Increased intestinal permeability means the lining of your gut is letting more through than it should. The change itself is real and measurable, and research has identified specific pressures that produce it. Anti-inflammatory painkillers, heavy drinking, gut infections, an imbalance in gut bacteria, some features of modern diets, psychological stress and very long, hard exercise all have human evidence behind them. That is a separate question from leaky gut, its symptoms and what may help, because a measurable change in the gut wall is not the same thing as a diagnosis. This guide covers how the barrier normally works, what each of those pressures does to it, and where the evidence is still unsettled.
How does the gut barrier control what gets through?
A single layer of cells lines the intestine, coated by mucus on the inside and backed by immune defences underneath. Between neighbouring cells sit tight junctions, protein seals that work less like glue and more like adjustable gates. They loosen a little to let water and nutrients pass and close up to keep bacteria and larger food fragments in the gut, where they belong. A 2019 review in the journal Gut set out how permeability rises when those gates are held open too wide, when lining cells are damaged or die faster than they are replaced, or when material is carried through the cells rather than between them.
A 2025 review reached a similar map of the territory. The routes to a more permeable gut run through signals that open the tight junctions, injury to the lining cells, and changes in the gut bacteria that support the barrier from the inside. Every cause on this page works through one or more of those routes.
What is zonulin?
Zonulin is a protein made in the gut that tells tight junctions to open. A 2011 review by the researcher who identified it describes it as the only signal so far shown to regulate the junctions as part of normal physiology, and proposes that an overactive zonulin pathway in people with a genetic susceptibility may contribute to autoimmune and inflammatory conditions. That second part is a hypothesis under investigation, not a settled finding, and much of the surrounding research remains contested.
The protein's fame has outrun its measurement. Blood tests sold as zonulin tests rely heavily on one commercial laboratory kit, and when German researchers examined that kit in 2018, it did not recognise zonulin at all. It reacted instead to other proteins with a similar structure. A zonulin result from such a kit is measuring something, but not the protein it is named after, which is a reason to treat both the tests and studies built on them with caution.
How do anti-inflammatory painkillers increase permeability?
NSAIDs, the family of painkillers that includes ibuprofen, naproxen and aspirin, are the best documented everyday cause of a more permeable small intestine. A 2018 review in Gastroenterology set out a double mechanism. On direct contact the drugs work into the fatty surface of the lining cells and disturb the way those cells produce energy, weakening them from within. Once absorbed, they also block the protective prostaglandins that maintain blood flow and mucus. The result is a barrier that lets more through, followed by low-grade inflammation along the small intestine.
The rise is measurable after a single dose, on a scale researchers use as a benchmark for other causes. What this means in practice, which painkillers are hardest on the gut and how to take one more safely, is covered in our guide to how NSAIDs affect the stomach and gut. If an anti-inflammatory has been prescribed for you, keep taking it as prescribed and raise any concerns with your pharmacist or GP rather than changing anything yourself.
Does alcohol make the gut more permeable?
Alcohol reaches the gut lining at full strength before the body dilutes it, and the barrier registers the hit quickly. In a study of healthy adults, a single session of heavy drinking was followed within hours by a rise in blood levels of endotoxin, a fragment shed by gut bacteria that has no business being in the bloodstream. Bacterial DNA rose alongside it, and levels ran higher in women than in men. Material was crossing a barrier that normally holds it back, in healthy people, from one binge.
Drinking heavily and often turns that short-lived breach into a standing one. A 2023 review lists alcohol among the exposures consistently linked with a weakened barrier, both through direct damage to the lining and through the changes it makes to gut bacteria. The barrier recovers when the pressure comes off, which is one more reason the NHS advice to keep within 14 units a week earns its place.
Can a gut infection leave the barrier more permeable?
A bout of gastroenteritis inflames and injures the gut lining while it lasts. The longer-term question got an unusual answer from a Canadian town. In 2000, the water supply in Walkerton, Ontario was contaminated with bacteria and thousands of residents became ill. Two years on, researchers compared 132 local people who had developed IBS with 86 who had not, using a drink of two test sugars whose appearance in urine measures how much slips through the small intestine.
The people with IBS let measurably more through. The raised permeability tracked the IBS symptoms themselves rather than whether someone had been ill in the outbreak, so a leakier lining looks like part of how IBS works, in the gut of people whose symptoms began after an infection and in others too. It also shows the change persisting years after any original trigger, which is what separates infection from the short-lived causes on this page.
How does an imbalance in gut bacteria weaken the barrier?
The bacteria in your colon feed the wall that contains them. Fibre-fermenting species produce short-chain fatty acids, compounds made when bacteria break down fibre, and one of them, butyrate, is the main fuel for the cells lining the colon. The helpful species also crowd out harmful ones and support the mucus layer above the cells. The same 2023 review describes what happens when that community shifts. Fewer of the species that feed the lining, more of the species that shed endotoxin, and a barrier weakened from the inside, which is why dysbiosis, an imbalance in your gut bacteria, appears in almost every account of increased permeability. The signs of an imbalance and what drives one, from antibiotics to diet, are covered in our guide to gut imbalance symptoms and causes.
No home test measures intestinal permeability itself, and no test diagnoses leaky gut. What can be measured from a home stool sample is the state of your gut bacteria, one of the few factors on this page you can actually see. A gut microbiome test for leaky gut shows the balance and variety of species present, including the fibre-fermenting groups that feed the gut lining.
Which diet patterns are linked with higher permeability?
Single foods matter less than patterns. A diet low in fibre leaves the fibre-fermenting bacteria underfed, with the knock-on effects on the barrier described above the strongest dietary lever researchers point to. The 2019 review in Gut also names surfactants, molecules that loosen the fatty surfaces of cells, among the pressures that raise permeability. Bile acids released to digest fatty meals act this way, and so do some emulsifiers added to processed foods to stop ingredients separating.
Emulsifiers have now been tested directly in people. In a controlled feeding study, 16 healthy adults ate identical diets for 11 days with or without a common emulsifier called carboxymethylcellulose. The emulsifier group ended with a narrower mix of gut bacteria and lower levels of short-chain fatty acids, and in two of the seven who ate it, bacteria had moved into the inner mucus layer that normally stays clear of them. This is a small, short study. It shows the mechanism operating in people, and it cannot say how large the risk from everyday amounts is.
Can stress physically open the gut barrier?
It can, and the experiment that showed it used nothing more than a speech. Belgian researchers asked 23 healthy volunteers to speak in public while their small-intestinal permeability was tracked with a sugar test. Permeability rose within two hours, and only in the volunteers whose cortisol, the main stress hormone, had also risen. Injecting the hormone that triggers the stress response reproduced the effect without any speech, and a medicine that stabilises mast cells, immune cells stationed in the gut wall, blocked it. The pathway runs from stress hormone to immune cell to tight junction.
For scale, the researchers also gave volunteers a dose of the anti-inflammatory painkiller indomethacin, which more than doubled the same measurement. One stressful hour moved the barrier a real but smaller amount. What repeated, unrelieved stress does over months is harder to measure and remains an open question, though the short-term mechanism is now established.
Does hard exercise affect the gut barrier?
Moderate exercise sits on the helpful side of the ledger for gut health. The barrier problem appears at the extremes. A systematic review of exercise and the gut found that markers of gut injury, permeability and bacterial fragments in the blood rise consistently once exercise passes roughly two hours at a hard, sustained intensity, regardless of how fit the person is. Heat pushes the changes further, and running produces more disturbance than cycling. Gut symptoms are a recognised limiter in marathon and ultra-distance events for exactly this reason.
The same review found these changes reverse with recovery. A long run opens the barrier temporarily and it closes again, so for most people exercising at ordinary intensities the gut barrier is not a reason for concern, and in people with gut conditions moderate exercise is linked with benefit rather than harm.
What can the evidence not yet say?
Three limits are worth stating plainly. First, leaky gut syndrome, the stand-alone illness described on wellness sites as the hidden cause of symptoms across the whole body, is not a diagnosis any UK clinician can give, because no medical body recognises it. The measurable change is real. The syndrome built on top of it is not established.
Second, direction. Raised permeability is found in IBS, coeliac disease, Crohn's disease and a list of conditions beyond the gut, but in most of them nobody yet knows whether the leakier lining helps cause the condition, results from it, or both.
Third, reversal. The 2019 review in Gut reached a conclusion that most pages on this subject leave out. Researchers have not demonstrated that repairing the barrier cures any disease. Reducing the pressures on this page is sensible and low risk, and several of them, like swapping to gentler pain relief or drinking less, are worth doing for other reasons anyway. Anyone selling a product that claims to seal the gut and fix the conditions linked to it is ahead of the evidence.
When should you see a GP?
Gut symptoms with a warning sign attached need a doctor, not a theory about permeability. See your GP promptly if you have blood in your stool or black, tarry stools, unexplained weight loss, a bowel habit that has changed and stayed changed for three weeks or more, difficulty swallowing, persistent vomiting, ongoing tummy pain, signs of iron-deficiency anaemia such as constant tiredness and breathlessness, gut symptoms that first appear after the age of 50, or bowel or ovarian cancer in your close family. Painkiller users have one extra flag. Black stools or vomit that looks like ground coffee while taking an NSAID needs same-day advice, because it can mean bleeding in the gut.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms.
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