L-glutamine is the most abundant, naturally-occurring, free amino acid found within skeletal muscle and blood. L-glutamine has many functions in the body, including protein synthesis. L-glutamine is also essential to proper function of the immune system. In addition, L-glutamine appears to play a role in brain function and digestion.
In sports nutrition it is mainly used as part of post-exercise recovery products, because it is said to help reduce the catabolic effects of muscle breakdown.