Leaky gut: separating the science from the product
The real science, described properly
The gut lining is a single layer of epithelial cells sealed together by tight junctions, sitting under a mucus layer, backed by an immune system. It has to be selectively permeable, because absorbing nutrients is the entire job. "Permeability" is not a defect. It is a regulated property, and the question is always about degree.
That degree can be measured. The best-established research method gives someone two sugars, lactulose and mannitol, and measures the ratio recovered in urine. Because the larger molecule only crosses in meaningful amounts when the barrier is compromised, the ratio estimates barrier integrity. Newer serum markers exist, and zonulin, a protein that regulates tight junctions, is the most widely cited.
The findings are genuine. Permeability is consistently elevated in Crohn's disease and ulcerative colitis compared with healthy controls. It is elevated in untreated coeliac disease and improves on a gluten-free diet. It is an active research topic in disorders of gut-brain interaction, and Gastroenterology published a bench-to-bedside review on exactly that in 2024. This is mainstream gastroenterology.
The question nobody has answered
Everything above is an association. The step that the commercial version takes for granted is the one the field has not taken.
If permeability is elevated in Crohn's disease, there are three readings. It might be an early driver that lets antigens through and triggers the immune response. It might be a downstream consequence of inflammation already damaging the epithelium. It might be a parallel marker of a disturbed gut that does nothing on its own. The evidence does not currently distinguish between these, and different researchers weigh them differently.
This matters enormously in practice, because only the first reading supports the idea that sealing the barrier treats the disease. The other two make barrier repair a bit like treating a fever by cooling the thermometer.
What the experts actually caution
The clinical literature is not dismissive, it is precise. Reviews stress that assessment methods lack consensus on accuracy, and that a positive permeability result should not be communicated to a patient as confirmation of "leaky gut" as a disease entity. They also warn that public-facing information on this topic needs confirmation before it is used to justify dietary exclusions or supplements.
The Canadian Society of Intestinal Research is blunter, publishing a piece specifically debunking "leaky gut syndrome" as a marketed diagnosis while accepting the underlying physiology.
That gap, between a real measurable phenomenon and a diagnosis with a product attached, is the whole of it.
The tests being sold
| Test | Status | What to know |
|---|---|---|
| Lactulose:mannitol ratio | Legitimate research tool | Well founded in principle. Affected by transit time, kidney function, collection accuracy and what you ate. Not validated for individual diagnosis or for guiding treatment. |
| Serum zonulin | Contested | Widely marketed. Commercial assays have been criticised for cross-reacting with proteins that are not zonulin, which undermines interpretation of the results built on them. |
| "Leaky gut panels" | Not validated | Typically bundle zonulin with antibody markers and produce a score. There is no evidence that the score predicts anything or that acting on it improves outcomes. |
What genuinely increases permeability
This list is well documented and is, oddly, far more actionable than anything on a supplement label:
- NSAIDs. Ibuprofen and related drugs measurably increase permeability, reliably enough that they are used to induce it in studies. Regular use is worth reviewing.
- Alcohol. Both acutely and chronically.
- Untreated coeliac disease. Which is a reason to get tested, not to supplement.
- Chemotherapy, radiotherapy, severe burns and critical illness. Where the clinical nutrition evidence, including most of the glutamine research, actually lives.
- Prolonged strenuous endurance exercise, particularly in heat. This is real, well studied, and usually transient.
The supplements, rated honestly
- L-glutamine. The most defensible of the group, and still thin for this purpose. The strong evidence is in critical illness, burns and mucosal injury from cancer treatment. Small trials in post-infectious IBS have been encouraging. It is not established for general gut symptoms.
- Zinc. Deficiency genuinely impairs barrier function. Correcting a documented deficiency is sound. Supplementing without one is not, and high-dose zinc depletes copper.
- Collagen and bone broth. Marketing extrapolation. Protein is digested to amino acids and distributed by need, not delivered to the gut lining because of where it came from.
- Slippery elm, marshmallow root, aloe. Demulcents with a long traditional use and very little trial evidence for this. Low risk, low expectation.
A more useful framing
If you have persistent gut symptoms, the productive question is not "is my gut leaky". It is not answerable, and answering it would not change what you do. The productive questions are the ones with defined answers: have the treatable conditions been excluded, and which specific things reliably precede your own symptoms. Those are boring by comparison and they are the ones that get people their diets back.
Minthe is a wellness and self-tracking tool, not a medical device. It does not diagnose or treat any condition and isn't a substitute for professional medical advice. If you have red-flag symptoms such as blood in your stool, unintentional weight loss, difficulty swallowing, persistent vomiting or a fever alongside gut symptoms, see a clinician rather than tracking them.
Sources
- Intestinal permeability in disorders of gut-brain interaction: from bench to bedside. Gastroenterology, 2024.
- Camilleri M. Leaky gut: mechanisms, measurement and clinical implications in humans. Gut, 2019;68(8):1516-1526.
- Intestinal permeability disturbances: causes, diseases and therapy. 2024 review.
- Canadian Society of Intestinal Research. Debunking the myth of leaky gut syndrome.
Frequently asked
Is leaky gut real?
Intestinal permeability is real, measurable, and actively researched. It is consistently elevated in coeliac disease and in inflammatory bowel disease, and has been studied in disorders of gut-brain interaction including IBS. What is not established is "leaky gut syndrome" as a distinct diagnosis that explains a broad set of unrelated symptoms and is corrected by supplements. Those are two different claims and only the first has support.
Does increased permeability cause disease, or result from it?
This is the central unresolved question and the reason the topic stays contested. Elevated permeability is found alongside several diseases, but whether it initiates them, results from the inflammation they cause, or accompanies them without driving anything is not settled. Most of the commercial framing assumes the first, which is the least supported reading.
Are leaky gut tests worth paying for?
Not for individual diagnosis. The lactulose-to-mannitol ratio is a legitimate research tool, and zonulin is a widely used but contested marker whose commercial assays have been criticised for measuring proteins other than zonulin. Experts caution specifically that a positive permeability result should not be communicated to a patient as confirmation of leaky gut as a disease entity. That is close to exactly what the direct-to-consumer versions do.
Do glutamine, collagen or bone broth repair the gut lining?
The evidence in ordinary adults with gut symptoms is weak. L-glutamine has the most research behind it, mostly in specific clinical settings such as critical illness, burns and chemotherapy-related mucosal injury, with some small IBS trials. Collagen and bone broth are largely marketing extrapolation: dietary protein is digested into amino acids and is not routed preferentially to the gut lining. Zinc deficiency does impair barrier function, which makes correcting an actual deficiency reasonable and supplementing without one much less so.
What genuinely does increase intestinal permeability?
Several well-documented things: NSAIDs such as ibuprofen, alcohol, chemotherapy and radiotherapy, severe burns and critical illness, and prolonged strenuous endurance exercise particularly in the heat. Coeliac disease does, and it improves on a gluten-free diet. This is worth knowing because it is a far more actionable list than the supplement aisle.