What Is L-Glutamine? The Body's Most Abundant Amino Acid, Explained

L-glutamine is the amino acid you have most of. It circulates in your blood at a higher concentration than any other, your muscles hold a large store of it, and your body makes it fresh every day from the protein in your diet. It is also a fuel source for the cells that line your gut, which is why it is so often sold in tubs with the word "gut" or "recovery" somewhere on the label, and that is where most people first meet it. This article covers what glutamine is, how it is made, what it does, where it occurs in food, what studies have looked at, and where the research in healthy adults currently stands.

What Is L-Glutamine?

Glutamine is an amino acid, one of the twenty the body uses as building blocks for protein. Of those twenty it is the one you carry the most of in free form, circulating in the blood rather than locked into a protein. Skeletal muscle holds the largest store, and the concentration in blood plasma is higher than that of any other amino acid.⊕

The "L" is worth settling early. Most amino acids exist in two mirror-image forms, labelled L and D. The L form is the one found in nature and used to build protein; the D form plays no part in human protein.⊕ So "L-glutamine" is the full name of the glutamine your body makes and eats, and "glutamine" on its own means the same molecule.

Glutamine is classed as non-essential, meaning the body can make it and does not depend on the diet for it. Researchers add a qualifier, "conditionally essential": in healthy adults the body keeps up comfortably, but in serious illness, after major surgery or trauma, demand can rise faster than supply.⊕ That is where glutamine first attracted clinical attention.

Because glutamine is one of the amino acids proteins are built from, it is worth knowing where the authorised claims actually sit. Protein as a whole carries two relevant claims in the UK and EU register: "Protein contributes to a growth in muscle mass" and "Protein contributes to the maintenance of muscle mass".⊕ Both belong to protein, the complete nutrient, in foods that are a source of protein, and not to glutamine on its own. Glutamine carries no authorised claim of its own, for reasons covered below.

How Does the Body Make Glutamine?

Your body builds glutamine from two things it has in abundance: glutamate, another amino acid, and ammonia, a nitrogen-containing by-product of normal metabolism. An enzyme called glutamine synthetase joins the two. Many tissues can run the reaction, but skeletal muscle does most of the work, accounting for roughly 90 per cent of the glutamine the body makes.⊕

Muscle is also the warehouse. It holds a large pool of free glutamine and releases it steadily into the bloodstream, where the gut, the kidneys and active immune cells are among the heaviest users.⊕ Made in muscle, spent elsewhere. That flow is why the "conditionally essential" label exists: in a prolonged critical illness, muscle can be asked for more than it can make. For a healthy adult eating enough protein, the pool generally stays topped up. Protein needs vary from person to person, and in serious illness or after injury the demand for glutamine can rise. Whether hard training does the same in healthy adults is one of the questions the research below has tested.

What Does Glutamine Do in the Body?

Here is a breakdown of how L-glutamine works in the body. These are the jobs the molecule does inside you every day, whether it came from your diet or from your own muscle.

Glutamine carries nitrogen. Every amino acid contains it, and the body needs a safe way to move nitrogen between tissues without letting free ammonia build up. Glutamine is the main carrier, which goes a long way to explaining why there is so much of it in the blood.⊕

Glutamine is also a fuel for fast-dividing cells. The cells lining the small intestine and the lymphocytes of the immune system replace themselves quickly, and both use glutamine readily as an energy source alongside glucose.⊕ This is the biochemical root of the gut and immune interest in glutamine, and why the supplement is so often marketed with the gut in mind.

Glutamine is a raw material. The body uses it to make nucleotides, the building blocks of DNA and RNA, and other amino acids. It is also the main source of the glutamate needed to make glutathione, an antioxidant the body produces itself.⊕ The kidneys, for their part, break glutamine down to release ammonia when the body needs to shed acid, part of how they keep acid-base balance.⊕

All of that happens continuously, in everyone, using glutamine the body has made or eaten. What researchers have asked is whether adding more makes a measurable difference, and in whom.

Which Foods Contain Glutamine?

Any food that contains protein contains glutamine, because glutamine is part of the protein. In its 2009 review, the European Food Safety Authority listed glutamine's dietary sources as beef, chicken, fish, eggs, milk and dairy products, wheat, cabbage, beets, beans, spinach and parsley.⊕ The proportion varies a lot. A 2009 analysis in the European Journal of Clinical Nutrition, which built a glutamine food composition database from protein sequencing data, found that glutamine makes up anywhere from about 1 to 33 per cent of the protein in a given food, with wheat and dairy proteins at the richer end.⊕

The same analysis estimated how much people actually eat. In a US cohort of more than 70,000 women, average glutamine intake from food worked out at roughly 6.8 g a day.⊕ That is a useful number to hold against the supplement tubs. If you eat enough protein across the day, from meat, fish, eggs, dairy, soya, pulses or grains, you are eating several grams of glutamine, and your muscles are making more.

What Has the Research Looked At?

The supplement has a specific history. In the 1990s, researchers observed that plasma glutamine falls after very prolonged, hard exercise such as a marathon, and proposed that this dip might be linked to the minor infections endurance athletes often report.⊕ Bodybuilders picked up a related idea, that heavy training drains muscle glutamine and replacing it might affect recovery. Glutamine powder, and later "glutamine peptides" (glutamine bound to other amino acids in short chains), became a sports-nutrition staple on the strength of those hypotheses. There is no UK or EU reference intake; the amounts in products are set by manufacturers.

The research that followed is large, and the clearest way through it is to ask what was studied, in whom, and what was reported. Three threads account for most of it.

Muscle and performance in trained adults. The bodybuilding hypothesis was tested directly. One often-cited trial gave 31 young adults either glutamine, at a high dose scaled to lean body mass, or a placebo during six weeks of resistance training, and reported no significant difference between the groups in muscle performance, body composition or muscle protein breakdown.⊕ A 2019 review of 55 studies in the journal Nutrients reported that most of them observed changes in some fatigue-related markers, such as glycogen synthesis and ammonia accumulation, but that supplementation did not increase physical performance.⊕ Researchers have looked at similar questions for leucine and its metabolite HMB.

The gut lining. Most of the gut evidence comes from hospital-based cohorts, where glutamine has been studied as part of feeding for critically ill or post-surgical patients, and from laboratory work in animals and cultured intestinal cells.⊕ A small number of trials have looked at irritable bowel syndrome. A 2019 trial in the journal Gut randomised 115 adults with post-infectious IBS to glutamine, 5 g three times a day, or a placebo for eight weeks; of the 106 who completed it, a far larger share of the glutamine group reached the study's threshold for improvement on a symptom score.⊕ A 2021 trial combined a low-FODMAP diet with 15 g of glutamine a day in 50 IBS patients for six weeks and reported a larger fall in symptom scores than diet alone.⊕ Both are small trials in people with a diagnosed condition, and both reported better symptom scores with glutamine. Together with the hospital work, they are why researchers keep returning to glutamine in illness and recovery. Research into what it does in healthy adults is still emerging. "Leaky gut" is the popular phrase here; researchers say "intestinal permeability", and whether glutamine affects it in healthy people is one of the open questions.

Immune cells and infections in athletes. The marathon observation led to a 1996 study in which endurance athletes were given a glutamine drink or a placebo straight after a race and again two hours later, then asked to record any infections over the following week. The glutamine group reported fewer.⊕ The evidence since then has been mixed. That study rested on self-reported symptoms rather than confirmed infections, and later studies measuring immune-cell function after exercise, with and without glutamine, did not find that keeping plasma glutamine steady changed the usual post-exercise picture.⊕ The immune question in athletes is best described as open rather than answered.

Thirty years of work, then: a well-described molecule, promising signals in people under physical or gut-related stress, and a healthy-adult literature that is still mixed and would need larger trials before anyone could generalise from it.

One piece of regulatory context belongs here. The European Food Safety Authority assessed proposed glutamine claims in 2009 (immune support, gut integrity and permeability)⊕ and again in 2011 (muscle mass, glycogen restoration, muscle repair, cognition, gut and immune function)⊕ and concluded in each case that a cause-and-effect relationship had not been established, largely because the trials were in patient groups rather than healthy adults, or rested on self-reported outcomes. So no health claim for glutamine is currently authorised in the UK or EU, which is why this article says "research has looked at" rather than "glutamine does". "Not established" does not mean the studies found nothing; it means the evidence EFSA reviewed came mostly from patient groups or self-reported outcomes, and did not show an effect in healthy adults.

Is Glutamine Safe?

Safety is one of the clearer parts of the picture, partly because glutamine is something the body handles in gram quantities every day. A 2008 review of glutamine in sport summarised that single intakes of around 20 to 30 g appeared to be without ill effect in healthy adults, and that one study in which athletes took 28 g a day for two weeks reported no harm.⊕ Across short-term trials in patient groups, the only negatives recorded were loose stools, abdominal discomfort and headache.⊕ Longer-term data in healthy people are thin.

If you are pregnant or breastfeeding, taking medication, managing a kidney or liver condition, or undergoing cancer treatment, speak to your GP or a registered dietitian before adding glutamine.

Is Glutamine the Same as Creatine?

No, although the two share a shelf and often a product. Creatine is not an amino acid. The body makes it from three amino acids, arginine, glycine and methionine, and stores it in muscle largely as phosphocreatine, where it is involved in regenerating energy for short, intense efforts. It is one of the most thoroughly researched supplements there is, and what creatine does and why it is so well studied has its own article.

The regulatory difference is the clearest one. Creatine has authorised health claims in the EU and UK, tied to a daily intake of 3 g.⊕ Glutamine, as above, has none yet. If a combination product carries a health claim, that claim belongs to the creatine.

L-Glutamine FAQs

Is L-Glutamine the Same as Glutamine?

Yes, in practice. L-glutamine is the naturally occurring form of the amino acid, the one your body makes, eats and uses. "Glutamine" on a label or in a study almost always means L-glutamine.

Is Glutamine the Same as Glutathione?

No. Glutathione is an antioxidant, a small molecule built from three amino acids: glutamate, cysteine and glycine. Glutamine is not one of the three, but it is the body's main source of the glutamate that goes into it. Related, not interchangeable.

How Much Glutamine Was Used in Studies?

Trials in athletes have commonly used around 5 to 15 g a day, with a few using larger amounts scaled to body weight. The IBS trials used 15 g a day. That describes the research, not a recommendation. There is no UK or EU reference intake for glutamine.

Is L-Glutamine Worth Taking?

For most healthy adults, the body and a normal diet supply several grams of glutamine a day. The clearest signals in the research so far have come from people under strain, such as hospital patients and people with IBS, while trials in healthy, trained people have been mixed. Whether it is worth trying is a question for your own situation, and a GP or registered dietitian can help you weigh it up.

The Bottom Line

Glutamine is a molecule worth understanding on its own terms. Your body carries more of it than any other amino acid, makes it fresh in muscle every day, and spends it on some of its most demanding jobs: moving nitrogen safely, fuelling the cells that line your gut and the immune cells that defend it, building glutathione and DNA. Demand for it rises when the body is under strain, and that is where researchers have looked hardest, with the most promising signals so far in hospital patients and people with IBS. Research in healthy adults is still emerging, and no claim is authorised in the UK or EU yet, so this article describes what has been studied rather than what to expect. A protein-rich diet is the foundation, and it is the part everyone can act on; whether glutamine on top of that suits you is a conversation worth having with a GP or dietitian.