Zonulin: pathway, marker and measurement are not the same.

The deceptively simple chain

Zonulin occupies an unusual position in discussions about intestinal permeability. It is discussed as a biological regulator, a biomarker, a laboratory measurement and sometimes as though a single number could describe the state of the intestinal barrier.

This often produces a simple linear interpretation:

increased zonulin → increased intestinal permeability → pathology

That chain is inadequate. It turns a dynamic regulatory mechanism into a disease marker by collapsing several separate claims into one conclusion.

A regulatory mechanism is not a disease switch

Within the cited mechanistic literature, human zonulin has been identified as pre-haptoglobin-2 and studied in relation to reversible regulation of intercellular tight junctions and paracellular permeability.

Experimental work has also shown stimulus-responsive zonulin secretion after bacterial exposure.

This places zonulin within a regulatory pathway. It does not make every increase in production, secretion, signalling or measured concentration pathological. Nor does it establish that all such observations represent the same biological event.

An increase may occur in different contexts and may be measured in different ways. Before it can support a pathological interpretation, the observation itself and each inferential step must be established.

“Increased zonulin” is not one observation

Production, secretion, pathway activity, tissue expression, circulating concentration and a commercial assay signal are not interchangeable.

A biological pathway describes a process. An assay produces a measured value. Before interpreting a result labelled zonulin, it is necessary to know what molecular target the assay detects, in which specimen, how reliably and for what intended use.

That distinction is not theoretical. A widely used commercial ELISA was reported not to detect pre-haptoglobin-2 and appeared to recognise other proteins. The name printed on an assay therefore cannot establish that its value represents activity in the biological pathway.

The canonical technical account is available in Zonulin as pathway, biomarker and measurement.

Each arrow requires evidence

The first arrow—from increased zonulin to increased permeability—requires evidence that the relevant zonulin construct was actually measured and that the result represents permeability under the conditions studied.

The second—from increased permeability to pathology—requires evidence that the permeability change was inappropriate, persistent or consequential in that context. Permeability itself is part of normal physiology. An increase alone does not establish barrier damage, diagnosis or disease.

The intestinal barrier is also not a single variable. It includes epithelial cells, junctional regulation, mucus, immune systems, microbial communities and multiple routes of passage. A result related to one pathway cannot automatically represent the entire system.

What different studies can support

My systematic review and meta-analysis examined observational studies of zonulin-dependent intestinal permeability in children and adolescents diagnosed with mental disorders. The evidence was limited and heterogeneous. It could identify associations in the populations studied, but it could not establish causal direction, a diagnostic test, treatment effects or interchangeability among different measurements.

Animal models answer a different question. My doctoral work used zonulin-transgenic mice and transcriptomic analysis to investigate biological patterns under experimental conditions. Such work can contribute mechanistic insight, but it does not establish human clinical effects.

Different designs become misleading when their findings are compressed into one linear conclusion.

From dynamic regulation to linear disease interpretation

Zonulin is a concrete example of a wider scientific problem: the conversion of a dynamic regulatory mechanism into a simple linear disease model.

The existence of a mechanism does not validate every assay. An assay result does not automatically represent the whole intestinal barrier. An association does not establish causal direction. A biomarker does not become clinically useful simply because the biology behind its name is compelling.

Pathway, marker, measurement and interpretation belong to the same scientific conversation. They are not the same thing. Keeping those distinctions visible is necessary before the evidence can earn a pathological or clinical conclusion.