Intestinal permeabilityLeaky gut
How much passes through the gut lining: real biology, oversold as a diagnosis.
Intestinal permeability describes how readily substances cross the gut lining. The lining is not a sealed wall: it is a single layer of cells joined by tight junctions that open and close in a regulated way, allowing nutrients through while keeping bacteria and larger molecules out.
That regulation is normal physiology. Permeability changes with meals, exercise, alcohol, stress and illness, and increased permeability is a real, measurable phenomenon documented in coeliac disease, inflammatory bowel disease, and in the acutely unwell.
The popular version, leaky gut syndrome, goes considerably further. It proposes that increased permeability causes a long list of unrelated conditions in otherwise healthy people, and that a specific protocol of supplements and elimination diets will seal the gut.
That leap is where the evidence stops. A review in Gut concluded that the biology is real, the measurement is imperfect, and the causal role in the conditions commonly attributed to it is not established. In several diseases increased permeability appears to be a consequence rather than a cause.
There is also no validated consumer test. Kits sold for it are not standardised or clinically interpretable, and the treatments sold alongside them are not established treatments for anything.
The practical harm is delay and restriction. Someone attributing symptoms to leaky gut may not get investigated for coeliac disease, inflammatory bowel disease or IBS, all of which have real diagnoses and real management.
In practice, persistent gut symptoms deserve a GP rather than a protocol, and the diet advice with actual evidence remains fibre, variety and, where indicated, a supervised low FODMAP trial.