The short version
- The intestinal barrier comprises surface mucus, the epithelial layer and immune defences, and permeability is measured in people with oral probe molecules such as lactulose with mannitol or rhamnose.
- Endurance exercise, non-steroidal anti-inflammatory drugs, pregnancy and surfactants including bile acids and dietary emulsifiers all increase intestinal permeability in the absence of disease.
- A 2013 review concluded that human and mouse studies show barrier loss alone is insufficient to initiate disease, leaving it uncertain whether raised permeability is a cause or an effect.
- A 2019 review in Gut states that no disease can be cured by simply normalising intestinal barrier function, and that it remains unproven that restoring the barrier improves clinical outcomes.
- No proposed barrier-restoring drug target has been proven effective in clinical trials, and no supplement has been shown to cure disease by repairing the barrier.
There is a version of this subject that is careful, measurable and taught in gastroenterology, and a version that sells a 30-day protocol. They share a name and almost nothing else. Being able to tell them apart is genuinely useful, so here is the seam.
The real thing
The intestinal barrier is not one layer. Camilleri's 2019 review in Gut describes it as surface mucus, the epithelial layer, and immune defences working together, and makes the point that assessing the barrier requires measurements beyond the epithelium alone. Epithelial permeability itself can increase through three different routes: paracellular transport between cells, apoptosis, and transcellular permeability through them.
It is measurable. In people, the standard approach is to swallow probe molecules — lactulose with mannitol or rhamnose — and measure the ratio that appears in urine. Larger sugar gets through only where the barrier is leaky; smaller sugar is the internal control. Mucosal biopsies and cell-layer work fill in the rest.
And it changes under conditions that have nothing to do with disease. Camilleri lists endurance exercise, non-steroidal anti-inflammatory drugs, pregnancy, and surfactants including bile acids and dietary emulsifiers as "stress" states that increase permeability. A marathon does it. Ibuprofen does it. That alone should tell you that raised permeability is not a diagnosis.
The sentence the protocols are built on, and the sentence that follows it
Increased intestinal permeability really is associated with a long list of conditions, intestinal and systemic. That association is the foundation of everything sold under the leaky gut banner, and it is not fabricated.
What gets left off is what the same reviewers say next. Odenwald and Turner, writing in Clinical Gastroenterology and Hepatology, note that most available data are correlative, that human and mouse studies show barrier loss alone is insufficient to initiate disease, and that it therefore remains uncertain whether increased permeability in these patients is a cause or an effect of the underlying disorder. On treatment they are blunt: proponents claim barrier restoration will cure underlying disease, but this has not been demonstrated in clinical trials, and although drug targets have been proposed, none have been proven effective.
Camilleri reaches the same place from the other direction. Inflammatory or ulcerating intestinal diseases do result in a leaky gut — and no such disease can be cured by simply normalising barrier function. It is still unproven, he writes, that restoring barrier function can improve clinical manifestations in gastrointestinal or systemic disease.
That is the whole gap. The barrier is real, the measurements are real, the associations are real, and the therapeutic claim — fix the barrier, fix the person — is the one part that has never been demonstrated.
What a genuine finding in this space looks like
It is narrow and it is boring, which is how you recognise one.
Zhou and colleagues took people whose diarrhoea-predominant IBS began after an enteric infection and who had elevated urinary lactulose/mannitol ratios — a defined population with a measured abnormality. Eight weeks of 5 g of glutamine three times daily normalised the ratio and produced a large symptom improvement against placebo. The authors then asked for large trials to validate it.
Compare the shape of that to a protocol sold to anyone with fatigue and bloating on the basis that their gut is leaking. One selected a population by test result. The other selects a population by search history.
The supplement with the most human permeability data behind it is probably bovine colostrum, and it is worth knowing that the data point in both directions — including one trial where eight weeks of it in runners raised permeability substantially. That is not a scandal; it may be the compound doing what colostrum does. It is a reason not to assume "gut repair" means what the label implies.
What we would tell you
We are not going to sell you a leaky gut product, because we would have to make a claim the literature does not support to justify it. Nothing here treats, cures or prevents any disease, and anyone telling you their powder seals a leaky gut is describing a marketing document rather than a trial.
If you have symptoms, the productive move is a diagnosis rather than a mechanism. Coeliac disease, inflammatory bowel disease and microscopic colitis all genuinely damage the barrier, and all three are found with tests and treated properly. IBS is a positive diagnosis, not a leftover. And if your complaint is swelling rather than a diagnosis, the differential for bloating has a running order and leaky gut is not on it.
The unglamorous things that plausibly help the barrier are the ones nobody can charge for: not taking NSAIDs you do not need, not drinking heavily, eating enough fibre, and treating whatever the actual underlying condition turns out to be. The gut-brain axis is a real and useful clinical lever here too, and it is further along than the supplement version of this story.
Good questions
Is leaky gut a real diagnosis?
Increased intestinal permeability is real and measurable; leaky gut syndrome as a standalone diagnosis is not. Permeability rises with exercise, NSAIDs, pregnancy and several diseases, which makes it a finding rather than a condition. Doctors do not treat the measurement, they treat what caused it. If you have symptoms, the useful question is what the underlying diagnosis is.
Can a supplement heal a leaky gut?
No supplement has been shown to do that. Reviewers in both Gut and Clinical Gastroenterology and Hepatology state that restoring barrier function has not been demonstrated to improve disease, and that no proposed treatment has been proven effective. We sell nothing for this because we would have to make a claim the evidence does not support in order to justify it.
How is intestinal permeability actually measured?
Usually by swallowing two sugar probes, typically lactulose with mannitol or rhamnose, and measuring the ratio excreted in urine. The larger sugar crosses only where the barrier is leaky and the smaller one acts as a control. Mucosal biopsies and blood markers are also used. Home tests marketed as leaky gut panels are not the same thing and are not diagnostic.
Does exercise cause leaky gut?
Endurance exercise does raise measured permeability, and it is listed among the recognised stress states that do. That is a normal physiological response, not damage in the everyday sense, and it is one reason a raised permeability reading on its own does not tell you much. It is also why most human permeability trials are run in athletes.
If I have IBS, is leaky gut the cause?
It may be part of the picture in some people and it has not been shown to be the cause. The most convincing trial in this area selected people whose IBS started after a gut infection and who had a measured permeability abnormality, which is a specific subgroup rather than IBS in general. Get a positive IBS diagnosis first, with coeliac disease excluded.
What actually helps the gut barrier?
Mostly things nobody can sell you: avoiding NSAIDs you do not need, moderating alcohol, eating enough fibre, and treating the underlying condition if there is one. Coeliac disease, inflammatory bowel disease and microscopic colitis all damage the barrier and all three have real treatments. That is a less satisfying answer than a protocol, and it is the one with evidence behind it.
Sources
- Camilleri M. Leaky gut: mechanisms, measurement and clinical implications in humans. Gut, 2019. View study
- Odenwald MA, Turner JR. Intestinal permeability defects: is it time to treat? Clin Gastroenterol Hepatol, 2013. View study
- Zhou Q, Verne ML, Fields JZ, Lefante JJ, Basra S, Salameh H, Verne GN. Randomised placebo-controlled trial of dietary glutamine supplements for postinfectious irritable bowel syndrome. Gut, 2019. View study
- Buckley JD, Butler RN, Southcott E, Brinkworth GD. Bovine colostrum supplementation during running training increases intestinal permeability. Nutrients, 2009. View study
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
