Glutamine is the most abundant free amino acid circulating in the blood as well as stored in the skeletal muscles. Glutamine is a diverse amino acid that is utilized in numerous vital functions. 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 is an important fuel for some immune cells, such as lymphocytes and macrophages, that may have specific immunostimulatory effects.
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.  Glutamine supplements cause a rapid rise in cellular glutamine levels and glutamine stores in muscle.