Intestinal permeability is measurable and real. 'Leaky gut' as a diagnosis is not
A review in Gut says no intestinal disease can be cured by normalising barrier function, and that it is unproven whether restoring the barrier improves any clinical outcome at all.
Intestinal permeability is a real, measurable property of the human gut, and it does increase under certain conditions. What does not exist is evidence that increasing or restoring it explains or resolves the conditions "leaky gut" is popularly blamed for. A review written for clinicians and published in Gut states the position directly: whereas inflammatory or ulcerating intestinal diseases result in a leaky gut, no such disease can be cured by simply normalising intestinal barrier function, and it remains unproven that restoring barrier function can ameliorate clinical manifestations in gastrointestinal or systemic diseases [s1].
What the barrier actually is
The review's first correction is that the intestinal barrier is not a single layer with holes in it. It comprises surface mucus, an epithelial layer, and immune defences [s1]. Epithelial permeability itself has three separate routes — increased paracellular transport between cells, apoptosis, and transcellular permeability through cells [s1]. Assessing the barrier, the review notes, requires measurements beyond the epithelial layer [s1].
That structure matters because almost every consumer explanation of leaky gut describes one mechanism — gaps opening between cells — as though it were the whole thing.
How it is measured, and why the numbers vary
Barrier function can be tested in living people using orally administered probe molecules, whose appearance in urine indicates how much crossed the gut wall [s1]. It can also be tested outside the body, using mucosal biopsies from humans, or by exposing colonic mucosa from rats or mice, or cell layers, to extracts of colonic mucosa or stool from human patients [s1].
Those are different measurements of different things, and the review's framing throughout is that information on "healthy" or "leaky" gut circulating in the public domain requires confirmation before it is used to endorse dietary exclusions, replacement with non-irritating foods such as fermented foods, or supplements to repair the damage [s1].
What genuinely increases permeability
This is the part of the concept with the firmest footing. The review identifies a set of "stress" states that increase permeability: endurance exercise, administration of non-steroidal anti-inflammatory drugs, pregnancy, and surfactants — a category that includes bile acids and dietary factors such as emulsifiers [s1].
Two things follow. First, increased intestinal permeability is not rare or pathological by definition; running a marathon or being pregnant will do it. Second, dietary factors can reverse intestinal leakiness and mucosal damage in those stress disorders, which the review states directly [s1].
So the mechanism is real, it responds to diet, and the honest question is what that response achieves.
What the supplement trials actually measured
A systematic review and meta-analysis published in Pharmacological Research pooled trials of probiotics, synbiotics and prebiotics against markers of intestinal permeability, drawing on 46 published studies of pro- and synbiotics and 22 studies of prebiotics from searches running from 1961 to January 2023 [s2].
Pooling 24 trials with 28 effect sizes in 1,603 people, probiotic and synbiotic consumption was associated with a significant reduction in lipopolysaccharide levels — but with high heterogeneity and very low certainty of evidence (standardised mean difference −0.54, 95% CI −1.01 to −0.07, I² = 94.4%) [s2]. Thirteen trials with 15 effect sizes in 778 people showed significantly lowered zonulin, with high heterogeneity and moderate certainty (SMD −0.49, 95% CI −0.79 to −0.18, I² = 74.9%) [s2]. For prebiotics, 16 randomised trials in 792 people showed reduced lipopolysaccharide with high certainty of evidence but high heterogeneity (SMD −0.88, 95% CI −1.28 to −0.47, I² = 85.7%) [s2].
The authors describe these as promising findings regarding efficacy in alleviating "leaky gut" — their own quotation marks [s2].
Read carefully, though, every one of those outcomes is a blood marker. Not a symptom, not a diagnosis, not a hospital admission. An I² of 94.4% means the trials disagreed with each other almost completely, and "very low certainty" is GRADE's bottom rung. What this meta-analysis establishes is that supplements can move biomarkers associated with permeability. It does not establish that moving them changes how anyone feels or what happens to them.
Which returns to the review's own conclusion: it is unproven that restoring barrier function ameliorates clinical manifestations of disease [s1].
The honest position
There is a genuine science here, and it is more interesting than the version sold with it. Clinicians, the review argues, should be aware of the potential for barrier dysfunction in gastrointestinal diseases, and of the barrier as a target for future therapy [s1]. "Future therapy" is the operative phrase — it names a research direction, not an available treatment.
What the evidence does not support is the consumer syllogism: that a defined condition called leaky gut exists as a diagnosis, that it causes fatigue, skin problems, autoimmune disease or brain fog, and that a supplement seals it. The first premise is unavailable — there is no validated clinical diagnosis here — and the last step has been tested only against biomarkers with heterogeneity so high the trials barely agree [s2].
Meanwhile, the conditions that reliably do produce a leaky gut are inflammatory and ulcerating intestinal diseases [s1] — conditions with their own diagnoses, their own tests, and their own treatments, none of which are a powder.
This article is informational and is not medical advice. Persistent gastrointestinal symptoms deserve assessment for the conditions that have established diagnoses, rather than for one that does not.
Sources
Sources
- Leaky gut: mechanisms, measurement and clinical implications in humans — Gut , May 10, 2019
- Reinforcing gut integrity: A systematic review and meta-analysis of clinical trials assessing probiotics, synbiotics, and prebiotics on intestinal permeability markers — Pharmacological Research , May 15, 2025
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