agricultural Research, the National Cattlemen's Beef Association, the Beef Board, and Kraft Foods.