Skeletal Muscle

In the skeletal muscle glutamine is needed to maintain protein levels, immune system function, and glucose-glycogen metabolism, all of which can be very significant for an active individual such as an athlete.28 Much of the glutamine is stored in skeletal muscle, and athletes who overuse these muscles may deplete their glutamine stores and increase their susceptibility to infection or slow their rate of recovery from injuries. In the postabsorptive state, skeletal muscle glutamine is a major contributor (48%) to the amino acid nitrogen released into the circulation.29 Furthermore, 4 h after a mixed meal, skeletal glutamine accounts for 71% of the amino acid released and 82% of the nitrogen released from the muscle.29 Plasma glutamine concentrations decreased 10 to 25% after moderate- to high-intensity exercise (50 to 80% VO2max), which could be due to an increase in muscle protein synthesis during recovery.29 Glutamine may also be a stimulator of post-exercise glycogen synthesis in the muscle, probably due to the direct conversion of glutamine (carbon skeleton) to glycogen or glutamine-induced cell swelling, which stimulates glycogen synthesis.30 However, addition of protein to carbohydrate supplements post-exercise has not been consistently observed to increase glycogen synthesis beyond that produced with carbohydrate only.3031 A high correlation exists between muscle glutamate concentrations and muscle glutathione concentrations, suggesting that glutamate plays a determining role in the glutathione synthesis process, and thus oxidation pathway.32 During exercise, glutamate plays a critical role in energy provision because it participates in the tricarboxylic acid cycle and the purine nucleotide cycles critical for protein synthesis. Kargotich et al.33 observed high-intensity exercise in well-trained swimmers to result in a significant reduction in plasma glutamine during the post-exercise period, suggesting that glutamine could be a marker of overtraining.

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