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The aim of this study was to evaluate the effect of a high-protein intake compared with a “normal”-protein intake on energy substrate utilization (burning protein, fat or carbohydrate for energy). Fourteen men were placed on a standardized diet and exercise regimen for six days. Then, six of the men were placed in a “high-protein" group, lowering carbohydrate content from 58% to 33% (consuming 2.5 grams of protein per kilogram bodyweight) while eight were placed in a “normal-protein” group (consuming 1 gram of protein per kilogram of bodyweight). All subjects exercised for 90 minutes each day on a stationary bicycle. On days when their blood was tested, subjects engaged in two 90-minute bike workouts. When comparing diets, the high-protein, lower-carbohydrate group used more protein for energy during exercise. However, there was no effect on total 24-hour protein utilization. Subjects on the high-protein diet used more fat for energy during exercise as well as during recovery and at rest, compared with the “normal”-protein group. In addition to burning fat for energy, subjects on the high-protein lower- carbohydrate diet were also in positive nitrogen balance, which prevents the body from breaking down body tissue, such as muscle.