Rushall notes (1989).

Only under extreme conditions, such as in starvation or prolonged exercise-induced carbohydrate depletion, does their energy contribution approach a 10 percent value. In that role, the main function of proteins is to provide the liver with substrates from which glycogen can be produced. Amino acids are extracted from proteins in the digestive process and are transported to the liver where they may serve as precursors for glucose and glycogen production. Proteins are used mainly for muscle movement. They are used by all body tissues, and in particular, the muscles, to replace proteins that are continually being broken down.

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