L-glutamine is available in powder form and is a popular supplement. The bioavailability of oral glutamine has been questioned because much of the oral glutamine is utilised as a source of energy by the enterocytes of the small intestine. Glutamine can also be released from skeletal muscle in large amounts and this too it utilised by the enterocytes of the small intestine. For this reason oral glutamine is seen as being useful to those with particular gastrointestinal disorders where enterocytes may require a sufficient supply of energy. Cabbage is a good source of glutamine, and its use in the treatment of gastrointestinal lesions is well reported. Oral glutamine may protect the enterocytes from radiation, and rat experiments have shown that oral glutamine can protect irradiated rats from some of the symptoms associated with radiation poisoning including diarrhea and intestinal bleeding. The gut healing properties of oral glutamine are therefore quite well established.
Because of the use of oral glutamine by the gut, little of the orally taken glutamine makes it to the circulation. However, by ingesting oral glutamine, some of the glutamine released from the skeletal muscle may not be used by the gut cells and in this way oral glutamine may raise plasma levels of glutamine indirectly. In this regard oral glutamine before exercise may significantly increase plasma levels of glutamine, as this is a time when skeletal muscle catabolism and release of glutamine to the blood is elevated. The release of glutamine from skeletal muscle is a result of the metabolic acidosis caused by intense exercise. In this role the glutamine is used for base generation by the kidneys. Oral glutamine can therefore decrease muscle catabolism following exercise as there is less need for the muscle glutamine as endogenous supplies are able to supply the substrate for base generation. Animal and human studies show that oral glutamine may therefore be an effective ergogenic aid.
Oral glutamine may also enhance the activity of natural killer cells, and thus have an immune stimulatory effect. The use of glutamine in large amounts by other immune cells including lymphocytes and macrophages suggests that immunity benefits from oral glutamine supplementation if plasma levels rise. Because oral glutamine has an anti-catabolic effect on skeletal muscle, it may be a useful supplement for those with chronic wasting diseases. However, the daily production of glutamine in humans is thought to exceed 100 grams in some cases and therefore small doses orally, most of which would be consumed by the enterocytes of the gut, may not be effective. Glutamine is interconvertible to glutamic acid through loss of a nitrogen group (to form ammonia). Food labels tend not to list glutamine levels separately but list the glutamic acid content as including glutamine. Roughly 50 % of the glutamic acid in animal protein is glutamine, and around 80 % of the glutamic acid in plant proteins are glutamine.