The Zonulin Test Cannot See Your Gut Wall. Here Is What Can

A zonulin test promises a number for the state of your gut wall. Two independent laboratories took the most common commercial kits apart and found they do not detect zonulin at all. That matters, because people are cutting foods, buying supplements and building a theory of their own illness on that number.

The video above is Chapter 4 of The Angry Gut, by Dr. Gurpreet Singh Padda, MD, MBA, MHP, and Ami Michelle Grimes. It tells the story of the wall itself. What follows goes deeper into measurement: what the kit actually reads, what a real permeability test prints, and what a result near the line does and does not mean.

What a zonulin test actually binds

Zonulin was defined as one protein, pre-haptoglobin-2. Some people carry haptoglobin genes that leave them unable to make it. An honest kit should read low in those people.

It did not. In 376 people who were genotyped and tested with the standard kit, the results ignored the genes. Handed the pure protein, the kit saw nothing at any concentration tested. What it bound instead looked like properdin, an unrelated immune protein (Scheffler et al., 2018). A second group ran two other kits and identified the bound material by mass spectrometry. It was haptoglobin and a complement protein, not zonulin (Ajamian et al., 2019).

A third check came from healthy relatives of people with Crohn’s disease. Among 36 of them with an imputed haptoglobin type, readings did not differ by genotype, p = 0.729. Set against the direct sugar test in the same people, serum zonulin explained essentially nothing, with an r2 of 0.004 (Power et al., 2021).

The kit measures something real. That something has never been tied to your gut wall.

The most repeatable number measures the wrong thing

Across three eight-day periods in 24 adults, plasma zonulin was highly repeatable, scoring above 0.9 on reliability. The sugar test, which actually probes the wall, scored 0.53, rated only fair (Kuzma et al., 2019). A perfectly steady reading of an unidentified protein is still a reading of an unidentified protein.

When six candidate markers were lined up against the sugar test in 78 people, 51 healthy and 27 with obesity, only plasma LBP tracked it in every group (Seethaler et al., 2021). Plasma zonulin and plasma I-FABP, the two you can order online, did not make the list. LBP is an acute-phase protein, so inflammation from any cause pushes it up.

So why does the panel survive? Because a single number on a printout sells, and a timed urine collection does not. The market rewards the convenient answer. The patient pays twice: once for the kit, and again in the foods and months it costs.

How gut permeability is measured in a living body

You test a wall by trying to push something through it. The patient drinks sugars of different sizes, and urine is collected on a clock. Small sugars cross a healthy lining. Large ones mostly should not. Timing tells you where the leak sits, with the early hours reporting on the small bowel and the later collection on the colon.

Two details decide whether the result means anything. Ordinary mannitol is present in food, and in the validation study it turned up in baseline urine from all 60 participants before anyone took a dose, so the labeled form is required (Khoshbin et al., 2021). And the familiar lactulose-to-mannitol ratio can hide a real leak. In inflammatory bowel disease, each sugar measured alone separated patients from healthy volunteers, at P = .003 and P = .006, while the ratio found nothing (Dunleavy et al., 2025). Ask whether the individual sugars are reported, not only the ratio.

Why one abnormal result is not a verdict

Even the right test has a wide normal. Sixty healthy adults on a tightly controlled diet recovered a median of about 30% of the labeled mannitol and 0.32% of the lactulose over 24 hours (Khoshbin et al., 2021). Between people, the median coefficient of variation ran to 76.5%. Sex, age and body mass index did not explain the spread. Individuals did.

The usual cutoffs sit close to that noise. With the abnormal line at a ratio of 0.025, 14 of 39 healthy young volunteers flagged (Power et al., 2021). More than a third of healthy people would have been told they leak.

A result near the line is a reason to repeat the test under controlled conditions. It is not a reason to strip the diet.

A wall that fails before the disease does

None of this makes leaky gut fiction. The wall is real, and measured properly, its failure carries weight. In 1,420 healthy first-degree relatives of people with Crohn’s disease, an abnormal sugar test at baseline predicted a later diagnosis at a hazard ratio of 3.03 over a median of 7.8 years. Limited to tests done more than three years before diagnosis, the link still held at 1.62 (Turpin et al., 2020).

The same literature also cuts against me, and I owe you that. When eight studies of permeability in obesity and metabolic syndrome were graded formally, certainty came back very low to low, with no clear relationship found (Bona et al., 2022). Six methods across eight studies, nothing poolable. The honest position is narrower. The wall can be measured, abnormal results preceded disease in a genetically loaded cohort, and the metabolic link rests mostly on mechanism. That is the evidence tier, and it is enough to act on the gates you control.

Repairing the terrain behind the test

The gut lining is the outer surface of the first brain, and a breach there is where metaflammation begins: low, steady, metabolic inflammation whose bill later arrives at the liver, the pancreas and the arteries. Two biological engines run together. Bacterial fragments slip through loosened claudin seals, the real mortar between cells. Behind the wall, an immune garrison reads those fragments as invasion and raises the inflammatory tone of the whole body. For how that signal travels up the vagus to the second brain, see the previous chapter on the vagal tether.

Repair starts with the gates, not the grocery list. The exposures shown to open the human gut wall include anti-inflammatory drugs, endurance exercise, pregnancy, and surfactants such as bile acids and dietary emulsifiers (Camilleri, 2019). The same review warns against cutting foods or buying supplements on the strength of public claims about leaky gut. The physiological reason is simple: an elimination diet does nothing to the seal if the drug, the drink or the stress hormone opening it is still arriving every day. If you take an anti-inflammatory daily, raise that with your physician rather than changing it on your own.

Measurement is part of repair, not a gate before it. A wall measured with a validated test can be measured again after the terrain changes. That is also why the same injection works in one body and fails in another, a point developed in why identical treatments give different results, and why no procedure outruns a broken metabolism. Every study and every figure above, with what each one does and does not show, is laid out in the Chapter 4 Deep Dive.

Frequently asked questions

Is a zonulin test accurate for leaky gut?

No, not as a measure of the gut wall. Two independent laboratories found that common commercial kits do not detect the protein they are named for, and one kit appears to bind properdin instead. In healthy relatives of Crohn’s patients, serum zonulin showed essentially no relationship to a direct sugar permeability test. For tests that do earn a place in a metabolic workup, read the metabolic audit of biomarkers worth ordering.

What is the best test for intestinal permeability?

A timed urine collection after drinking sugars of different sizes, ordered by a physician. It should use labeled mannitol, because ordinary mannitol in food contaminates the baseline, and the report should include each sugar on its own, since the ratio alone can miss a real leak. How blood markers of inflammation fit beside it is covered in our guide to hsCRP and inflammation biomarkers.

Can a healthy person test positive for leaky gut?

Yes, often. In one group of healthy young volunteers, 14 of 39 sat at or above a commonly used abnormal cutoff. Normal variation between people is large, so a single borderline result is a reason to repeat the test under controlled conditions, not a diagnosis. The gut still matters to how the rest of the body feels, as the gut and joint pain connection explains.

What damages the gut lining?

The measured culprits are ordinary. Anti-inflammatory drugs, psychological stress, heavy drinking, a hard rise in core temperature, endurance exercise and dietary emulsifiers have all been shown to open the human gut wall. None of them is exotic, and most people meet several in a normal week. If a daily painkiller is part of your routine, the long-term data on daily ibuprofen is worth reading before you talk with your physician.

Does leaky gut affect the liver?

The liver is the first organ to receive what crosses the gut wall, because blood leaving the bowel travels there before it reaches the rest of the body. A loosened wall means more bacterial fragments arriving at a filter that already works around the clock. That delivery, and the fat and alcohol it brings with it, is the subject of the next chapter on the fatty liver.

Sources

1. Scheffler, L., Crane, A., Heyne, H., Tonjes, A., Schleinitz, D., Ihling, C. H., Stumvoll, M., Freire, R., Fiorentino, M., Fasano, A., Kovacs, P., & Heiker, J. T. (2018). Widely used commercial ELISA does not detect precursor of haptoglobin2, but recognizes properdin as a potential second member of the zonulin family. Frontiers in Endocrinology, 9, 22. https://doi.org/10.3389/fendo.2018.00022
2. Ajamian, M., Steer, D., Rosella, G., & Gibson, P. R. (2019). Serum zonulin as a marker of intestinal mucosal barrier function: May not be what it seems. PLoS One, 14(1), e0210728. https://doi.org/10.1371/journal.pone.0210728
3. Power, N., Turpin, W., Espin-Garcia, O., Smith, M. I., CCC GEM Project Research Consortium, & Croitoru, K. (2021). Serum zonulin measured by commercial kit fails to correlate with physiologic measures of altered gut permeability in first degree relatives of Crohn’s disease patients. Frontiers in Physiology, 12, 645303. https://doi.org/10.3389/fphys.2021.645303
4. Kuzma, J. N., Hagman, D. K., Cromer, G., Breymeyer, K. L., Roth, C. L., Foster-Schubert, K. E., Holte, S. E., Weigle, D. S., & Kratz, M. (2019). Intraindividual variation in markers of intestinal permeability and adipose tissue inflammation in healthy normal-weight to obese adults. Cancer Epidemiology, Biomarkers & Prevention, 28(3), 610–615. https://doi.org/10.1158/1055-9965.EPI-18-0641
5. Seethaler, B., Basrai, M., Neyrinck, A. M., Nazare, J. A., Walter, J., Delzenne, N. M., & Bischoff, S. C. (2021). Biomarkers for assessment of intestinal permeability in clinical practice. American Journal of Physiology. Gastrointestinal and Liver Physiology, 321(1), G11–G17. https://doi.org/10.1152/ajpgi.00113.2021
6. Khoshbin, K., Khanna, L., Maselli, D., Atieh, J., Breen-Lyles, M., Arndt, K., Rhoten, D., Dyer, R. B., Singh, R. J., Nayar, S., Bjerkness, S., Harmsen, W. S., Busciglio, I., & Camilleri, M. (2021). Development and validation of test for “leaky gut” small intestinal and colonic permeability using sugars in healthy adults. Gastroenterology, 161(2), 463–475.e13. https://doi.org/10.1053/j.gastro.2021.04.020
7. Dunleavy, K. A., Rypstra, C. R., Busciglio, I., Eckert, D., Ryks, M., Omerdic, E., Chedid, V. G., Raffals, L. E., & Camilleri, M. (2025). Intestinal permeability in vivo in patients with inflammatory bowel disease: comparison of active disease and remission. Inflammatory Bowel Diseases, 31(9), 2511–2520. https://doi.org/10.1093/ibd/izaf043
8. Turpin, W., Lee, S.-H., Raygoza Garay, J. A., Madsen, K. L., Meddings, J. B., Bedrani, L., Power, N., Espin-Garcia, O., Xu, W., Smith, M. I., Griffiths, A. M., Moayyedi, P., Turner, D., Seidman, E. G., Steinhart, A. H., Marshall, J. K., Jacobson, K., Mack, D., Huynh, H., … Croitoru, K. (2020). Increased intestinal permeability is associated with later development of Crohn’s disease. Gastroenterology, 159(6), 2092–2100.e5. https://doi.org/10.1053/j.gastro.2020.08.005
9. Bona, M. D., Torres, C. H. de M., Lima, S. C. V. C., Morais, A. H. de A., Lima, A. Â. M., & Maciel, B. L. L. (2022). Intestinal barrier permeability in obese individuals with or without metabolic syndrome: a systematic review. Nutrients, 14(17), 3649. https://doi.org/10.3390/nu14173649
10. Camilleri, M. (2019). Leaky gut: Mechanisms, measurement and clinical implications in humans. Gut, 68(8), 1516–1526. https://doi.org/10.1136/gutjnl-2019-318427

Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, board certified in anesthesiology, pain medicine, interventional pain management, addiction medicine and obesity medicine. Adapted from Chapter 4 of The Angry Gut by Dr. Padda and Ami Michelle Grimes. Last reviewed September 2026.