Glutamine, taken either in the diet or as a supplement, preserves muscle function and improves nitrogen balance. In addition, it crosses the blood-brain barrier and acts as a precursor to neurotransmitters. In theory, this makes it beneficial in athletes who subject their bodies to high levels of physical and mental stress and whose requirement for glutamine far exceeds that of a non-training person. Glutamine deficiencies can contribute to sub-optimal performance and slow recovery times.