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Protein May Keep Muscle, Burn Fat continued...

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.

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