Zonulin and leaky gut: what the test can show
The gut barrier is real, measurable and well studied. The popular blood marker for it is the weakest part of the story.
A test comes back with one raised marker, zonulin, and a printed conclusion: increased intestinal permeability. The person has been bloated for a year and now has a name for it, a supplement plan, and a number to retest in twelve weeks.
The barrier they are describing is real and well studied. Whether that number measured it is a separate question, and the two get run together constantly.
What does “leaky gut” describe?
Your gut lining is a single layer of cells, sealed to each other by protein junctions. It has to be selective: nutrients cross, most of the bacterial traffic in the gut does not. When those junctions loosen, more material crosses than should, and the immune tissue sitting behind the wall meets things it does not usually meet.
Researchers call this intestinal permeability, and it is measurable in laboratories. Increased permeability is a real finding, documented in coeliac disease, inflammatory bowel disease, and after major burns, heavy alcohol intake and intense endurance exercise.
The argument is not about whether the barrier can loosen. It is about what loosening causes, and how you would know.
What is zonulin, and what does the test measure?
Zonulin is a protein that loosens those junctions. It was identified in the early 2000s and gave the field something it badly wanted: a single blood marker for barrier function.
The difficulty is technical. The widely used commercial kits do not reliably measure the zonulin molecule itself; several published analyses found they react with other, related proteins in blood. That means two laboratories can report different values on one sample, and a change on retesting may not be a change in the person.
Retesting zonulin at twelve weeks and reading a fall as proof the plan worked. With current kits, a 20 per cent change is within what the assay itself can produce.
If the blood marker is shaky, it is fair to ask what the research uses instead.
How is permeability measured properly?
The established method is a sugar test. You drink two sugars the body does not metabolise: one small, one larger. The small one crosses the lining easily; the larger one should barely cross at all. Urine is collected for several hours and the ratio between them is measured.
A high ratio of the larger sugar to the smaller one means more is crossing than should. Lactulose and mannitol are the classic pair; lactulose with rhamnose is used too. The ratio is what matters, because it cancels out how fast the person absorbed, drank and passed urine.
It is more trouble than a blood draw, which is exactly why the blood marker became popular.
Measurement aside, the question people actually care about is what a loose barrier does.
Does increased permeability cause disease?
In coeliac disease and Crohn’s disease, permeability is increased and tracks with activity. In some studies, relatives of people with Crohn’s show raised permeability before any disease appears, which is why the question stays open and interesting.
For most other conditions the honest position is that permeability is found alongside the condition, and which came first is usually unknown. Inflammation loosens junctions, and loosened junctions feed inflammation, so a loop is easy to demonstrate and hard to order.
That intestinal permeability increases in coeliac disease, inflammatory bowel disease, serious burns, heavy alcohol use and prolonged intense exercise is well established. The dual sugar test is the validated way to measure it. Whether raised permeability drives conditions beyond the gut is being studied, and the data are early. Commercial zonulin kits have a documented specificity problem, which is a measurement issue rather than a claim about the barrier.
Which leaves what to do when the picture fits, whatever the test says.
What helps the barrier?
The reliable moves are unremarkable. Treat what is irritating the lining: coeliac disease if present, heavy alcohol, long courses of anti-inflammatory painkillers, and untreated infection. Feed the lining, which means enough protein and enough fermentable fibre for the bacteria that produce butyrate, the short-chain fat that colon cells use as fuel.
Most barrier improvement follows from treating the cause, not from a supplement aimed at the junctions. Glutamine has some supporting data in specific settings, including after burns; the broad “gut repair” blends sold for this have no controlled evidence behind them, and some carry doses of individual ingredients well above what studies used.
- The barrier is real and measurable; the popular blood marker is the weak link.
- Ask for the sugar ratio if permeability genuinely needs measuring.
- Do not read a retest of zonulin as progress.
- Look for coeliac disease, alcohol, anti-inflammatory painkillers and infection first.
- Protein and fermentable fibre do more for the lining than most products aimed at it.
The harder case is the person with convincing symptoms, a normal sugar ratio, and a year of supplements behind them. That is where the Deep Dive on leaky gut picks the thread up.
