Utilization of Protein and Fat by Muscle

The metabolism of protein in the body is ordinarily measured by determining the amount of nitrogen excreted in the urine. The fact that exercise increases the excretion of nitrogen has been confirmed by later workers, but the conclusion that this indicates utilization of protein as a fuel by the muscles has been questioned since there is no direct relationship between the amount of nitrogen excreted and the amount of work done.

The possibility that the increased nitrogen excretion may be due to altered kidney function is suggested by the finding of albuminuria following severe exercise. The rise in urinary nitrogen after exercise was abolished by feeding carbohydrate before the exercise. It is possible that in long-continued work, exhaustion of the carbohydrate supplies results in the conversion of protein into carbohydrate, thus making protein an indirect fuel for muscular work.

From a practical viewpoint, the available evidence indicates that protein is of minor importance as a source of muscular energy. There is no scientific support for the idea that athletes and workers doing hard physical labor require a larger protein intake than do sedentary persons. Only when tissues are being built or repaired at an increased rate, as in growing children and convalescent patients, is an increased protein intake necessary. By tile same token, when total muscle mass is being increased by a course of muscle training, there may be some necessity for increasing the protein content of tile diet.