What Is Leaky Gut Syndrome — and Can It Be Fixed?

Originally published March 2026 · Updated May 2026 with the September 2024 Jagiellonian University Medical College intestinal permeability review, the May 2025 ScienceDirect meta-analysis of 68 clinical trials on probiotics and gut barrier function, and the Cleveland Clinic 2025 position on leaky gut syndrome

Few health concepts have generated as much simultaneous clinical interest and wellness industry noise as leaky gut. Supplement companies have built entire product lines around it. Functional medicine practitioners cite it as the root cause of dozens of disparate conditions. And many conventional physicians dismiss it entirely as an invented diagnosis.

The reality is more nuanced than either extreme. Increased intestinal permeability — the biological phenomenon underlying what is popularly called leaky gut — is real, measurable, and increasingly well-understood. Its relationship to disease is the subject of serious ongoing research. The broader claim that it is a discrete syndrome causing everything from autoimmune conditions to depression is where the evidence gets considerably thinner.

Understanding the distinction between these two things — the real biology and the overextended claim — is the most useful thing anyone can know about leaky gut.

TL;DR

• Increased intestinal permeability — sometimes called leaky gut — is a real, measurable condition in which the gut lining allows substances to pass through that a healthy gut barrier would block. According to the Cleveland Clinic, the condition of having intestinal permeability or a leaky gut is real, but leaky gut syndrome as a distinct diagnosis is hypothetical.
• The gut barrier depends on proteins called tight junctions — claudins, occludins, and zonulin-regulated complexes — that control what passes between gut cells. When these proteins are disrupted, permeability increases.
• Leaky gut is consistently associated with inflammatory bowel disease, MASLD, type 2 diabetes, coeliac disease, and sepsis. Whether it causes these conditions or results from them — or both — is an area of active research.
• A September 2024 review in Clinical and Experimental Medicine confirmed that damage to the intestinal barrier leads to dysbiosis, translocation of microorganisms into intestinal tissue, immune response, and development of chronic inflammation.
• A May 2025 meta-analysis of 68 clinical trials found that probiotics, synbiotics, and prebiotics measurably reduce key intestinal permeability markers.
• The most evidence-backed approach combines removing known disruptors — ultra-processed food, excess alcohol, NSAIDs, chronic stress — with actively supporting barrier repair through dietary diversity, fermented foods, and adequate sleep.

What the Gut Barrier Actually Is

The intestinal lining is one of the body's most demanding structures — a single layer of epithelial cells covering a surface area of approximately 30 square metres, separating the contents of the gut from the rest of the body while simultaneously absorbing nutrients and water.

This barrier works through two mechanisms. The transcellular route — through the cells themselves — is highly selective, allowing specific nutrients to pass via dedicated transport proteins. The paracellular route — between cells — is regulated by protein complexes called tight junctions.

Tight junctions are the primary determinant of intestinal permeability. According to a 2024 review published in Clinical and Experimental Medicine by researchers at Jagiellonian University Medical College, an increase in intestinal permeability is caused primarily by changes in the functioning and expression levels of tight junction proteins. The key proteins involved include claudins, occludins, and zonula occludens proteins. When these are disrupted — by inflammation, dietary insults, pathogens, or stress — the spaces between cells widen and substances that should remain in the gut can pass through.

What substances? Bacterial lipopolysaccharides — fragments of the outer membrane of gram-negative bacteria — are among the most clinically significant. According to the 2024 Jagiellonian review, damage to the intestinal barrier leads to dysbiosis, translocation of microorganisms deep into intestinal tissue, immune response, and development of chronic inflammation. When LPS reaches systemic circulation, it activates toll-like receptors on immune cells, triggering the inflammatory signalling that is consistently found in the conditions associated with increased intestinal permeability.

The Zonulin Question

Zonulin is a protein produced by intestinal cells that regulates tight junction opening — essentially the body's own mechanism for temporarily increasing permeability. It was identified by Alessio Fasano and colleagues and became a focus of leaky gut research because elevated serum zonulin is measurable in blood and has been proposed as a biomarker for intestinal permeability.

Gliadin — the protein component of gluten — activates zonulin release in the gut. This was initially proposed as a mechanism through which gluten exposure increases intestinal permeability in people without coeliac disease — a significant claim that drove much of the non-coeliac gluten sensitivity debate.

The zonulin story has become more complicated with subsequent research. Questions have been raised about whether standard zonulin assays are measuring zonulin specifically or other proteins in the same family. The clinical significance of elevated serum zonulin in people without inflammatory bowel disease or coeliac disease is still being established. It remains a biologically plausible and actively studied marker but is not yet a validated diagnostic test for increased intestinal permeability in general clinical use.

The Conditions It Is Associated With

The strongest and most consistently replicated evidence for increased intestinal permeability is in specific clinical conditions — not as a general wellness phenomenon.

Inflammatory bowel disease — Crohn's disease and ulcerative colitis are the conditions with the best-established intestinal permeability connection. According to a review in Clinical Gastroenterology and Hepatology, people with IBD display several defects in mucosal barrier components, from mucus layer composition to tight junction protein expression. In Crohn's disease, increased intestinal permeability may represent a primary dysfunction — present before clinical disease — rather than merely a consequence of inflammation. In first-degree relatives of people with Crohn's disease who do not themselves have IBD, elevated intestinal permeability has been detected, suggesting a genetic contribution to barrier function.

MASLD and liver disease — the gut-liver axis connects intestinal permeability directly to liver disease. When bacterial LPS and other microbial products translocate through a permeable gut wall into the portal circulation, they reach the liver and activate the hepatic inflammatory responses that drive MASLD progression, alcoholic liver disease, and cirrhosis. According to the 2024 Jagiellonian review, leaky gut is associated with alcoholic liver disease, non-alcoholic fatty liver disease, steatohepatitis, and liver cirrhosis.

Type 2 diabetes — multiple studies have demonstrated altered tight junction protein expression and elevated markers of intestinal permeability in people with type 2 diabetes. The relationship is likely bidirectional — hyperglycaemia damages the intestinal epithelium and its tight junctions, and the endotoxaemia that results from increased permeability drives the systemic inflammation that worsens insulin resistance.

Coeliac disease — intestinal permeability is a core feature of active coeliac disease, driven by the immune response to gliadin that destroys intestinal villi and disrupts epithelial barrier function. In this case, the cause is well understood and the intervention — strict gluten avoidance — produces measurable improvements in permeability markers as the gut lining heals.

Sepsis — according to a review in Frontiers in Medicine, both a leaky gut and gut dysbiosis are intrinsic to sepsis. While sepsis itself can cause dysbiosis, dysbiosis can worsen sepsis. The leaky gut syndrome refers to a status with increased intestinal permeability allowing the translocation of microbial molecules from the gut into the blood circulation.

What Causes Intestinal Permeability to Increase

The 2024 Jagiellonian review identified the primary drivers of increased intestinal permeability as predominantly from the external environment — modifiable factors rather than inevitable biological processes.

Diet — ultra-processed food is among the most consistently identified dietary disruptors of gut barrier function. Emulsifiers — including polysorbate 80, carboxymethylcellulose, and carrageenan — directly disrupt the mucus layer that protects the gut epithelium from bacterial contact. The low fibre content of ultra-processed diets reduces the short-chain fatty acid production that fuels colonocytes and maintains tight junction protein expression. High-fat, high-sugar diets consistently reduce barrier function in both animal and human studies.

Alcohol — ethanol and its metabolite acetaldehyde directly damage tight junction proteins and increase intestinal permeability. Even moderate alcohol consumption produces measurable increases in gut permeability, with effects that become more significant with regular use. According to the Cleveland Clinic, chronic overuse of alcohol or NSAIDs, such as aspirin and ibuprofen are among the factors that can erode the intestinal lining.

NSAIDs — non-steroidal anti-inflammatory drugs including aspirin and ibuprofen reduce prostaglandin synthesis in the gut mucosa, impairing the protective mechanisms that maintain barrier integrity. Regular NSAID use consistently increases intestinal permeability in clinical studies.

Chronic stress — activates the HPA axis and produces cortisol, which alters gut motility, reduces mucus production, and increases mast cell activation in the gut wall. Psychological stress has measurable effects on intestinal permeability through these mechanisms — explaining one route through which chronic stress contributes to gastrointestinal symptoms.

Gut dysbiosis — the composition of the gut microbiome directly influences tight junction protein expression. Reduced butyrate-producing bacteria — consistently found in dysbiosis — leads to reduced butyrate availability for colonocytes, impairing the cellular energy and signalling required for tight junction maintenance.

Antibiotics — by disrupting gut microbial community composition, antibiotics alter the microbial signals that regulate tight junction expression. The extent and duration of this effect depend on the antibiotic class and the individual's microbiome resilience.

The "Syndrome" Question: Where the Evidence Gets Thin

The Cleveland Clinic's position is worth quoting directly: the theory of leaky gut syndrome suggests that anything that injures your gut lining can lead to intestinal permeability if the injury is persistent enough. Scientists aren't sure about this, but they do agree that these everyday factors may cause inflammation in your gut lining and uncomfortable GI symptoms.

The distinction between "intestinal permeability is real and clinically relevant" and "leaky gut syndrome is a distinct condition causing a wide range of non-gut diseases" is important.

The former has substantial evidence. The latter — specifically the claim that increased intestinal permeability is the primary cause of conditions as varied as autoimmune disease, mental health conditions, skin disorders, and fatigue — runs significantly ahead of the current evidence. These conditions are associated with gut dysbiosis and gut-immune interactions, and intestinal permeability may be one of several mechanisms involved. But the direct causal evidence that fixing intestinal permeability cures these conditions is not established.

This matters practically because it determines what interventions are evidence-based and what represents overreach. Addressing gut barrier health through dietary and lifestyle means that have evidence for barrier support is sensible. Taking expensive supplement stacks sold specifically to "heal leaky gut" based on testimonials rather than trials is a different proposition.

What Actually Helps: The Evidence Base

Dietary diversity and fibre — diverse plant foods provide the fermentable fibre substrates that produce butyrate, the primary fuel for colonocytes and the most important nutritional support for tight junction function. Aiming for 30 different plant foods per week maintains the microbial diversity required for consistent butyrate production.

Fermented foods — a May 2025 meta-analysis and systematic review of 68 clinical trials published in ScienceDirect found that probiotics, synbiotics, and prebiotics measurably reduce key intestinal permeability markers. The effects were found across diverse populations and conditions. Fermented foods — kefir, yoghurt, kimchi, sauerkraut — provide live bacterial strains alongside the fermentation byproducts that support gut barrier function.

Reducing ultra-processed food — removing the emulsifiers, additives, and low-fibre content of ultra-processed foods addresses multiple gut barrier disruption mechanisms simultaneously. This is likely the highest-impact single dietary change for gut barrier health.

Glutamine — the primary fuel source for enterocytes, the cells of the gut lining. Glutamine supplementation has evidence for maintaining gut barrier function in clinical populations under physiological stress — including post-surgery and critically ill patients. Evidence in healthy populations with no specific trigger for increased permeability is less robust.

Zinc — involved in tight junction protein structure and has evidence for reducing intestinal permeability in clinical studies, particularly in conditions like IBD. Found in meat, shellfish, legumes, and seeds. Deficiency is relatively common and contributes to impaired barrier function.

Vitamin D — has direct effects on tight junction protein expression and immune regulation in the gut mucosa. Deficiency — very common in the UK — is associated with reduced gut barrier integrity. Supplementing to maintain adequate vitamin D levels (above 75 nmol/L) supports gut barrier function alongside its other effects.

Sleep — the gut has its own circadian rhythms. Disrupted sleep disrupts gut microbial rhythms and reduces the SCFA production that maintains barrier function. Consistent, adequate sleep is one of the most underappreciated gut barrier interventions.

Stress management — reducing chronic HPA axis activation directly reduces cortisol-mediated gut barrier disruption. The gut-brain axis means this is not merely indirect — it operates through specific physiological pathways connecting psychological stress to intestinal permeability.

Reducing alcohol and NSAID use — the most direct pharmacological disruptors of gut barrier function. Reducing these removes a primary ongoing cause of permeability rather than simply attempting to repair damage they continue to cause.

Frequently Asked Questions

Is leaky gut syndrome a real condition?

Increased intestinal permeability — the biological phenomenon underlying leaky gut — is real and measurable. The Cleveland Clinic states that the condition of having intestinal permeability or a leaky gut is real, but leaky gut syndrome as a discrete diagnosis causing a wide range of conditions is more contested. The evidence for permeability being clinically significant in conditions like IBD, MASLD, coeliac disease, and type 2 diabetes is strong. The evidence for it being the primary cause of diverse non-gut conditions is considerably weaker.

What causes leaky gut?

The primary drivers identified in the current research include ultra-processed food — particularly emulsifiers that disrupt the gut mucus layer — chronic alcohol consumption, regular NSAID use, gut microbiome dysbiosis, chronic psychological stress, and certain medications including antibiotics. These factors reduce tight junction protein expression, impair colonocyte function, and disrupt the microbial signals that maintain barrier integrity.

What are the symptoms of leaky gut?

Increased intestinal permeability does not have a specific symptom set. The most commonly reported symptoms associated with gut barrier disruption include bloating, abdominal pain, food sensitivities, irregular bowel habits, and indigestion. However, these symptoms overlap with many other gut conditions. Fatigue, brain fog, and skin reactivity are frequently attributed to leaky gut in wellness contexts but have weaker direct evidence for intestinal permeability as the primary cause.

Can leaky gut be tested?

Several tests are used in research and some clinical settings. The lactulose-mannitol ratio test measures how much of these sugars passes through the gut wall into urine — an established research measure of intestinal permeability. Serum zonulin has been proposed as a blood biomarker but its clinical reliability is still being established. Confocal laser endomicroscopy allows direct visualisation of gut barrier integrity. None of these is yet a standard routine clinical test in NHS practice.

What foods heal leaky gut?

The foods with the strongest evidence for supporting gut barrier integrity are those that provide butyrate-producing bacterial substrates — diverse plant foods, legumes, oats, and other fermentable fibres. Fermented foods providing live bacterial strains support the microbial environment required for barrier function. Zinc-rich foods, vitamin D from oily fish and sunlight, and glutamine-containing protein sources support the cellular structures of the gut lining directly. Removing ultra-processed emulsifiers and reducing alcohol addresses the primary ongoing disruptors.

Can leaky gut cause autoimmune conditions?

The relationship between intestinal permeability and autoimmune disease is an active area of research rather than an established causal link. Increased intestinal permeability is associated with several autoimmune conditions including type 1 diabetes, rheumatoid arthritis, and multiple sclerosis. Whether permeability contributes to the development of these conditions or results from the same inflammatory processes — or both — is not yet clearly established. Researcher Alessio Fasano has proposed a model in which increased intestinal permeability is a necessary precursor for autoimmune disease in genetically predisposed individuals, but this remains a hypothesis under investigation rather than proven causation.

The Bottom Line

The gut barrier is genuinely important and genuinely modifiable. Increased intestinal permeability is real biology with measurable clinical consequences — particularly in conditions like IBD, liver disease, coeliac disease, and type 2 diabetes. The drivers of barrier disruption are largely dietary and lifestyle factors that are within reach of evidence-based intervention.

The wellness industry's version of leaky gut — as a singular syndrome responsible for everything from joint pain to depression, fixable with a specific supplement protocol — significantly overreaches the evidence. The biology is real; many of the specific claims built on top of it are not.

The most useful framing: gut barrier health is one important component of overall gut health, supported by the same foundations — dietary diversity, fermented foods, adequate sleep, stress management, and reduced ultra-processed food — that support the gut microbiome and the whole-body systems it influences.

For a structured approach to building the dietary and lifestyle habits that support gut barrier health, the Gut Reset from the Reset Series™ covers the evidence-based foundations most directly relevant to intestinal permeability and microbiome health. Pair it with the Reset Companion for personalised guidance as you put the principles into practice.

Related reading: The Science Behind the Gut Reset · Why Am I So Bloated? The Gut Science Behind Persistent Bloating · Diet, the Gut Microbiome and IBS: What the Evidence Now Shows

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