Glutamine is a nonessential amino acid that is the most abundant free amino acid in human muscle and plasma. It is necessary for rapidly dividing cells, including those of the gut and immune system. It is also used as an energy source in mitochondria, and is a precursor of the brain neurotransmitter glutamate. Glutamine is also needed for optimal nucleotide biosynthesis and protein synthesis.

The function of glutamine in stimulating glycogen synthase, the enzyme which controls the synthesis and storage of glycogen fuel in muscles and liver, may provide a mechanism by which glutamine supplements promote enhanced fuel stores. Glutamine supplements cause a rapid rise in cellular glutamine levels and glutamine stores in muscle.

Glutamine is a diverse amino acid that is utilized in numerous vital functions. People that do heavy exercising can lower their glutamine levels causing stress to their body and immune systems. For that reason, supplementing with glutamine can be important to a serious athlete, someone challenged with muscle loss, or who wants to help maintain optimal immune function under heavy stress.