What L-Glutamine Actually Does: Gut, Immunity and Recovery

Jasmine Meagher, founder of Ritua
Jasmine Meagher, BSc Food Science and Nutrition
Founder, Ritua · 12 min read

Glutamine turns up everywhere and gets explained almost nowhere. It sits in recovery blends, in gut formulas, in hospital feeding protocols and in single ingredient pouches, described in terms vague enough that it is hard to work out what is actually going on. So it is worth starting from the biochemistry rather than the marketing.

L-glutamine is an amino acid. It is the most abundant free amino acid in the human body, it is central to how the gut lining and the immune system run their metabolism, and in healthy well fed adults the body makes a large amount of it daily without any help. That last point decides most of the supplement question, and it is the one product pages skip. Here is what the research shows, including the parts that do not favour taking a supplement.

What glutamine is, and why there is so much of it

Glutamine is one of the twenty amino acids that make up human protein. The "L" simply refers to the naturally occurring form, which is the only one your body uses and the only one worth buying.

What makes it unusual is the sheer quantity floating around outside of protein. The 2018 review Glutamine: Metabolism and Immune Function, Supplementation and Clinical Translation by Cruzat and colleagues in Nutrients puts it plainly: in both plasma and tissues, glutamine concentration is ten to a hundred times higher than any other amino acid. A healthy person weighing around 70 kg carries roughly 70 to 80 g of free glutamine.

Around 80 per cent of that sits in skeletal muscle, at about thirty times the concentration found in plasma. Muscle is the body's glutamine reservoir, and the same review notes that its glutamine synthesis rate, in the order of 50 mmol per hour, is higher than for any other amino acid. Muscle makes it, releases it into the blood, and other tissues draw on it.

What "conditionally essential" actually means

Amino acids are usually sorted into essential, meaning you must eat them, and non-essential, meaning your body synthesises them. Glutamine is non-essential. You make it. For most people on most days, that is the whole answer.

The complication arrived in 1990, when Lacey and Wilmore published Is glutamine a conditionally essential amino acid? in Nutrition Reviews. Their argument is worth quoting for its logic rather than its buzzword: during stress, the body's requirement for glutamine appears to exceed the individual's ability to produce enough of it. Glutamine is non-essential right up until demand outruns supply, and at that point it behaves like an essential nutrient.

That is the crux of the whole supplement question. "Conditionally essential" is not a general upgrade in status, it is a claim about a condition. The useful question is never "is glutamine conditionally essential", it is "am I in the condition". Lacey and Wilmore were writing about the critically ill, and the application they proposed was supplemental glutamine in specialised enteral or parenteral feeding for seriously ill patients. That is a long way from a scoop in a smoothie.

The three jobs glutamine does

First, it is a fuel. Most cells run primarily on glucose. Rapidly dividing cells often do not. Enterocytes, the cells lining the small intestine, and lymphocytes, the white blood cells that proliferate during an immune response, both use glutamine heavily as a respiratory substrate. Cruzat and colleagues describe glutamine as quantitatively more relevant than glucose as an energy substrate for the gut, with roughly 75 per cent of it metabolised through the Krebs cycle. They also note that during infection or high catabolism, immune cells consume glutamine at a rate similar to or greater than glucose. What that fuel relationship does and does not mean for the gut barrier is covered in L-glutamine and gut health.

Second, it moves nitrogen around. Glutamine carries two amino groups rather than one, which makes it an efficient vehicle for shuttling nitrogen and ammonia between tissues. Unglamorous, and a large part of why muscle exports so much of it and the liver and kidneys take up so much.

Third, it is a building block. Glutamine donates carbon and nitrogen to the synthesis of purines, pyrimidines and amino sugars, the raw material for DNA and RNA, which matters most in tissues that are constantly replacing themselves. The glutamate derived from glutamine is also a substrate for glutathione, the main intracellular antioxidant. Lacey and Wilmore made the same list thirty five years ago: a preferred respiratory fuel for rapidly proliferating cells, a carrier of nitrogen between tissues, and a precursor of nucleic acids and nucleotides.

Where your body gets glutamine from

Two sources, and the second is much larger than most people assume.

Food supplies the first. In Glutamine and glutamate: Nonessential or essential amino acids?, Watford notes that glutamine and its close relative glutamate together make up somewhere between 5 and 15 per cent of dietary protein, and that they are the most abundant amino acids in milk. Meat, dairy, eggs, fish, legumes and wheat protein all contribute, so an adult eating a normal mixed diet with adequate protein takes in several grams a day without thinking about it.

Endogenous synthesis supplies the rest, and it dwarfs the dietary contribution. Cruzat and colleagues estimate endogenous production at 40 to 80 g per day. Watford puts plasma glutamine turnover at 60 to 80 g per day in healthy subjects.

That figure should anchor your expectations. A 3 g serve is a real amount of an amino acid, but it is a small addition to a pool that turns over dozens of grams daily and is actively regulated. That does not make it pointless. It does make dramatic claims implausible. The doses researchers have actually used, and what happens to one after you swallow it, are set out in L-glutamine dosing: how much, and when.

What happens under severe stress, and why the evidence lives there

The reason glutamine became a research subject at all is what happens to it in catabolic states: major burns, major surgery, serious trauma, sepsis and critical illness. In those situations, the system that normally keeps glutamine abundant stops keeping up.

In healthy adults, plasma glutamine sits at roughly 500 to 800 micromoles per litre after a 12 hour fast. In catabolic conditions it falls to 300 to 400 micromoles per litre, and in critically ill patients skeletal muscle glutamine concentration drops by around 80 per cent on average.

The mechanism is well described. In Glutamine metabolism in sepsis and infection, published in The Journal of Nutrition in 2001, Karinch and colleagues describe skeletal muscle roughly doubling its glutamine release during severe infection while synthesis also rises, and the muscle pool still becoming depleted because release outpaces production. Under those conditions, the authors write, glutamine availability can become rate limiting for functions such as phagocytosis and antibody production.

So the physiology is real. What is more interesting is how cautious clinical practice has become anyway. The ESPEN guideline on clinical nutrition in the intensive care unit recommends additional enteral glutamine at 0.3 to 0.5 g per kg per day for 10 to 15 days in patients with burns over 20 per cent of body surface area, and 0.2 to 0.3 g per kg per day for the first five days in critically ill trauma patients. For every other ICU patient it recommends that additional enteral glutamine should not be given, and in unstable and complex ICU patients, particularly those with liver or renal failure, that parenteral glutamine dipeptide shall not be administered.

That is a useful corrective. Even in the population where glutamine depletion is most severe and most measurable, the guidance is narrow, dose specific, time limited and explicitly restrictive outside two patient groups. An ingredient whose clinical case is drawn that tightly does not have a broad general mandate.

Healthy adults: what is plausible and what is thin

Very little of the clinical glutamine literature was generated in healthy, well fed people, and the healthy population is where most pouches are sold. Extrapolating from a burns unit to a gym bag is not a scientific argument, it is a hopeful one.

The most direct assessment of the athlete case is Michael Gleeson's 2008 review Dosing and Efficacy of Glutamine Supplementation in Human Exercise and Sport Training in The Journal of Nutrition. Gleeson accepts the starting premise, that prolonged exercise and heavy training blocks do lower plasma glutamine. He then reports that in feeding intervention studies, keeping plasma glutamine constant during and after prolonged strenuous exercise did not prevent the post exercise changes in several aspects of immune function, because although glutamine is essential for lymphocyte proliferation, the post exercise fall is not large enough to compromise it. His conclusion is blunt: the commonly suggested reasons for taking glutamine have received little support from well controlled scientific studies in healthy, well nourished humans.

More recent work has not overturned that. In Glutamine metabolism and optimal immune and CNS function, published in Proceedings of the Nutrition Society in 2023, Newsholme and colleagues describe glutamine as possibly the most widely recognised immunonutrient, with roles in the TCA cycle, heat shock protein responses and antioxidant systems. They also say plainly that the many studies testing glutamine replacement have produced results in poor agreement. Cruzat and colleagues land in the same place, noting that evidence for positive outcomes in living humans largely remains to be presented.

So the plausible case is narrower than the category implies. The mechanism is genuine and well characterised. The situations where measured depletion is large enough to matter are clinical. For a healthy adult eating enough protein, supplemental glutamine is going into a system already producing plenty of it, and the burden of proof for a specific benefit has not been met in that population.

The bottom line

L-glutamine is the most abundant free amino acid in the body, a preferred fuel for the cells lining the intestine and for proliferating immune cells, a carrier of nitrogen between tissues, and a precursor for glutathione and nucleotides. None of that is in dispute. It is textbook physiology, not a marketing position.

What is in dispute is whether adding a few grams a day changes anything in a person who is healthy and eating enough protein. The body makes 40 to 80 g of it daily. The clinical evidence sits in burns, trauma, surgery and critical illness, where the deficit is severe and measurable, and even there the guidance is deliberately narrow. In healthy, well nourished people, the well controlled trials have not supported the popular rationales.

That is not a reason nobody should take it. It is a reason to buy it with accurate expectations and to care about what is actually in the pouch, which is where a single ingredient product has an advantage over a blend with an undisclosed amount of glutamine in it. If you are weighing it against the other recovery products on the same shelf, glutamine vs BCAAs vs protein sets out what each one actually supplies. If you have decided glutamine is something you want to include, Ritua's L-Glutamine is 100 per cent pure micronised L-glutamine, third party lab tested, with a 3 g serve and no fillers, flavours or sweeteners. And if you are managing a health condition or recovering from surgery or injury, that is a conversation for your doctor or dietitian, not for a product page.

This article is general information, not medical advice, and is not a substitute for guidance from your doctor, particularly if you are managing an existing health condition or taking medication.

Jasmine Meagher, founder of Ritua

Jasmine Meagher

BSc Food Science and Nutrition · Founder, Ritua

Jasmine founded Ritua to bring evidence-based, TGA-compliant supplements to Australian women. She writes about the research behind every ingredient Ritua sells.

Common questions

Is L-glutamine an essential amino acid?

No. It is classified as non-essential, because the body synthesises it, mostly in skeletal muscle. It is often called "conditionally essential", a term Lacey and Wilmore proposed in 1990 for situations where demand exceeds the body's ability to produce enough. The condition they described was critical illness, not everyday life.

How much glutamine do you get from food?

Glutamine and glutamate together account for somewhere between 5 and 15 per cent of dietary protein, according to Watford's review, and they are the most abundant amino acids in milk. Meat, fish, eggs, dairy, legumes and wheat protein are all meaningful sources, so a mixed diet with adequate protein supplies several grams a day.

How much glutamine does the body make on its own?

Estimates of endogenous production sit at roughly 40 to 80 g per day, and plasma turnover in healthy people has been put at 60 to 80 g per day. Skeletal muscle does most of the synthesising and holds around 80 per cent of the body's free glutamine.

Why is so much of the glutamine research done in hospital patients?

Because that is where depletion is large enough to measure. In critical illness, skeletal muscle glutamine concentration can fall by around 80 per cent on average and plasma glutamine drops from roughly 500 to 800 micromoles per litre to 300 to 400. That is a very different situation to a healthy adult eating enough protein, which is why findings should not be transferred between the two.

Does hard training lower your glutamine levels?

Prolonged exercise and heavy training periods are associated with a fall in plasma glutamine, which Gleeson accepts in his 2008 review. The intervention studies he summarises found that holding plasma glutamine constant with supplementation did not prevent the post exercise changes in several aspects of immune function, and that the fall is not large enough to limit lymphocyte proliferation.

Is glutamine the same thing as glutamate?

Closely related, but not identical. Glutamine carries an extra amino group, which is what lets it act as a nitrogen carrier between tissues, and it can be converted to glutamate inside cells. That glutamate is then a substrate for glutathione synthesis. Nutritionally the two are usually discussed together, because both are abundant in dietary protein.

Sources

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