Persistent symptoms such as fatigue or brain fog can prompt people to look beyond the part of the body where the problem seems to appear. Gut health often enters that conversation because the intestinal tract sits at an important intersection between the outside environment and the body’s immune and microbial systems.
One molecule frequently discussed in this context is zonulin, a protein involved in regulating intestinal permeability. Research on zonulin has helped scientists explore how the intestinal barrier changes in conditions such as celiac disease. At the same time, commercial zonulin testing has important limitations, and an abnormal result cannot explain nonspecific symptoms on its own.
A systems-based approach considers the gut barrier as one part of whole person health rather than assuming it is the root cause of every inflammatory or systemic symptom.
Explore hol+ digestive health care.
What Is the Gut Barrier?
The gut barrier is a selectively permeable filter separating the contents of the intestine from the body’s internal environment. It needs to absorb nutrients and water while limiting exposure to pathogens and potentially harmful substances.
Research describes this barrier as a dynamic system made up of several interacting components (¹).
- The epithelial lining: A single layer of intestinal epithelial cells forms the physical boundary. Tight junctions between neighboring cells help regulate movement through the spaces between them.
- Mucus and immune defenses: Mucus helps separate microbes from the epithelial surface, while immune tissue monitors activity at the barrier.
- The microbiome: Microorganisms in the intestine interact with food, epithelial cells, mucus, and the immune system.
Intestinal permeability is a normal physiological function. Research now distinguishes regulated pathways through tight junctions from less selective permeability that can occur when the intestinal lining is damaged (²).
“A gut barrier that let nothing through would starve you. The question is never whether it’s permeable—it’s whether it’s regulated.”
Problems may arise when this regulation becomes disrupted by disease, infection, medication exposure, or other factors. That is different from thinking of the gut as simply “open” or “closed.”
Related: What Is Leaky Gut?
Zonulin: A Regulator of Gut Permeability
What is zonulin? Zonulin was originally identified as pre-haptoglobin-2, a protein involved in signaling pathways that can influence tight-junction permeability. It is better understood as one proposed regulator within a much larger barrier system than as a master switch.
“Zonulin isn’t a gate. It’s one hand among many resting on the latch.”
Zonulin has been studied extensively in celiac disease. In genetically susceptible people with celiac disease, gluten-derived gliadin peptides participate in an immune-mediated process that damages the small intestine. Increased intestinal permeability is part of the disease’s pathophysiology, and zonulin-related signaling is one mechanism under investigation (³).
This does not mean gluten causes harmful gut permeability in everyone. Findings from celiac disease cannot automatically be extended to people without the condition. Microbes, inflammatory signaling, diet, epithelial health, mucus, and other proteins also influence gut barrier function.
What Do High or Low Zonulin Levels Mean?
A high commercial zonulin result should be interpreted cautiously because some tests may not reliably reflect intestinal permeability. It cannot by itself confirm leaky gut or another underlying condition.
One widely cited investigation found that a commonly used commercial ELISA did not detect pre-haptoglobin 2 as intended and appeared to recognize other proteins instead (⁴). Recent reviews continue to raise questions about how consistently zonulin testing measures the intended protein across different methods (⁵).
Low zonulin levels are even less clearly understood. Low zonulin is not an established clinical disorder, and there is no accepted treatment goal of raising zonulin through food, supplements, probiotics, or exercise.
How Is Zonulin Tested?
Commercial tests may measure zonulin-related signals in serum or stool, often using antibody-based immunoassays. These methods are not interchangeable, and some assays marketed as measuring zonulin may detect proteins other than pre-haptoglobin 2 (⁴).
For this reason, a zonulin test is not considered a standalone intestinal permeability test. A 2026 systematic review found substantial variation among methods used to assess intestinal-barrier integrity and highlighted the continuing need for standardized, clinically useful tests (⁶).
Evaluation instead depends on the patient’s presentation. Appropriate testing may include celiac serology, blood counts, nutrient testing, selected stool studies, or gastroenterology assessment. Inflammatory markers such as hs-CRP can provide information about inflammation elsewhere in the body, but they cannot determine that the gut is its source.
From Gut Barrier Disruption to Immune Activation
Barrier disruption may alter how microbial products and dietary antigens interact with the immune system, although this does not mean increased permeability directly causes systemic inflammation.
1. Increased Intestinal Permeability
When epithelial junctions or the intestinal surface are disrupted, microbial products or dietary antigens may interact more closely with immune cells. In some disease settings, microbial components can also move beyond the intestinal lumen (⁷).
2. Immune System Activation
The gastrointestinal tract contains extensive immune tissue that continually monitors what passes through the gut and responds when potentially harmful material is detected. Changes in barrier function may alter that exposure and influence immune signaling.
3. Silent or Low-Grade Inflammation
Chronic low-grade inflammation, sometimes described as silent inflammation, can persist without the fever, swelling, or obvious flare associated with an acute inflammatory response.
It can also have many sources. Metabolic dysfunction, adipose tissue, chronic disease, infection, and other inflammatory processes may contribute. The gut is one potential contributor among several (⁸).
As Dr. Taz Bhatia explains, silent inflammation may be subtle, but symptoms such as fatigue or brain fog remain nonspecific. Even an elevated hs-CRP cannot identify where inflammatory activity originated.
Read: Warning Signs of Chronic Inflammation
Why Gut Barrier Dysfunction Can Go Unnoticed
Changes in intestinal barrier function do not necessarily produce a clear digestive symptom pattern. Some gastrointestinal diseases can also have effects beyond the intestinal tract.
Symptoms such as fatigue or brain fog are nonspecific and can have many possible causes. Similar concerns may also reflect another medical condition that needs to be evaluated separately.
Dr. Taz Bhatia describes the challenge this way: “Scattered symptoms tend to get scattered care. Sometimes the most useful thing a clinician does is put them on one page.”
This is where the hol+ concept of signal vs. static can be useful. The goal is not to label scattered symptoms as gut-driven. It is to examine possible relationships across body systems while ruling out other medical explanations.
How Gut Barrier Function Relates to Other Body Systems
Research continues to examine relationships among the microbiome, gut permeability, immune activity, metabolism, and other body systems. Many of these relationships run in both directions, and an association does not establish that barrier dysfunction caused a particular condition (⁷).
| Body system | Potential relationship | Important clinical context |
| Immune | Barrier disruption may influence exposure to microbial products and immune signaling. | This does not establish an autoimmune diagnosis. |
| Metabolic | The microbiome and inflammatory signaling may interact with metabolic health. | Insulin resistance and weight changes have many possible contributors. |
| Brain and nervous system | The gut-brain axis supports two-way communication between the digestive and nervous systems. | Brain fog and mood changes have many possible causes. |
| Hormonal | Metabolic and inflammatory states can interact with endocrine signaling. | Thyroid and reproductive concerns require condition-specific evaluation. |
| Skin | Gut, immune, and skin-barrier activity may be related in some inflammatory conditions. | Persistent skin symptoms still require appropriate evaluation. |
A systems-based medicine framework allows these connections to be considered without assuming that one system explains everything.
Factors That May Affect Gut Barrier Health
There is no single cause of increased intestinal permeability. Relevant contributors differ from one patient to another.
- Diagnosed gastrointestinal disease: Celiac disease and inflammatory bowel disease are established examples in which intestinal-barrier dysfunction can occur. Gastrointestinal infections may also affect the barrier.
- Gut dysbiosis: The microbiome and intestinal barrier influence one another, although a microbiome profile alone cannot establish the cause of symptoms (⁷).
- Medication exposure: NSAIDs can affect the small-intestinal lining and increase permeability in some circumstances. Patients should not stop prescribed or medically necessary medications without discussing them with a healthcare professional.
- Diet and nutrient status: Diet can influence the microbiome and intestinal environment, but no single food universally “opens” or “seals” the gut. Human evidence varies considerably by food and nutrient (1).
- Stress physiology and sleep: Chronic stress and inadequate sleep can affect digestive symptoms, immune signaling, and gut-brain communication.
- Environmental exposures: Some substances may affect gastrointestinal or immune health, but their relevance depends on the exposure, dose, and individual context.
Identifying the most relevant contributor is more useful than assuming every patient needs the same gut-health protocol.
Symptoms That May Warrant a Gut Health Evaluation
No symptom pattern can independently diagnose increased intestinal permeability. A broader gut health evaluation may be appropriate for:
- Persistent diarrhea or constipation
- Recurring abdominal discomfort or unexplained bloating
- Digestive changes following an infection
- Unexplained nutrient deficiencies
- Persistent digestive symptoms alongside fatigue, skin concerns, or other systemic symptoms
Brain fog and joint discomfort can also warrant medical evaluation, but they should not automatically be attributed to the gut.
When to Seek Prompt Medical Care
Seek timely medical evaluation for:
- Blood in the stool
- Black or tarry stool
- Severe abdominal pain
- Persistent vomiting
- Unintentional weight loss
- Persistent fever
- Anemia or signs of dehydration
The Root Cause Medicine Approach to Gut Barrier Health
At hol+, root cause medicine starts with the clinical picture rather than a predetermined gut diagnosis. A physician may review the patient’s history, current symptoms, and daily health patterns before deciding which tests can answer a meaningful clinical question.
Depending on the patient, evaluation may include:
- Standard blood work or nutrient testing
- Celiac serology when indicated
- Condition-specific stool testing
- Inflammatory markers, with the understanding that they do not identify the gut as the source
- Gastroenterology evaluation when symptoms raise concern for celiac disease, inflammatory bowel disease, bleeding, or another gastrointestinal disorder
People being evaluated for celiac disease should generally remain on a gluten-containing diet until appropriate testing is complete. Removing gluten beforehand can interfere with the accuracy of serology and biopsy findings (9).
Commercial zonulin and microbiome tests have important limitations. Some food-sensitivity panels, particularly IgG-based testing, are also not validated for diagnosing food allergy or intolerance. Testing should answer a specific clinical question rather than become a universal gut-health panel.
“‘Leaky gut’ describes a finding, not a diagnosis. The useful question is always what’s making it happen.”
This approach combines evidence-based evaluation with a whole-person health framework rather than relying on a single biomarker.
Supporting Intestinal Barrier Function
Supporting gut barrier health starts with the underlying diagnosis or contributor. Treatment may focus on a gastrointestinal condition, recovery after an infection, medication review, or correction of a nutritional deficiency rather than a universal “gut repair” protocol.
Care may include:
- Nutrition and microbiome support: Dietary changes can focus on nutritional adequacy, fiber, and food variety as tolerated. Probiotics or prebiotic strategies are not necessary for every patient.
- Nutrient repletion: Targeted nutrients, such as zinc and vitamin D, may be considered when testing or clinical history suggests a need. Supplement use should be individualized and reviewed for safety alongside the patient’s broader care plan.
- Stress resilience and sleep: Improving sleep and managing chronic stress may support digestive and whole-person health even when stress is not the primary cause of barrier dysfunction.
- Purposeful dietary changes: Restrictive diets should have a clear rationale and avoid unnecessary long-term limitations. Nutrition counseling can help maintain adequate intake.
L-glutamine has been studied in several gastrointestinal contexts, but it should not be considered a universal treatment for intestinal permeability.
Anyone who may have celiac disease should speak with a clinician before removing gluten from the diet because doing so can make diagnostic testing less reliable (⁹).
FAQs
Can a zonulin test diagnose leaky gut?
Not by itself. Commercial zonulin assays have important specificity and standardization limitations, and there is no single accepted zonulin cutoff that diagnoses increased intestinal permeability. The result needs to be interpreted alongside medical history, symptoms, and condition-specific testing (⁴,⁶).
Does high zonulin mean someone has celiac disease?
No. Increased permeability and zonulin-related signaling are studied in celiac disease, but a high commercial zonulin result does not diagnose it. Evaluation usually begins with celiac-specific blood tests, with biopsy used for confirmation in many patients. Testing should generally occur while gluten is still being consumed (³,⁹).
Can zonulin be high without digestive symptoms?
A commercial zonulin result may be elevated in someone without obvious gastrointestinal symptoms, but the result does not prove that intestinal permeability is causing fatigue, brain fog, skin changes, or other systemic concerns. Some gastrointestinal diseases can also occur with few digestive symptoms, which is why the broader clinical picture is important.
Lasting Health Starts With the Right Clinical Question
The gut barrier is a dynamic, selectively permeable system shaped by the health of the intestinal lining and its interactions with the microbiome and immune system. Its function can also change in response to disease or certain exposures. Zonulin offers one window into that biology, but commercial zonulin measurements do not provide a definitive picture of intestinal-barrier function.
At hol+, gut symptoms and possible barrier concerns are evaluated within a broader system-level health framework. Physician-led care looks for established gastrointestinal conditions and other relevant contributors before developing an individualized plan. If symptoms persist, a personalized evaluation can help identify the factors most relevant to your health and guide the next steps.
