Zonulin, "Leaky Gut" and What the Panel Really Shows

The syndrome sold under that name goes further: that a leaky gut is common in otherwise healthy people; that it is caused by modern diet, stress and toxins; that it produces fatigue, brain fog, joint pain, skin problems, food sensitivities and autoimmune disease; and that it can be diagnosed with a blood or stool marker and treated with supplements — glutamine, collagen, zinc, "gut-healing" protocols.

None of that has been established. No medical body recognizes leaky gut syndrome as a condition. Increased permeability has not been shown to cause any of the symptoms attributed to it in people who do not have an underlying gut disease; where permeability and symptoms coexist, the direction of cause is unknown; and no trial has shown that a supplement that "heals" the gut lining improves any of those symptoms. The idea is not absurd — it is a hypothesis with a mechanism — but it has been sold as a diagnosis for twenty years without acquiring the evidence a diagnosis needs.

Zonulin was identified in the early 2000s as a protein that, when released by the gut lining, signals the tight junctions to open — a regulator of permeability, and therefore a plausible marker of it. The commercial tests that followed measure "zonulin" in blood or stool with an antibody-based assay.

The problem, established in the last decade, is that the commercial assays do not measure zonulin. Independent work using purified protein showed that the widely used kits detect other proteins — properdin, and members of a related family — rather than the zonulin the assay is named for, and that the result does not correlate with permeability measured by the reference methods. A zonulin result from a stool panel is therefore a number whose relationship to the thing it claims to measure is unproven, produced by an assay that has been shown to bind something else. It is not a measurement of your gut's permeability, and no gastroenterology guideline uses it.

If zonulin is reported on the panel you receive, we read it as we read a horoscope: with interest and without action. It does not change what we recommend, it does not prompt a treatment, and a "high" zonulin is not a diagnosis of anything.

What the panel does show that bears on the intestinal barrier is more useful and less advertised. Calprotectin, which indicates inflammation of the lining and is validated for exactly that. Secretory IgA, the mucosal immune marker, which reflects the barrier's immune activity. The digestion and absorption markers, which show whether the gut is doing its job. And, where ordered, the pathogens whose presence would explain a genuinely disrupted barrier. A person with a normal calprotectin, normal absorption markers and no pathogen does not have a damaged gut lining, whatever a zonulin number says.

We will say that intestinal permeability is real biology, that it rises in defined diseases, and that if you have one of those diseases — celiac, Crohn's, ulcerative colitis — your gastroenterologist manages it. We will say that the fatigue, brain fog, joint pain and food intolerance attributed to leaky gut are real symptoms with real causes, and that the fatigue workup, the brain fog workup and, for food intolerance, a proper dietary assessment are where those causes are found.

We will not say you have leaky gut syndrome, because it is not a diagnosis. We will not sell a supplement protocol to heal it. And we will not tell you a zonulin number means your gut is leaking, because the assay does not support the claim.