Glutamine (abbreviated as Gln or Q) is the most abundant naturally occurring, non-essential amino acid in the human body.

Glutamine has a variety of biochemical functions including acting as a substrate for DNA synthesis, playing a major role in protein synthesis, acts as the primary source of fuel for enterocytes (cells lining the inside of the small intestine), aids in immune function, helps to regulate of acid-base balance in the kidney by producing ammonium, acts as an alternative source of fuel for the brain,  and helps to block cortisol-induced protein catabolism. 

In catabolic states of injury and illness, GLN becomes conditionally-essential (requiring intake from food or supplements). Glutamine has been studied extensively over the past 10-15 years and has been shown to be useful in treatment of serious illnesses, injury, trauma, burns, cancer and its treatment related side-effects as well as in wound healing for postoperative patients. That is why it is now also classified as a "nutraceutical". Glutamine is also marketed as a supplement used for muscle growth in weightlifting, bodybuilding, endurance and other sports.

Dietary sources of glutamine include beef, chicken, fish, eggs, milk, yogurt, ricotta cheese, cottage cheese, dairy products, cabbage, beets, beans, spinach, and parsley. Small amounts of free L-glutamine are also found in vegetable juices and fermented foods, such as miso.

References

Glutamine. Vitamins & health supplements. Retrieved on November 1, 2007.