Zonulin.

A protein that regulates the opening of intestinal tight junctions.

Zonulin is a protein described as a regulator of intestinal permeability: it acts on the tight junctions between the cells lining the gut, prompting them to loosen and allowing more material to pass between cells rather than through them. It is often measured in blood or stool and presented as a marker of what popular writing calls leaky gut. The underlying biology of intestinal permeability is real and well studied, and altered permeability is genuinely associated with several conditions. Where honesty is required is in how zonulin specifically is used. The identity of the protein measured by commercial zonulin assays has been contested in the research literature, with questions about whether the antibodies used detect the intended molecule or a related family member, which means results across studies are not straightforwardly comparable. It has become a popular commercial biomarker faster than it became a validated one. The term appears in peptide discussion because gut-barrier peptides are frequently promoted with reference to it. The reasonable position is that intestinal permeability is a legitimate research area, that zonulin is a proposed marker within it with real measurement controversy attached, and that a marker moving is not the same as a health outcome changing.