Glutamine

Glutamine (symbol Gln or Q) is one of the twenty amino acids used to build proteins, distinguished from the related glutamic acid by an amide group in place of a second carboxylic acid. In humans it is regarded as non-essential under normal conditions but conditionally essential during metabolic stress, because the body's own synthesis may not keep pace with demand. It is the most plentiful free amino acid in the circulation, and skeletal muscle is its principal site of production and storage. The naturally occurring form is L-glutamine.

Glutamine occupies a central place in nitrogen metabolism. It acts as a nontoxic vehicle for carrying ammonia and nitrogen among organs, moving in the fed state largely from the intestine to other tissues and, during fasting or stress, from muscle toward the liver and kidneys ^[1]. It supplies carbon to the citric acid cycle after conversion to alpha-ketoglutarate, donates nitrogen for the synthesis of purines and other nucleotides, and contributes to the production of glutathione, an important cellular antioxidant. Cells with high turnover, including the enterocytes of the gut lining, activated immune cells, and many tumor cells, consume glutamine avidly as both an energy source and a building block ^[4].

Glutamine is obtained from protein-rich foods such as meat, fish, eggs, dairy, and legumes, and it is manufactured on an industrial scale by microbial fermentation for use in nutrition and research. As a supplement it has been studied in critical illness, gut health, and sports nutrition. Evidence in critically ill patients is mixed; some formulations were once used routinely to support the gut and immune function, but later large trials raised caution, and current reviews advise against glutamine supplementation in patients with multiple organ failure ^[2]. In sickle cell disease a pharmaceutical-grade oral form was shown in a randomized phase 3 trial to modestly reduce the frequency of pain crises and hospitalizations, leading to its approval as a prescription therapy ^[3].

Glutamine is sold both as a dietary supplement, commonly in powder or capsule form, and, for sickle cell disease, as an approved prescription medicine. It is generally considered safe for healthy adults at customary intakes and is a normal component of the diet. Caution is warranted in people with liver disease or those receiving intravenous nutrition, and its role in feeding rapidly dividing cells has prompted ongoing research interest in how glutamine metabolism relates to cancer ^[4].

Very safe and well tolerated, even at fairly high doses; occasional mild GI upset is about it. People with liver or kidney disease should be cautious because of altered nitrogen handling. It's widely used in clinical nutrition without major issues.

No formal drug interaction studies have been carried out on glutamine, and the prescribing information for the sickle cell product says as much. What is documented is metabolic rather than pharmacokinetic.

Glutamine is deaminated in the gut and liver to glutamate and ammonia, and an oral glutamine load raises blood ammonia reliably enough that it is used clinically as a provocation test for minimal hepatic encephalopathy. In someone with cirrhosis or a urea cycle disorder, that ammonia rise is the interaction: it works directly against lactulose and rifaximin, and it adds to the hyperammonemia valproate can cause.

Beyond that, the concerns are theoretical. Glutamine is the precursor for brain glutamate, so an effect on seizure threshold is plausible but has not been shown in controlled work. Glutamine has no known effect on cytochrome P450 enzymes or on drug transporters.

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