HPC › Nutrition › Glossary
Leaky gut is the colloquial term for increased intestinal permeability: a measurable state in which tight junction protein complexes between adjacent gut epithelial cells widen, allowing bacteria, bacterial fragments, and dietary antigens to breach the intestinal barrier and pass into systemic circulation. The condition is assessed non-invasively using the lactulose:mannitol urinary excretion ratio.
'Leaky gut syndrome', promoted in wellness circles as a root cause of diverse symptoms, is not a recognised clinical diagnosis, and commercial serum zonulin testing lacks validated reliability.
How it works
The intestinal barrier is maintained by tight junction protein complexes, including claudins, occludins, and zonulin-sensitive junctional proteins, that connect adjacent epithelial cells and regulate paracellular transport. Turner et al. describe three distinct pathways: the pore pathway handles small ions; the leak pathway accommodates larger macromolecules; and the unrestricted pathway is activated during pathological states such as apoptotic cell shedding.4
Zonulin is the only known endogenous regulator of intercellular tight junctions.2 Its upregulation reversibly opens the paracellular space, increasing permeability to macromolecules. Dysregulation of the zonulin pathway is observed across multiple inflammatory, autoimmune, and metabolic conditions. When the barrier is compromised, bacterial lipopolysaccharide (LPS) enters systemic circulation, activating Toll-like receptor 4 (TLR4) signalling and sustaining low-grade endotoxaemia.2
Exogenous factors that disrupt tight junction integrity include NSAIDs, excess alcohol, certain luminal bacteria, and dietary patterns high in ultra-processed foods.3 These act through distinct molecular mechanisms, including cytokine-driven downregulation of claudins and direct enterocyte damage. The lactulose:mannitol urinary ratio remains the most widely used non-invasive permeability measure for research purposes, with an elevated ratio indicating paracellular barrier dysfunction, though no single biomarker is currently validated for routine clinical use.13
Example
A person presents to a functional-medicine clinic with fatigue, bloating, and brain fog. A serum zonulin test returns an elevated reading, and the clinician diagnoses 'leaky gut syndrome'. The same individual's lactulose:mannitol ratio, ordered by a gastroenterologist, falls within the normal range. The apparent conflict arises because commercial serum zonulin assays lack analytical validity and do not correlate reliably with barrier function in clinical populations.
The underlying physiology is real; the commercial serum test most widely used to diagnose it is not.
Why it matters
Increased intestinal permeability is associated with inflammatory bowel disease, irritable bowel syndrome, coeliac disease, type 1 diabetes, and graft-versus-host disease.3 For most conditions in this list, the evidence establishes association rather than causation: normalising barrier function alone has not been shown to resolve disease. The more actionable finding is that elevated permeability is detectable in first-degree relatives of Crohn's disease patients before symptom onset, suggesting that barrier disruption may precede disease rather than simply accompany it.1
Dietary approaches targeting barrier integrity centre on increasing fermentable fibre to support short-chain fatty acid production, which strengthens tight junctions, and on reducing NSAIDs, excess alcohol, and ultra-processed food.3 Randomised trial evidence for these interventions is emerging but not yet definitive. LPS-driven endotoxaemia links barrier dysfunction to metabolic syndrome and non-alcoholic fatty liver disease, extending the gut barrier's significance well beyond digestive symptoms.2
What is leaky gut and is it a real medical condition?+
"Leaky gut" is the colloquial label for increased intestinal permeability, a well-documented physiological state in which tight junction proteins between gut cells widen. The underlying biology is real; "leaky gut syndrome" as a formal clinical diagnosis is not recognised, and the wellness framing of it as a universal root cause lacks clinical support.
What causes increased intestinal permeability?+
Several factors disrupt the tight junction proteins that seal the gut lining. NSAIDs, excess alcohol, certain gut bacteria, and diets high in ultra-processed foods all compromise barrier integrity through distinct molecular mechanisms, including cytokine-driven downregulation of claudin proteins and direct damage to enterocytes.
Which health conditions are associated with a leaky gut?+
A compromised gut barrier is linked to inflammatory bowel disease, irritable bowel syndrome, coeliac disease, type 1 diabetes, and graft-versus-host disease, though for most the evidence shows association, not causation. LPS leaking into circulation also drives low-grade endotoxaemia associated with metabolic syndrome.
Can diet repair intestinal barrier function?+
The most evidence-backed approach is increasing fermentable fibre, which supports short-chain fatty acid production and tightens junctions, alongside reducing NSAIDs, alcohol, and ultra-processed food. Randomised trial evidence for these interventions is growing but not yet definitive; barrier restoration through diet alone has not been demonstrated.
