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Amino Acid in Animal Protein Burns Fat, Spares Muscle

Feb. 7, 2003 -- Research supporting high-protein diets keeps rolling in. And in the latest study, researchers suggest the key to losing fat and maintaining muscle can be found in one ingredient in protein-rich animal foods.

Since high-protein foods, such as meats, are often loaded in harmful saturated fats, some experts suggest shunning them to reduce both the waistline and heart disease. But in practice, these plans often translate to excess intake of the simple carbohydrates that boost risk of obesity and type 2 diabetes.

So the experts continue to try to determine the ideal combination of calories that should come from protein, carbohydrates, and fats for Americans hungry for weight-loss answers.

Nutritionist Donald Layman, PhD, says protein-rich foods high in the amino acid leucine help maintain muscle mass while reducing body fat during weight loss.

"In studying exercise and how muscle develops, we found that leucine has a particularly unique effect in that it spares muscle proteins during weight loss, so you only lose the fat and not the muscle," Layman, at the University of Illinois at Urbana-Champaign, tells WebMD. "While most dietary plans talk about protein as percent of total calories, we look at protein needs based on a person's individual body weight and projected leucine intake to lose weight without losing lean muscle."

Leucine, which isn't produced by the human body, is found in protein-rich animal foods such as beef, chicken, fish, dairy, and eggs.

In two studies in the February issue of the Journal of Nutrition, Layman compared a higher-protein, leucine-rich regimen with the typical high-carbohydrate American diet on 24 middle-aged women who averaged about 182 pounds. Both eating plans fall within the recommended intake ranges for protein, carbohydrates, and fat under guidelines issued last September by the National Academies' Institute of Medicine.

All the women consumed about 1,700 calories a day, but the protein-rich group got approximately 30% of their calories from protein, 41% from carbohydrates, and 29% from fat sources. They averaged about 125 grams of protein daily, with a goal of 0.73 grams for each pound of body weight.

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