"Leaky gut" — or increased intestinal permeability — has become one of the most discussed topics in functional and integrative medicine. While the concept was once controversial, research now clearly supports that the intestinal barrier can become compromised, allowing substances to pass through that should not, and that this contributes to a wide range of health conditions.
What Is Leaky Gut?
Your intestinal lining is held together by tight junction proteins that form a selective barrier. When functioning properly, this barrier allows nutrients to pass through into the bloodstream while keeping out harmful substances like undigested food particles, bacterial endotoxins (lipopolysaccharides), and pathogens. When these tight junctions become loose or damaged, the barrier becomes "leaky," allowing substances to pass through that trigger immune and inflammatory responses.
A note on terminology is worth making up front. "Leaky gut" is the popular name; in the scientific literature the phenomenon is described as increased intestinal permeability or intestinal barrier dysfunction. Researchers have documented that the barrier can indeed become more permeable, and that this is observed in conditions such as inflammatory bowel disease and celiac disease. What remains debated is how often increased permeability is a primary cause of symptoms versus a consequence of another underlying problem, and "leaky gut" is not itself a formal medical diagnosis. The practical takeaway is that barrier function is real and measurable, but it should be interpreted carefully and in context rather than treated as a catch-all explanation.
What Causes the Gut Barrier to Break Down?
Intestinal permeability has many potential contributors, which is part of why it can be hard to pin down. Commonly discussed factors include chronic gut infections and dysbiosis (an imbalance between beneficial and opportunistic microbes), diets low in fiber and high in ultra-processed foods, chronic stress, heavy or frequent alcohol use, certain medications such as NSAIDs, and depletion of the fiber-fermenting bacteria that produce butyrate — a short-chain fatty acid that serves as the primary fuel for the cells lining the colon. Because the causes overlap and feed into one another, identifying which factors are at play for a given person usually requires looking at the gut as a whole rather than at permeability alone.
Sign 1: Multiple Food Sensitivities
If you find yourself reacting to an increasing number of foods — especially foods that never bothered you before — this may indicate a compromised gut barrier. When undigested food particles cross the intestinal barrier, your immune system can develop antibodies against them, creating food sensitivities. Many people with leaky gut notice that their list of trigger foods keeps growing over time.
Sign 2: Autoimmune Conditions
Research led by Alessio Fasano and others has proposed that increased intestinal permeability is one of the contributing factors in the development of autoimmune disease in genetically susceptible individuals, alongside a genetic predisposition and an environmental trigger. If you have been diagnosed with an autoimmune condition — including Hashimoto's thyroiditis, rheumatoid arthritis, type 1 diabetes, celiac disease, or lupus — barrier function is one of the gut-related factors a practitioner may consider. This is a model of biological mechanism rather than a guarantee that addressing the gut will resolve an established autoimmune condition. We cover this relationship in more depth in our article on the gut-autoimmune connection.
Sign 3: Chronic Skin Issues
The gut-skin axis is well-documented. Conditions like eczema, psoriasis, acne, and rosacea have all been associated with gut dysfunction and increased intestinal permeability. When the gut barrier is compromised, the resulting systemic inflammation often manifests visibly in the skin.
Sign 4: Brain Fog and Fatigue
When bacterial endotoxins (LPS) cross a leaky gut barrier into the bloodstream, they trigger systemic inflammation that can affect the brain. This neuroinflammation can manifest as persistent brain fog, difficulty concentrating, poor memory, and chronic fatigue that is not explained by sleep quality or other factors.
Sign 5: Digestive Symptoms That Do Not Respond to Basic Treatment
If you have tried dietary changes, probiotics, and basic gut protocols without lasting improvement, a compromised gut barrier may be one of several underlying issues worth investigating before other interventions can be effective. Persistent bloating, irregular bowel habits, and food reactions that do not settle with simple measures are a reasonable prompt to look more systematically at what is happening in the gut.
A Word of Caution About Self-Diagnosis
It is important to recognize that every symptom on this list is non-specific — meaning it can be caused by many things that have nothing to do with intestinal permeability. Fatigue, brain fog, skin issues, and digestive complaints can stem from thyroid problems, anemia, sleep disorders, mental-health conditions, and a long list of other causes. The presence of these signs does not confirm leaky gut, and important conditions can be missed if symptoms are attributed to it prematurely. The sensible path is to treat these signs as a reason to investigate with a clinician, not as a self-diagnosis.
What May Support a Healthy Gut Barrier
Research on supporting intestinal barrier function is ongoing, but several themes recur in the literature. Adequate dietary fiber feeds the bacteria that produce butyrate, the short-chain fatty acid that fuels the cells of the colon lining. Diets rich in whole, minimally processed foods, managing chronic stress, moderating alcohol, and limiting unnecessary use of gut-irritating medications such as NSAIDs are all commonly discussed. Certain nutrients and probiotics have been studied for barrier support as well. None of these are guaranteed fixes, and the right combination depends on the individual — which is why identifying the specific drivers in your gut, ideally with testing and a knowledgeable practitioner, tends to be more productive than applying a generic protocol.
How Intestinal Permeability Is Assessed
There is no single perfect test for barrier function, and clinicians use several approaches. Historically, research has used the lactulose-mannitol urine test, which measures how much of two ingested sugar molecules cross the gut wall. More recently, blood and stool markers have been used as practical proxies. Zonulin — the protein that regulates tight-junction permeability — is among the most widely used of these markers, with elevated levels interpreted as a sign of increased permeability. It is worth knowing that zonulin measurement has limitations and is still an area of active research, so it is best read as one data point rather than a definitive verdict.
The GI-MAP™ + Zonulin package adds a direct zonulin measurement to the comprehensive panel. The advantage of combining the two is context: rather than measuring permeability in isolation, it lets a practitioner see the zonulin result alongside the gut infections, dysbiosis, low beneficial bacteria, and inflammation (via calprotectin) that may be driving barrier dysfunction in the first place. If your symptoms point more toward a specific infection, our guides on H. pylori and on how molecular testing compares to conventional stool culture may help you decide where to start. You can review options and pricing or order a kit online when you are ready. As always, interpret results with a qualified practitioner — barrier markers guide a plan, they do not replace clinical judgment.
Sources
This article summarizes peer-reviewed literature on intestinal permeability. "Leaky gut" is not a formal diagnosis; discuss persistent symptoms with a licensed healthcare provider.
- Alterations in Intestinal Permeability: The Role of the "Leaky Gut" in Health and Disease (PMC)
- The Intestinal Barrier and Current Techniques for the Assessment of Gut Permeability (PMC)
- Fasano A. Intestinal permeability and its regulation by zonulin: diagnostic and therapeutic implications (PubMed)
- Intestinal Permeability in Inflammatory Bowel Disease: Pathogenesis, Clinical Evaluation, and Therapy of Leaky Gut (PMC)