L-Glutamine for Gut Health: What the Trials Tested

L-glutamine is sold as the supplement for the gut lining. Plain powders promise to support it, and blends built around glutamine promise to repair it, restore it and tighten its junctions. The claim sounds like settled science. It rests almost entirely on one clinical trial in a narrow group of patients, on laboratory work, and on results borrowed from hospital medicine.

That one trial is worth understanding in detail, because its result was striking and nobody has repeated it. So is the gap between who was studied (adults with frequent, watery diarrhea that began after a gut infection, selected with a research test most people cannot get) and who buys the powder.

Why glutamine is sold for the gut

The pitch has a tidy logic. Glutamine is the most abundant amino acid in the blood. Cells lining the intestine burn it as fuel. In laboratory dishes, intestinal cells starved of it let the seals between them loosen. So more glutamine, the argument goes, should mean a better fed and tighter gut lining. On the product pages we read, plain powders mostly speak of supporting gut health or a healthy gut lining, although one also says it improves gut permeability. At least one blend named for leaky gut goes further and says its glutamine rebuilds intestinal cells and strengthens the junctions between them.

Three different kinds of evidence get blended together in that pitch: laboratory and animal work on what gut cells do with glutamine, trials in hospital patients with burns, major surgery or chemotherapy, and one trial in IBS. Each answers a different question. None of them asks whether a daily scoop does anything for a healthy adult with ordinary digestive complaints, and we found no trial that did.

What glutamine is, and what the gut fuel claim rests on

Glutamine is an amino acid that the body makes for itself, mostly in muscle. One review estimates production at 40 to 80 g a day, and an average diet adds about 7 g more (a mean of 6.84 g a day in a study of more than 70,000 US women). It is often called conditionally essential, a term that comes from a 1990 proposal about critically ill patients, not from any finding in healthy people. We found no study documenting a glutamine deficiency in healthy, fed adults.

Intestinal cells do use glutamine as a fuel. How much is known mainly from animals: in fed piglets, glutamine from the blood accounted for 19% of the carbon dioxide the gut produced (a measure of the fuel it burned), while glutamate from food, a closely related amino acid, accounted for 36%. In healthy adults, one study that tracked labeled glutamine delivered into the stomach by tube found that the gut and liver together removed more than half of it before it reached the rest of the body (54% in 14 fasted people). Specific figures, such as glutamine supplying 60 to 70% of the gut's energy, do not trace to any human study we could find.

The tight junction evidence is also from the laboratory. When a human colon cancer cell line was deprived of glutamine and its own glutamine-making enzyme was blocked, the cells lost junction proteins. The same research group had earlier reported that the barrier was no more sensitive to a lack of glutamine than to the cells' overall protein levels, and suggested that the gut's dependence on glutamine probably involves roles other than the barrier itself. In one study, colon biopsies from 12 people with diarrhea-predominant IBS made more of one junction protein when bathed in glutamine at more than ten times the level found in blood. These experiments show that cells need some glutamine. They do not show that adding more to a normally supplied gut tightens anything, and a swallowed dose of about 5 g raised blood glutamine for only about two hours in one small study.

The evidence at a glance

These summaries describe research, most of it in people with a diagnosed condition. They are not claims about what a supplement does. A confidence interval that includes zero, or a P value at or above .05, means the difference was not statistically significant.

The one IBS trial behind the claim

What was done. The trial was published in the journal Gut in 2019 by a US research group and was funded by the National Institutes of Health and the Department of Veterans Affairs. The researchers screened 287 adults with diarrhea-predominant IBS that had started after a gut infection. Each took a lactulose and mannitol urine test, a research measure of intestinal permeability, meaning how easily substances pass through the gut lining. The person drinks a solution of the two sugars, and the amounts that reach the urine over the next 24 hours are compared. Of those screened, 145 had a normal result and were excluded. In the end 115 people were randomized to L-glutamine powder, 5 g three times a day, or to the same amount of whey protein powder, for eight weeks.

Who was studied. These were not people with mild digestive discomfort. They averaged more than five loose, watery stools a day, and their average age was about 32. People with liver disease, raised liver enzymes, reduced kidney function or celiac disease were excluded, as were pregnant and breastfeeding women.

What was found. Among those who finished, 43 of 54 on glutamine (79.6%) improved by at least 50 points on a 500-point symptom severity scale, compared with 3 of 52 (5.8%) on whey protein. Stools fell from an average of 5.4 a day to 2.9 on glutamine and were unchanged on whey (5.3 before and after). The permeability test reading fell as well, from an average ratio of 0.11 to 0.05 on glutamine, against 0.11 to 0.10 on whey (the trial's cutoff for a raised result was 0.07). Side effects were reported by 3.8% of each group.

Why one trial is not enough. Several features of this result call for caution, and none of them is a judgment on the researchers.

• It has not been repeated. As of October 12, 2026, we found no completed trial that repeats it in PubMed or on ClinicalTrials.gov (the one other completed trial of glutamine alone in IBS that we found, described below, asked a different question). An independent French trial of the same dose is recruiting, with primary completion estimated for March 2029. The same sponsor is also recruiting for a trial of 10 g three times a day in adults with obesity, insulin resistance and IBS. Its main outcome is insulin resistance, an IBS symptom score is a secondary outcome, and no results had been posted when we checked.
• The comparison group hardly moved. Only 5.8% of the whey group improved. Across 73 IBS drug trials, a 2021 meta-analysis put the average placebo response at 27.3% for overall improvement. In a 2021 Boston trial that used the same scale and the same 50-point threshold in a different group of IBS patients over six weeks, 70.4% improved on placebo pills and 54.2% improved with no pill at all. A comparison group that flat is unusual, and it makes the gap look larger.
• Only people who finished were analyzed. That was 106 of the 115 randomized. Counting the nine who dropped out as not improved would not change the picture much (43 of 59 against 3 of 56), so this does not explain the result.
• The comparator was whey protein, a dairy protein and not an inert placebo.
• The group was narrow. Half of the people screened did not have a raised permeability test and were never studied. The authors say their findings may not apply to IBS with a normal test, to constipation-predominant IBS, to children or to pregnant women.
• The registration changed. From 2011 until at least March 2015, the trial's public record on ClinicalTrials.gov described glutamine 10 g three times a day in 100 people with diarrhea-predominant IBS and a raised permeability test, with no requirement that the IBS began after an infection. The 5 g dose and the post-infection group first appear in the version submitted with the results in August 2017, after the trial had ended. In a 2015 letter, two of the investigators described preliminary results from a trial in which 61 patients were randomized to glutamine or placebo, each at 10 g three times a day. The 2019 paper, which we read as the author manuscript on PubMed Central, does not say how these fit together. The yardstick was not new: the first registered version, in 2011, already named an improvement of 50 points or more on the symptom scale as the primary outcome measure.
• There was no follow-up after week eight, so nothing is known about what happened next.

A letter published in the same journal raised some of the same concerns, including how narrowly the participants were selected and that the analysis left out people who dropped out. None of this shows the result is wrong. The trial was randomized, double-blind and publicly funded, and its authors called for larger trials themselves. It does mean that 79.6% against 5.8% should be read as one unrepeated finding in a narrow group, not as an established effect. IBS needs a clinician's diagnosis, and persistent diarrhea needs a medical evaluation before a supplement. People with liver or kidney disease, anyone seriously ill or hospitalized, people in cancer treatment, and anyone pregnant or breastfeeding should ask a clinician before taking glutamine.

The second trial. The only other randomized trial of glutamine in IBS that we found was run in Tehran and published in 2021. Fifty adults with any type of IBS all followed a dietitian-led low FODMAP diet, which limits certain fermentable carbohydrates, and were randomized to glutamine 15 g a day or whey protein 15 g a day for six weeks. The total symptom score fell further with glutamine (from 308 to 128 points, against 279 to 152). Stool frequency, stool consistency, pain intensity, bloating and quality of life were not significantly different between the groups. The headline responder figure it reports, an improvement of more than 45% in 22 of 25 people against 15 of 25, uses a threshold that its methods section does not define. It asked a different question from the first trial and is not a repeat of it.

What the guidelines say. Nothing. The American College of Gastroenterology, British Society of Gastroenterology and American Gastroenterological Association IBS guidelines that we checked do not mention glutamine. A 2024 review by US gastroenterologists concluded that more data are needed before any nutritional supplement, glutamine included, can be recommended in practice. For the supplements that guidelines do discuss, see our reviews of peppermint oil for IBS and probiotic strains for IBS.

Leaky gut: what can be measured, and what the glutamine trials found

Two different things share one name. Increased intestinal permeability is a real, measurable finding in specific settings: celiac disease, Crohn's disease, injury from anti-inflammatory painkillers, heavy alcohol use, chemotherapy, severe burns, hard exercise in the heat, and some people with IBS. Leaky gut syndrome is the idea that a leaky gut is itself a disease lying behind fatigue, skin problems, mood symptoms and autoimmune conditions. That is not an accepted medical diagnosis, and in the words of a 2024 review, "no validated test currently exists to make this diagnosis." Our guide to supplements for leaky gut covers that debate in more detail.

Even where permeability really is raised, lowering it has not been shown to make people better. A 2019 review in Gut put it this way: "It is still unproven that restoring barrier function can ameliorate clinical manifestations." European food regulators made the same point when they assessed glutamine claims in 2009 and 2011. They concluded that a cause and effect relationship had not been established for the intestinal lining or for gut permeability, and noted that a lower permeability reading is not a health benefit in itself unless it is linked to an outcome.

Testing is the practical obstacle. The sugar test used in the IBS trial, in which a person drinks two sugars and has them measured in urine, is a research tool that has not become part of routine care. The blood and stool tests for zonulin, a protein promoted as a marker of a leaky gut, are not a substitute. Two independent laboratory studies found that widely used commercial blood test kits did not reliably detect zonulin, and the 2024 review quoted above describes the stool tests sold online as unregulated, with minimal evidence to support their use. So a reader has no reliable way to find out whether they resemble the people in the trial. Published studies have found raised permeability in roughly 37 to 62% of people with diarrhea-predominant IBS, which leaves a large share without it.

The pooled analysis. A 2024 meta-analysis pooled 10 to 11 trials (the paper gives both numbers) that measured permeability in people given glutamine. All were in patients: people with severe burns, sepsis, pancreatitis, major surgery, cancer treatment, HIV or Crohn's disease. None was in healthy adults, and the IBS trial was not included. Overall there was no change: the pooled difference between the glutamine and comparison groups (a weighted mean difference) was -0.00, with a 95% confidence interval of -0.04 to 0.03, and the trials disagreed with each other widely. Some burn, surgery and chemotherapy trials reported lower readings. Trials in sepsis, pancreatitis and Crohn's disease did not.

That paper's abstract also reports a benefit at high doses taken for under two weeks, and that line has been repeated as a dose threshold of 30 g a day. It should not be. The paper's own table prints that subgroup with numbers that cannot both be right (the estimate falls outside its own confidence interval), and the dose and duration subgroups were analyzed separately, not together. Only the overall result is safe to rely on.

Hard exercise in the heat

Long, hard exercise in hot conditions temporarily raises intestinal permeability, and this is where glutamine has its other positive findings. Three small crossover studies, in 8, 7 and 10 people, reported a smaller rise in a permeability marker when volunteers took glutamine before running for an hour at 30 °C (86 °F). The doses were large: 0.25 to 0.9 g per kilogram of fat-free body mass, which works out to roughly 15 to 54 g for a 70 kg (154 lb) person with 15% body fat, taken as a single dose or, in one study, as a daily amount for a week.

Two later studies from another laboratory, each in 10 men, tested a dose near the low end of that range, about 20 g, before exercise in the heat. Neither found protection, and in one the permeability marker was higher after glutamine than after placebo (0.043 against 0.034, P = 0.02). None of these studies reported fewer gut symptoms. In a 2026 trial, 19 collegiate cross-country skiers took 10 g a day for four weeks with no significant effect on gut symptoms, stool form or a stool marker of inflammation. Large single doses also have a cost: in a tolerance study of 14 healthy men, gut symptoms were greater at about 59 g than at lower doses.

Crohn's disease, hospital care and two prescription products

Crohn's disease. A 2016 Cochrane review found two small trials with 42 patients in total. In the one with remission data, a trial in 18 children with active Crohn's disease, remission occurred in 4 of 9 on a glutamine-enriched diet and 5 of 9 on a standard diet, and a disease activity score improved significantly more on the standard diet (P = 0.002). The reviewers judged the evidence insufficient and of low quality. In a trial of 30 people whose Crohn's disease was in remission, a permeability test improved about as much on whey protein as on glutamine. In another, 14 patients took 21 g a day or a comparison amino acid for four weeks, with no significant change in a permeability test.

Hospital care. Glutamine has been studied most in critically ill, burned and surgical patients, where it is given as medical nutrition by feeding tube or vein. The largest trials were disappointing. In a trial of 1,223 intensive care patients on ventilators with failing organs, randomized to very high doses of glutamine by vein and feeding tube together, antioxidants, both or placebo, 28-day mortality was 32.4% with glutamine and 27.2% without (P = 0.05, just short of the trial's threshold for significance), and deaths in the hospital and at six months were significantly higher with glutamine. In a trial that analyzed 1,200 patients with severe burns, glutamine by feeding tube or mouth made no significant difference to how soon patients left the hospital alive (a median of 40 days against 38). The intensive care trial's authors warn against applying their result to other settings, and it says nothing direct about a healthy adult's scoop. It is the reason seriously ill people should not self-supplement.

Prescription products. The FDA has approved two brand-name prescription L-glutamine powders, and generic versions have followed. None is for IBS or the gut lining. One brand, approved in 2017, is for reducing acute complications of sickle cell disease at 10 to 30 g a day depending on body weight. European regulators assessed the same product for that use and recommended refusing it, judging that the main study had not shown effectiveness, and the company withdrew its application in 2019. The other, approved in 2004, was for short bowel syndrome in people dependent on specialized nutritional support, at 30 g a day and only together with growth hormone and a specialized diet. In its trial of 41 adults, the group given glutamine without growth hormone improved least, and the FDA now lists the product as discontinued. Those are specialist uses in serious disease. They are not evidence about a supplement, and a supplement is not a substitute for a prescription.

Dose: what the trials used and what a scoop holds

• The IBS trials: 5 g three times a day, 15 g a day in total, for six to eight weeks.
• The exercise studies: roughly 15 to 54 g, as a single dose before exercise or, in one study, as a daily amount for a week.
• Prescription labels, for other conditions: 10 to 30 g a day.
• A typical supplement label: 5 g per scoop, once or twice a day.
• Capsules: most often 500 mg each, so 15 g is 30 capsules.

A 5 g label serving matches one of the three daily doses in the IBS trials. Taken once or twice a day, as most labels suggest, it comes to 5 to 10 g a day, a third to two thirds of the trials' 15 g. That does not mean a scoop is too little or that three scoops is right: we found no trial of either amount in healthy adults with ordinary digestive complaints.

The label for the prescription powder used in sickle cell disease, written for that product and its patients, gives some handling detail. It says food made no significant difference to blood levels, so the powder can be taken with or without food (the short bowel product's label says with meals or snacks). It tells patients to mix each dose into a cold or room-temperature drink or food, not a hot one, and to take it right away, and it says the powder need not dissolve completely. Those are a prescription product's directions, not directions for a supplement. No trial we found compared taking it on an empty stomach with taking it at meals for any gut outcome.

Dipeptide forms such as L-alanyl-L-glutamine (sold as Sustamine), in which glutamine is bonded to another amino acid, were developed for hospital nutrition solutions because free glutamine is unstable under heat sterilization and has limited solubility, according to a 2001 review. Glutamine peptides (PepForm) are another branded form. In one study of 8 men, L-alanyl-L-glutamine raised blood glutamine more than the same amount of free glutamine did. No trial we found compared these forms on a gut outcome, and both IBS trials used plain L-glutamine powder. As for food, ordinary protein foods supply glutamine, and we found no published measurement of how much is in bone broth. These are descriptions of what studies and labels used, not dosing advice. People with liver or kidney disease, anyone seriously ill or hospitalized, people in cancer treatment, and anyone pregnant or breastfeeding should ask a clinician before taking glutamine at any dose, and persistent gut symptoms need a diagnosis before a supplement.

Side effects and who should ask a clinician first

In the main IBS trial, side effects were reported by 3.8% of each group: one person with abdominal pain and one with bloating on each side. That was eight weeks, in adults screened to exclude liver and kidney disease. The larger body of safety data comes from the prescription powder for sickle cell disease, taken at up to 30 g a day for 48 weeks. There, constipation was reported by 21% on glutamine and 18% on placebo, nausea by 19% and 14%, headache by 18% and 15%, and abdominal pain by 17% and 16%.

There is no official upper limit. An industry-authored risk assessment judged 14 g a day to be well documented as safe in healthy adults and said the data above that level were not enough for confidence about long-term use. The IBS trial dose sits just above that figure. The longest controlled exposures are 8 weeks in IBS, 16 weeks on the short bowel label and 48 weeks in sickle cell disease, and we found no long-term study in healthy adults.

Single case reports describe liver injury in a supplement user and kidney injury in a person taking 18 g a day. Case reports cannot show cause, but they are a reason not to treat an amino acid as automatically harmless at any dose.

Frequently asked questions

Does L-glutamine help IBS?

The evidence is one striking trial that has not been repeated. In adults whose diarrhea-predominant IBS began after a gut infection, and who also had a raised intestinal permeability test, 43 of 54 people who took 5 g three times a day for eight weeks improved by at least 50 points on a 500-point symptom scale, compared with 3 of 52 on a whey protein comparator. Only people who finished were analyzed, the comparison group barely changed, and the trial's registration listed a different dose and a broader group until the results were posted. A second, 50-person trial that added glutamine to a low FODMAP diet found a lower total symptom score but no significant difference in stool frequency, pain intensity or bloating. The IBS guidelines we checked do not mention glutamine. IBS needs a clinician's diagnosis.

Does L-glutamine heal leaky gut?

No trial we found has tested that in healthy adults with ordinary digestive complaints. Increased intestinal permeability is a measurable finding in specific conditions, but leaky gut syndrome is not an accepted medical diagnosis and no validated test for it exists. A 2024 pooled analysis of trials in hospital and disease populations found no overall change in permeability tests with glutamine. European food regulators reviewed the claim in 2009 and 2011 and concluded that a cause and effect relationship had not been established. Persistent gut symptoms need a diagnosis, not a supplement.

How much L-glutamine did the gut studies use?

The two IBS trials used 5 g three times a day, 15 g a day in total, for six to eight weeks. Exercise studies used roughly 15 to 54 g, as a single dose or, in one study, daily for a week. Most supplement labels suggest 5 g once or twice a day, a third to two thirds of the IBS trials' daily amount, and 15 g is about 30 capsules of 500 mg. We found no trial of either amount in healthy adults with ordinary digestive complaints. One trial of a label amount, 5 g twice a day for four weeks in 19 collegiate skiers, found no significant effect on gut symptoms. This describes what studies and labels used, not a dose recommendation. People with liver or kidney disease, anyone seriously ill or hospitalized, people in cancer treatment, and anyone pregnant or breastfeeding should not take glutamine without their clinician. Persistent gut symptoms need a diagnosis before a supplement.

How long does L-glutamine take to work?

No study we found answers this. The main IBS trial measured its results once, at eight weeks, with no follow-up afterward, and the diet trial measured at six weeks. In one small study, a swallowed dose of about 5 g raised blood glutamine for about two hours. We found no study that shows a timeline for healthy adults.

Should you take L-glutamine on an empty stomach or with food?

No trial we found compared timing on a gut outcome. The prescribing information for the prescription L-glutamine powder used in sickle cell disease says food made no significant difference to blood levels, so it can be taken with or without food. The same label says to mix the powder in a cold or room-temperature drink, not a hot one, and to take it right away.

What are the side effects of L-glutamine?

In the main IBS trial, side effects were reported by 3.8% of people in each group: one case each of abdominal pain and bloating. In the trials of the prescription powder for sickle cell disease, at up to 30 g a day, constipation was reported by 21% on glutamine and 18% on placebo, nausea by 19% and 14%, and abdominal pain by 17% and 16%. Single doses near 60 g caused more nausea and belching in healthy men. Single case reports describe liver injury and kidney injury in supplement users, with causation unproven. People with liver or kidney disease, anyone seriously ill or hospitalized, people in cancer treatment, and anyone pregnant or breastfeeding should not take it without their clinician. Persistent gut symptoms need a diagnosis before a supplement.

Is it safe to take L-glutamine every day?

Short term, it was tolerated in trials of screened adults. Long term is unknown. The longest controlled exposures are 8 weeks in IBS, 16 weeks on the short bowel prescription label, and 48 weeks in sickle cell disease. We found no long-term study in healthy adults. An industry-authored risk assessment judged 14 g a day to be well documented as safe in healthy adults and said the data above that were not enough for confidence about long-term use. There is no official upper limit. Liver disease, kidney disease, serious illness, cancer treatment, pregnancy and breastfeeding are reasons to ask a clinician first. Persistent gut symptoms need a diagnosis before a supplement.

Who should not take L-glutamine without a clinician?

People with liver disease or cirrhosis, because single 10 to 20 g doses raised blood ammonia in people with cirrhosis. People with kidney disease. Anyone who is critically ill or hospitalized, because a large intensive care trial of very high doses found higher in-hospital and six-month mortality with glutamine. People in cancer treatment, who should ask their oncology team. Pregnant or breastfeeding women, for whom the prescription labels report no data. Children, because we found no trial for gut complaints in otherwise healthy children. People with seizure disorders or bipolar disorder, where the concern is theoretical. And anyone prescribed L-glutamine for sickle cell disease should not swap it for a supplement. Blood in the stool, black stools, unintended weight loss or diarrhea that is mainly at night need a medical evaluation first.

The bottom line

L-glutamine's reputation as a gut supplement rests on one striking trial. In adults whose diarrhea-predominant IBS began after an infection, and who also had a raised permeability test, far more people improved on 5 g three times a day for eight weeks than on a whey protein comparator. That result has never been repeated, the comparison group barely changed, and the trial's registration listed a different dose and a broader group until the results were posted. Elsewhere the evidence is thin: no trial that we found in healthy adults with ordinary digestive complaints, no overall change in a pooled analysis of permeability tests, no significant difference in remission or permeability tests in small Crohn's disease trials, and exercise studies that disagree. Label directions come to a third to two thirds of the 15 g a day used in the IBS trial, and we found no trial of either amount in healthy adults with ordinary digestive complaints. People with liver or kidney disease, or who are seriously ill, pregnant, breastfeeding or in cancer treatment, should ask a clinician first, and persistent gut symptoms need a diagnosis before a supplement. For other options people ask about, see our IBS supplement hub.