Glutamine is one of the 20 fundamental amino acids involved in protein synthesis, and it is also the most abundant, forming approximately one-fifth of the amino acid pool circulating in the human bloodstream. It is classified as "non-essential" because it can be synthesized from another amino acid, namely glutamic acid.

Glutamine participates in a wide variety of essential functions, including: optimal operation of the kidneys, pancreas, gall bladder, liver, the small and large intestines, and the brain. Glutamine is also the most important of all amino acids in maintaining nitrogen balance within the body, accounting for a full one-third of all amino acid nitrogen transported in the human bloodstream.

Although all amino acids release an atom of nitrogen into the bloodstream after they are broken down in the liver, glutamine's higher concentration in the bloodstream combined with the fact that it has two nitrogen side-chains is the reason for this. Furthermore, it is a precursor for the body's most powerful endogenous anti-oxidant, glutathione; it also plays a role in pituitary's GH production and seems to be important for proper glycogen deposition as well.

Characteristically glutamine is quickly depleted during the course of catabolic stress such as injury, infection, or even strenuous exercise. While the average person is likely to consume approximately 10 grams of glutamine daily, a conservative estimate for the needs of someone in such a stressful state is 20-40 grams of glutamine daily.

• Digestion: Studies among laboratory rats have revealed that the intestinal tract and other portal-drained viscera account for approximately 40% of all the glutamine utilized by the whole body. The vast majority of this glutamine is consumed by the epithelial cells which convert it into energy for use by the mucosa, the lining of the small intestine. This importance of glutamine to the intestinal tract (and by extension to glucose metabolism) was only discovered in the last twenty years. This holds important implications for cases on enteral nutritional support, intestinal tract difficulties, percutaneous endoscopic jejunostomy (PEJ) tube is inserted into the small intestine to provide nutritional support. Enteral diets enriched with glutamine are well-tolerated, with better immunological aspects, and less expansion of mucositis in post-chemotherapy patients.
• Athletic Performance: Variety of amino acids are precursor to the GH production by pituitary gland including glutamine. During exercise the skeletal muscle amino acid concentration of which usually includes 60% glutamine, falls. It takes minimum of 24 hours for the body to recover the glutamine levels to the pre-exercise level after a single bout of resistance training
• Cellular Health: Since cancer has been described as a nitrogen trap, considering the essential role of glutamine in nitrogen transport, glutamine deprivation seems like the first course of action, but it is not practical due to the adverse effects it would cause the host. However, glutamine is a precursor for the synthesis of glutathione which impacts the numbers and activity of glutathione-dependent natural killer cells. Glutamine play a role in revitalizing natural killer cells via the restoration of glutathione.
• Immunity: Infections modifies the distribution of glutamine among the organs of the body. Inflammatory infections (sepsis) causes a drop in IGF-1 levels lowering glutamine absorption by the bowel, and depletes glutamine reserves in cells, especially from the skeletal muscle and the lungs, and liver, Therefore, there will be less supply available for the immune system which is consumer of glutamine to produce of antibodies and glutathione. based on the recent studies, supplemental glutamine may help overcome this condition.