Leaky Gut: What Is Measured, and What Is Sold

Two claims travel under the same name, and only one of them has a research literature.

The first is intestinal permeability: a measurable property of the gut barrier, quantified in humans with defined probe molecules, with normative data and known sources of variation. The second is “leaky gut syndrome”: a consumer-facing explanation in which a permeable gut is the root cause of fatigue, brain fog, joint pain, skin conditions, autoimmunity, and mood, and in which a supplement protocol seals it shut.

The first is real. The second is not supported by the literature that the first produced. Both things can be true at once, and both are.

What the barrier is, and how it is measured

A review in Gut written for clinicians describes the intestinal barrier as more than a sheet of cells: it includes surface mucus, the epithelial layer, and immune defences. Epithelial permeability can result from increased paracellular transport, from apoptosis, or from transcellular permeability. The review is explicit that assessing the intestinal barrier requires measurements beyond the epithelial layer alone.¹

Barrier function is most meaningfully tested in vivo using orally administered probe molecules. Other approaches are performed in vitro 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 patients.²

Permeability also genuinely changes under conditions that have nothing to do with a supplement aisle. The same review lists stress states that increase permeability: endurance exercise, administration of non-steroidal anti-inflammatory drugs, pregnancy, and surfactants such as bile acids and dietary emulsifiers.¹

So the underlying phenomenon is not in dispute. Barriers vary, and measurable things change them.

What a properly validated permeability test looks like

It is worth seeing what the research-grade version of this measurement actually involves, because it sets the standard against which a mail-order test should be judged.

A study in Gastroenterology enrolled 60 healthy adults aged 18 to 70 across three randomized studies on standardized diets. At each test, participants ingested 12C-mannitol, 13C-mannitol, rhamnose, sucralose, and lactulose. Meals were standardized from 24 hours before the sugars through 24 hours after, with three timed urine collections at 0 to 2, 2 to 8, and 8 to 24 hours, and sugars measured by high-performance liquid chromatography with tandem mass spectrometry.³

Median 24-hour excretions as a percentage of the administered dose were roughly 30 percent for 13C-mannitol, roughly 15 percent for rhamnose, 0.32 percent for lactulose, and 2.3 percent for sucralose. Within a given individual, saccharide excretion was consistent, with only minor differences between a 16.25 gram and a 32.5 gram fiber diet. Between individuals, the median coefficient of variation was 76.5 percent, with a 10th to 90th percentile range of 34.6 to 111.0. The study also found that baseline 12C-mannitol excretion precludes its use as a probe, and identified 13C-mannitol as the preferred probe for small intestinal permeability.³

Read that variation figure again. Healthy people, standardized diets, controlled collection, and a median between-person coefficient of variation of 76.5 percent. A single reading against a population reference range, in a population that spread out that widely, carries much less information than a number on a report implies.

The zonulin problem

Serum zonulin is described as a marker of intestinal permeability and identified with pre-haptoglobin 2, and it is measured by a commercial ELISA kit that the research literature itself describes as widely used.⁴ We have not counted how many of the permeability panels sold to consumers include it, so we are not going to tell you that most of them do. Two findings complicate the marker itself.

A 2018 study measured serum zonulin with a widely used commercial ELISA kit in 376 subjects from a well-characterized cohort, and also determined each subject’s haptoglobin genotype. Zonulin concentrations did not correlate with haptoglobin genotype, which prompted the authors to investigate the assay itself using antibody capture experiments, mass spectrometry, and Western blot. They found several proteins likely captured by the kit’s antibody, none of which corresponded to pre-haptoglobin 2. When they applied increasing concentrations of recombinant pre-haptoglobin 2 directly, the kit did not detect it. They identified properdin as another likely member of what they term the zonulin family, and concluded that the assay does not recognize pre-haptoglobin 2 but rather structural analogues.⁴

Separately, a pilot study in 34 healthy adults measured small intestinal permeability by the urinary lactulose-rhamnose ratio alongside serum zonulin. The two showed no correlation at all, with a correlation coefficient of minus 0.016 and a p value of 0.929. The lactulose-rhamnose ratio correlated with lipopolysaccharide; zonulin instead correlated moderately with waist circumference in female participants and with high-sensitivity C-reactive protein overall.⁵

That is a small study and its authors present it as a pilot warranting larger work. Taken with the assay findings, the position is that a widely used blood assay may not be measuring the protein it names, and did not track the sugar-probe measurement it is meant to stand in for.

The claim that does not hold

Here is where the honest reading has to be blunt, because this is the load-bearing part of the consumer story.

Inflammatory or ulcerating intestinal diseases do result in leakiness of the gut barrier. That direction of the relationship is established. But no such disease has been resolved by simply normalizing intestinal barrier function. And it remains unproven that restoring barrier function can ameliorate clinical manifestations in non-ulcerating gastrointestinal disease, or in systemic or neurological disease.²

The 2019 review states the same conclusion and adds a direct instruction about consumer information: material on “healthy” or “leaky” gut in the public domain requires confirmation before endorsing dietary exclusions, replacement with non-irritating foods such as fermented foods, or use of supplements to repair damage. It also notes that clinicians should be aware of barrier dysfunction in gastrointestinal disease and of the barrier as a target for future therapy.¹

That phrase, a target for future therapy, is the accurate status. Not a mechanism you can currently buy your way out of.

This is not a case of researchers dismissing a popular idea out of conservatism. It is that permeability has mostly been observed as a downstream feature of disease states, and the step from there to “and correcting it resolves the disease” has not been demonstrated. Marketing has taken that step anyway.

What has actually been shown about diet and the barrier

It would be an overcorrection to say nothing influences the barrier. The 2021 review reports that dietary factors can influence intestinal leakiness, listing vitamins A and D, zinc, short-chain fatty acids, methionine, glutamine, and probiotics among factors reported to fortify the barrier, and fat, bile acids, emulsifiers, and gliadin among those reported to weaken it. It also notes that intestinal mucosal leakiness in stress disorders such as major burns normalizes with enteral glutamine.²

Notice the setting on that last one: major burns, enteral nutrition, a hospital. That is a long way from a capsule taken for bloating, and citing it as support for the capsule is exactly the move this article exists to flag.

The practical read

Intestinal permeability is a legitimate research variable with validated probes, published normative data, and wide between-person variation.³ “Leaky gut syndrome” as sold, a single root cause with a supplement answer, is not something the permeability literature supports, and the specific claim that restoring barrier function improves clinical outcomes in systemic disease is described in the reviews as unproven.²

If you have been sold a permeability panel, it is fair to ask which probe or assay it used, whether that assay has been validated against a sugar-probe measurement, and what reference range it is scoring you against.⁴⁵ If you have symptoms, the symptoms are the thing worth taking to a clinician. The barrier is a research question that has not yet become a clinical answer.