The Zonulin Discovery
Zonulin (pre-haptoglobin 2) is a protein identified by Alessio Fasano's group as a modulator of intestinal tight junctions. It was discovered through research on cholera toxin — which causes diarrhoea partly by opening intestinal tight junctions. Zonulin mimics this effect in a regulated, physiological manner: it is released from intestinal epithelial cells in response to specific stimuli (gliadin peptides, certain bacteria) and reversibly increases paracellular permeability by disassembling the tight junction protein complex.
The Proposed Pathway
In the proposed model, luminal stimuli trigger zonulin release from the epithelial apical surface. Zonulin then binds to PAR2 and EGFR receptors on the same or adjacent epithelial cells, activating intracellular signalling cascades that phosphorylate ZO-1 and cause its dissociation from the tight junction complex. This opens the paracellular space, permitting passage of macromolecules (including gliadin peptides and bacterial antigens) from lumen to lamina propria. In coeliac disease, this creates a feed-forward loop: gliadin triggers zonulin release, which increases permeability, which permits more gliadin to access immune cells.
Legitimate Clinical Associations
Elevated serum zonulin has been reported in coeliac disease, type 1 diabetes, IBD, obesity, NAFLD, and autoimmune conditions. Fasano proposed that increased intestinal permeability — mediated partly by zonulin — is an early, potentially causal step in autoimmune disease development, occurring before clinical symptoms manifest. Larazotide acetate, a zonulin receptor antagonist, has been tested in coeliac disease clinical trials as an adjunct to the GFD, with some evidence of reduced symptoms and permeability markers.
The Controversy
Zonulin science has attracted significant criticism. A 2020 study in Gut found that the most widely used commercial zonulin ELISA assays do not actually measure zonulin (pre-haptoglobin 2) — they cross-react with complement C3, a completely different protein. This raises serious questions about the validity of studies using these assays. Furthermore, zonulin's proposed mechanism has not been fully replicated in independent laboratories, and some researchers question whether zonulin plays the central role in tight junction regulation that was initially proposed.
What Commercial Tests Measure
Many functional medicine practitioners offer serum zonulin testing as a measure of "leaky gut." Given that the most common commercial assays may not measure true zonulin, and that reference ranges are not standardised, these results should be interpreted with extreme caution. An elevated "zonulin" result on a commercial test does not necessarily confirm increased intestinal permeability, and certainly does not constitute a diagnosis.
The Bigger Picture
Intestinal permeability is a real and measurable physiological parameter with relevance to multiple diseases. However, the tools available for its clinical measurement — including zonulin assays, lactulose-mannitol tests, and serum I-FABP — are imperfect, lack standardisation, and are not recommended for routine clinical use by any major gastroenterology society. Research continues, but translating permeability science into validated clinical diagnostics requires overcoming significant methodological challenges.