Leaky gut refers to a condition where the lining of your small intestine becomes more porous than it should be, allowing partially digested food particles, bacteria, and other substances to pass through into your bloodstream. The medical term is “increased intestinal permeability,” and while it’s a measurable biological phenomenon, the broader “leaky gut syndrome” remains a subject of debate among doctors.
Understanding the difference matters. Increased intestinal permeability is real, testable, and well-documented in conditions like celiac disease and inflammatory bowel disease. What’s less settled is whether a leaky gut on its own causes the wide range of health problems sometimes attributed to it.
How Your Gut Lining Normally Works
The inside of your small intestine is lined with a single layer of cells packed tightly together. Between those cells are structures called tight junctions, which act like gatekeepers. They allow water and nutrients to pass through while keeping larger molecules, bacteria, and toxins locked inside the intestinal tube.
Your body actually has a built-in system for opening and closing these gates. A protein called zonulin is the only human protein identified so far that reversibly controls tight junction openness. When zonulin is released, it triggers a chain reaction: the protein scaffolding that holds cells together gets loosened, and the gaps between cells temporarily widen. Once the signal stops, the junctions tighten back up. In a healthy gut, this process is tightly regulated. Problems start when zonulin stays elevated or when other factors damage the lining faster than it can repair itself.
What Triggers Increased Permeability
Several well-studied factors can physically degrade the intestinal barrier or keep those tight junctions open longer than they should be.
Common pain relievers (NSAIDs): Drugs like ibuprofen and aspirin damage the gut through what researchers describe as a “three-hit” process. First, they directly injure the membrane of mucosal cells. Then they damage the energy-producing structures inside those cells, leading to a buildup of damaging free radicals. Finally, this disrupts the junctions between cells, increases permeability, and allows gut contents to trigger inflammation. NSAIDs also reduce the protective mucus that coats the intestinal wall by blocking the chemical signals that tell your body to produce it.
Alcohol: Chronic alcohol use is a well-established cause of gut barrier damage, though the mechanism works somewhat differently, involving direct toxic effects on intestinal cells and changes to the gut’s microbial balance.
Food additives in ultra-processed foods: Emulsifiers, the compounds that give processed foods their smooth texture, can thin the protective mucus layer in your gut, reduce bacterial diversity, and directly impair tight junction proteins. Common examples include polysorbate 80, carboxymethylcellulose, and various gums used in low-fat and shelf-stable products. Artificial sweeteners present a similar concern. Acesulfame K consistently reduces microbial diversity and promotes inflammation in preclinical studies, while sucralose has shown mixed results. Aspartame tends to shift the gut microbiome toward a less favorable profile.
Chronic stress, infections, and existing digestive disease: Conditions like celiac disease and inflammatory bowel disease (Crohn’s and ulcerative colitis) are strongly linked to increased permeability. In celiac disease, gluten directly triggers zonulin release in susceptible people, opening tight junctions. In ulcerative colitis, studies using intestinal tissue samples have found a 50% decrease in the electrical resistance of the gut wall compared to healthy tissue, a direct measure of barrier breakdown.
Symptoms Linked to a Permeable Gut
There’s no single symptom that points directly to increased intestinal permeability. Instead, the symptoms you notice usually come from whatever is injuring your gut lining in the first place, or from the downstream effects of substances crossing into your bloodstream.
Local gut symptoms include bloating, gas, diarrhea, a burning sensation in the stomach or intestines, painful indigestion, and food sensitivities. Many people describe feeling generally unwell after eating, with reactions to foods that didn’t previously bother them.
The systemic picture is broader and less well-defined. When bacterial fragments and other molecules cross the intestinal barrier, they can trigger a low-grade inflammatory response throughout the body. This type of chronic, simmering inflammation has been linked to fatigue, joint pain, skin problems, brain fog, and mood changes. Research has documented increased intestinal permeability in a wide range of conditions, including non-alcoholic fatty liver disease, type 2 diabetes, cardiovascular disease, anxiety and depression, and even cognitive decline associated with dementia. Fatigue and depression have been specifically correlated with increased immune cell activity in the gut lining of people with irritable bowel syndrome.
The unresolved question is causation versus correlation. It’s clear that a leaky gut accompanies many of these conditions. Whether it’s the spark that starts them or a consequence of the same underlying process is still being worked out.
How Intestinal Permeability Is Tested
If your doctor wants to measure gut permeability directly, the standard research method is the lactulose-mannitol test. You drink a solution containing two sugars: lactulose (a larger molecule that shouldn’t cross a healthy gut barrier easily) and mannitol (a smaller one that crosses freely). You then collect urine for several hours, and the ratio of the two sugars in your urine reflects how permeable your gut is. A higher ratio means more of the large sugar leaked through.
In practice, this test is used mainly in research settings. There’s no universally agreed-upon cutoff for “too permeable,” and results vary depending on collection timing. Research suggests the most reliable window for urine collection is between two and a half to four hours after drinking the sugar solution. Some doctors may use blood tests for zonulin levels or markers of bacterial translocation, but these aren’t standardized either.
The Medical Debate Around “Leaky Gut Syndrome”
This is where things get complicated. Increased intestinal permeability is an accepted, measurable phenomenon in gastroenterology. But “leaky gut syndrome” as a standalone diagnosis, the idea that a permeable gut is the root cause of dozens of seemingly unrelated conditions, is not recognized as a formal medical diagnosis by most mainstream medical organizations.
Cleveland Clinic describes the situation plainly: many people who believe they have leaky gut are experiencing real gastrointestinal symptoms like abdominal pain, bloating, food sensitivities, and indigestion. The disagreement is over whether increased permeability is the primary driver or a secondary effect of something else going on. Chronic low-grade inflammation may indeed connect gut permeability to metabolic disorders, arthritis, chronic fatigue syndrome, asthma, and fibromyalgia, but the causal chain isn’t fully mapped.
Supporting Gut Barrier Repair
While there’s no single “leaky gut cure,” several approaches have evidence behind them for supporting intestinal barrier function.
Reducing known triggers: Cutting back on NSAIDs, alcohol, and heavily processed foods addresses the most common sources of barrier damage. For people with celiac disease, strict gluten avoidance is essential. In one clinical trial, a gluten challenge caused a 70% increase in intestinal permeability in the placebo group, while those given a zonulin-blocking compound maintained normal permeability.
Glutamine: This amino acid is the primary fuel source for the cells lining your intestine. In a controlled study of exercise-induced gut permeability (heat and intense activity reliably increase it), glutamine supplementation reduced permeability in a dose-dependent manner. Even the lowest dose tested reduced markers of leakiness, while the highest dose cut permeability by about 33%. Glutamine is available as a supplement, often in powder form.
Probiotics: Specific strains of Bifidobacterium have shown the ability to directly strengthen tight junctions. B. infantis, in particular, has been studied extensively. In animal models, it preserved the localization of key tight junction proteins (occludin and claudin-4) and reduced intestinal permeability. B. bifidum and B. longum subspecies infantis reduced inflammation markers and enhanced barrier integrity in models of gut injury. A broader review of probiotic research found that Lactobacillus and Bifidobacterium species, in general, enrich the gut, reduce inflammation, and improve barrier function.
Dietary fiber and whole foods: Beneficial gut bacteria ferment dietary fiber into short-chain fatty acids like butyrate, which directly nourish intestinal lining cells and help maintain tight junction integrity. A diet built around vegetables, fruit, legumes, and whole grains supports this process, while diets high in ultra-processed foods do the opposite.