"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," said Donald Layman, a U of I professor of food science and human nutrition.
A higher-carbohydrate, lower-protein diet based on the USDA food guide pyramid actually reduced the effectiveness of exercise, Layman said.
Forty-eight adult women participated in Layman's 4-month study, published in the August 2005 issue of the Journal of Nutrition. One group ate a protein-rich diet designed to contain specific levels of leucine, one of the essential amino acids. A second group consumed a diet based on the food guide pyramid, which contained higher amounts of carbohydrates.
Both groups consumed the same number of calories, but the first group substituted high-quality protein foods, such as meats, dairy, eggs, and nuts, for foods high in carbohydrates, such as breads, rice, cereal, pasta, and potatoes.
"Both diets work because, when you restrict calories, you lose weight. But the people on the higher-protein diet lost more weight. Some people refer to this as the metabolic advantage of a protein-rich diet," said Layman.
The study included two levels of exercise. "For one group, we recommended that they add walking to their lives. They usually walked two to three times a week, less than 100 minutes of added exercise," the researcher said.
The other group was required to engage in five 30-minute walking sessions and two 30-minute weightlifting sessions per week. In both groups of dieters, the required exercise program helped spare lean muscle tissue and target fat loss. But, in the protein-rich, high-exercise group, Layman noted a statistically significant effect. That group lost even more weight, and almost 100 percent of the weight loss was fat, Layman said. In the high-carbohydrate, high-exercise group, as much as 25 to 30 percent of the weight lost was muscle.
While this protein-rich diet works for everyone, it seems to be even more effective for people who have high triglyceride levels and carry excess weight in their midsection--a combination of health problems known as Syndrome X.
"The protein-rich diet dramatically lowered triglycerides and had a statistically significant effect on trunk fat, both risk factors associated with heart disease," he said. "Exercise helped dieters lose an even greater percentage of body fat from the abdominal area."
The protein-rich diet works so well because it contains a high level of the amino acid leucine. Leucine, working together with insulin, helps stimulate protein synthesis in muscle. "The diet works because the extra protein reduces muscle loss while the low-carbohydrate component gives you low insulin, allowing you to burn fat," he said.
"We believe a diet based on the food guide pyramid actually does not provide enough leucine for adults to maintain healthy muscles. The average American diet contains 4 or 5 grams of leucine, but to get the metabolic effects we're seeing, you need 9 or 10 grams," he noted.
To achieve that leucine level, the researcher recommended adding dairy, meat, and eggs, all high-quality proteins, to the diet. According to Layman, losing weight doesn't have to mean relying on supplements to fill in nutritional gaps in your diet. "If you use a high-quality protein approach to your diet, you can actually improve the overall quality of your diet while losing weight," he said.
Other researchers involved in the study are Ellen Evans, Jamie I. Baum, Jennifer Seyler, Donna J. Erickson, and Richard A. Boileau, all of the University of Illinois. The study was funded by the Illinois Council on Food and Agricultural Research (C-FAR), the National Cattlemen's Beef Association, the Beef Board, and Kraft Foods.
Journal of Nutrition