Muscle proteins turn over slowly and there are minimal diurnal changes in the size of the muscle protein pool in response to feeding and fasting. Nitrogen balance and tracer studies indicate that protein oxidation and net protein breakdown (degradation — synthesis) is not increased during dynamic exercise at intensities of ≤ 70% VO_2max. An imbalance between muscle protein synthesis and degradation does exist during one leg knee extensor exercise and during two legged cycling in patients with glycogen Phosphorylase deficiency. In these latter cases amino acids liberated from the protein pool are used for synthesis of TCA-cycle intermediates and glutamine.