What intestinal permeability actually means
The intestinal lining is a single layer of cells held together by tight junctions that regulate what passes between the gut interior and the bloodstream. The phrase leaky gut refers to increased intestinal permeability, a measurable state in which these junctions loosen and allow more luminal contents through than normal. The protein zonulin is one studied regulator of junction tightness.
Increased permeability is documented in conditions such as celiac disease, inflammatory bowel disease, and some infections, where it can contribute to local and systemic immune activation.
Separating mechanism from marketing
While altered permeability is real and measurable, leaky gut syndrome as a stand-alone diagnosis causing a long list of unrelated symptoms is not an established medical entity, and the claim that specific supplements seal the gut and resolve such symptoms is largely unproven in humans.
This is general educational information, not medical advice. Permeability changes are usually a feature of an underlying condition rather than a free-standing illness, so symptoms attributed to leaky gut should be evaluated by a clinician rather than self-treated with supplements.