Leaky gut: what is real and what is being sold

Search for leaky gut and you land in one of two internets. In one, a failing gut barrier explains fatigue, eczema, aching joints and low mood, and a powder will seal it. In the other, the whole idea is invention. Between those two sits real physiology, well studied, and nothing like either.

By the end of this article you will be able to sort three things. What is established about the gut barrier, what is genuinely unresolved, and what is simply being sold.

Is the gut barrier a solid wall?

Start with two sentences of anatomy, because everything rests on them. The contents of your gut are technically outside your body. One single layer of cells stands between that content and the largest mass of immune tissue you have.

The stitching between those cells is the interesting part. Neighbouring cells are joined near their upper edges by protein complexes called tight junctions. They are not cement. The cell assembles and dismantles them continuously. Different stretches of gut are leakier or tighter by design. The small intestine is deliberately leakier than the colon, because that is where water and nutrients move across.

Junctions also loosen for legitimate reasons. Fluid shifts after a meal. Immune cells cross through to do their work. How much passes between cells is called permeability, and it is a regulated variable with a working range, more like blood pressure than like a broken window. The barrier is not a wall that is either intact or breached. It is a set of doors under constant regulation, and some opening is ordinary physiology.

A regulated system needs a regulator. The best known one is also the one printed on the test brochures.

What is zonulin, and what does the test measure?

Zonulin is a human protein. When it is released into the gut, it signals the junctions to loosen. Two triggers reproduce reliably in studies: gliadin, a fragment of gluten, and bacteria arriving in the small intestine in numbers that do not belong there. Raised levels are reported in coeliac disease and type 1 diabetes.

The complication arrived when independent laboratories examined the commercial test kits. Several widely used kits appear not to detect zonulin itself. They bind other proteins in the same family instead. A number printed under the word zonulin may be measuring something real, but not the thing it names. Zonulin is a genuine signalling pathway. The commercial tests sold to measure it have been credibly questioned for detecting something else.

Set the marker aside and ask a better question. What actually opens the junctions?

What genuinely raises permeability?

The best documented everyday cause surprises most people: anti-inflammatory painkillers. In studies of healthy volunteers, short courses of NSAIDs, the drug family that includes ibuprofen, raise measured permeability within days. Camera examinations of long-term users find small bowel damage far more often than symptoms suggest.

Alcohol does it too, in a single heavy session and in long heavy use. Hard endurance exercise in heat raises it for a few hours, then it settles. Acute infection and critical illness raise it substantially. Permeability also runs high in coeliac disease and inflammatory bowel disease, and that is where the subject stops being tidy.

The obvious next thought runs like this. A loose barrier lets material through, the immune system reacts, and disease follows. That is a hypothesis, not a finding. In Crohn’s disease, some healthy close relatives show increased permeability years before any disease appears. That is the strongest hint anywhere that the change can come first. Elsewhere, inflammation is usually present at the same time, and inflammation opens junctions by itself. So a raised reading could be cause, consequence or bystander. Whether increased permeability starts these conditions or follows from them is unresolved in most of them. That open question is the honest centre of this subject.

Which brings us to what most people came for. Can it be measured?

Can intestinal permeability be measured?

In research, yes. The standard is a sugar probe test. You drink two sugars. Lactulose is a large molecule that can only pass between cells. Mannitol is a small one that crosses through them. Both leave the body unchanged in urine, and the ratio between them over the next hours is the reading. Using a ratio cancels out stomach emptying, kidney function and incomplete collection.

The test works in research, and it travels badly. Sugar doses, collection windows and laboratory methods differ between centres. Results move within the same person from day to day. And no threshold is tied to an outcome that would change what anyone does next.

No validated, routinely useful clinical test of intestinal permeability exists for general use. The supplement blends sold to correct it have no controlled evidence in people for that purpose. That deserves saying without contempt. The biology is genuine, and the questions people bring to it are reasonable. The commercial products simply moved ahead of what the research can yet support.

What actually supports the barrier?

Nothing on the supported list is exotic. Adequate fibre and a wide range of plants feed the microbes whose products the colon lining runs on, butyrate among them. Protein matters more than people expect, because the gut lining replaces itself every few days out of amino acids. Moderating alcohol removes one well documented cause outright. Sleep shows up here as it does everywhere.

Where a condition already exists, treating it is the step shown to move permeability. In coeliac disease, a strict gluten-free diet returns the junctions toward normal over months. Long or unnecessary NSAID courses matter for the same reason, though decisions about anyone’s medication belong with their own doctor. The measures with the best support are unglamorous, and not one of them is sold as barrier repair.

Sorting a claim into the right column is what this field keeps asking of you. The Gut and Immunity knowledge check will show you how fast you can do it.