Although considered a "non-essential" amino acid, glutamine is considered a "brain food" and helps build and maintain muscles (useful in weight loss and bodybuilding). It has been used treat arthritis, autoimmune diseases, fibrosis, intestinal disorders, ulcers, connective tissues disease, tissue damage from chemotherapy/radiation treatments, fatigue, impotence, schizophrenia and senility. Glutamine is utilized as a source of energy and for nucleotide synthesis by all rapidly dividing cells, such as the cells of the intestinal lining and certain immune cells (thymocytes, lymphocytes and macrophages). Without sufficient glutamine, the intestines atrophy and the immune function breaks down.

In the brain, glutamine is a substrate for the production of both excitatory and inhibitory neurotransmitters (glutamate and GABA). Glutamine is also an important source of energy for the nervous system. If the brain is not receiving enough glucose, it compensates by increasing glutamine metabolism for energy, thus the popular perception of glutamine as "brain food" and its use as a pick-me-up. Glutamine users often report more energy, less fatigue and better mood.

Glutamine also plays a part in maintaining proper blood glucose levels and the right pH range. The body has an exquisite mechanism for maintaining pH homeostasis. If the pH of the blood is too acidic, more glutamine is directed to the kidneys, where a certain type of glutamine results in the release of bicarbonate ions to correct acidosis. If the pH is too alkaline, more glutamine is sent to the liver, where a different kind of metabolism releases hydrogen ions to correct alkalosis.

Glutamine is plentiful in both animal and plant protein. The typical American diet provides between 3.5gm and 7gm of glutamine. Glutamine is the most abundant free amino acid in the serum, muscle and cerebrospinal fluid. It constitutes 50% of all amino acids in the serum, and more than 60% of free amino acids within the body.

Most clinicians who are using the L-glutamine to support repair of the intestinal tract are recommending that this supplement be taken on an empty stomach. If however, significant intestinal problems are noted when taken alone then using with foods may be better tolerated.

The major use for high-dose glutamine would be to repair gastrointestinal injury. In such cases, short-term use is advised.
Glutamine has recently been shown to produce extreme hypoglycemia, even more so than leucine, which is known to produce fatal hypoglycemia in infants.

The reason Chinese Restaurant Syndrome is not seen with glutamine challenge is that the glutamate receptors in the lungs and esophagus are stimulated by glutamate, not glutamine. The glutamine must be converted first and this occurs primarily in the brain.

The only safe situation for long-term glutamine use is in the vigorous athlete. Glutamine is used as a muscle fuel, so that vigorous exercise will consume most of the glutamine before it can accumulate in the brain.