Intestinal Permeability —
The Leaky Gut Explained
Intestinal permeability is a measurable physiological state — not a speculative concept. When the gut lining is compromised, it drives food sensitivity, systemic inflammation, immune dysregulation, and symptoms that appear far removed from the gut. Here is what it is, what causes it, what it drives, and how it is assessed in UK clinical practice.
What Intestinal Permeability Actually Is
The gut lining is a single cell layer thick — roughly the surface area of a tennis court — held together by tight junction proteins that act as gatekeepers between the gut contents and the bloodstream. These proteins — including occludin, claudin, and zonulin-regulated junctions — control what passes through: nutrients in, everything else out.
When this barrier is compromised, the tight junctions open beyond their normal parameters. Partially digested food particles, bacterial fragments, lipopolysaccharides (LPS) from gram-negative bacteria, and other substances cross into circulation that should not. The immune system — 70-80% of which is located in the gut — encounters these substances and mounts a response. Over time, this drives chronic low-grade inflammation that can manifest as symptoms far removed from the gut itself.
The term "leaky gut" is informal but the physiology is not speculative. Zonulin — the primary protein regulating tight junction opening — is measurable. Elevated zonulin is documented in peer-reviewed literature in association with coeliac disease, type 1 diabetes, IBS, inflammatory bowel disease, non-alcoholic fatty liver disease, and several autoimmune conditions. The mechanism is real. The question is what is driving it and how significantly.
The mainstream medical hesitation around leaky gut is not about the physiology — intestinal permeability as a measurable state is well established. The hesitation is about causation: whether increased permeability is a driver of systemic disease or a consequence of it, and whether treating it as a primary clinical target produces meaningful outcomes. The functional medicine position is that it is a clinically significant modifiable factor — one piece of a picture that also includes the microbiome, the immune system, the neuroendocrine axis, and the person's full history. It is not treated as a single cause. It is investigated as one component of a systemic pattern.
Symptoms Associated with Increased Permeability
The symptoms associated with intestinal permeability are often diffuse and non-specific — which is part of why it goes unrecognised through conventional routes. The common thread is systemic immune activation: the body responding to substances that have crossed a barrier they should not have crossed.
- Bloating and distension
- Alternating bowel habits
- Food reactions and intolerances
- Nausea after eating
- Abdominal discomfort
- Chronic fatigue
- Recurrent infections
- Autoimmune flares
- Joint pain and inflammation
- Skin conditions — eczema, psoriasis, rosacea
- Brain fog and poor concentration
- Anxiety and mood dysregulation
- Headaches
- Poor sleep quality
- Low motivation
These symptoms overlap significantly with many other conditions — which is precisely why intestinal permeability requires testing rather than assumption, and why a positive finding in isolation never tells the whole clinical story.
The Causes of Leaky Gut
Intestinal permeability is rarely the result of a single cause. In clinical practice it typically reflects the accumulated effect of multiple factors operating simultaneously — which is why it responds best to an approach that addresses the full picture rather than a single intervention.
Cortisol directly affects tight junction protein expression — chronic HPA axis activation increases gut permeability. The gut-brain axis runs both directions: stress compromises the barrier, and a compromised barrier drives further neuroinflammation and mood dysregulation.
An imbalanced microbiome — depleted beneficial bacteria, overgrowth of opportunistic bacteria or Candida — produces metabolites and toxins that damage the epithelial lining and disrupt tight junction integrity. H. pylori, SIBO, and Candida are all associated with increased permeability.
Gluten triggers zonulin release in susceptible individuals — opening tight junctions even in people without coeliac disease. Ultra-processed foods, emulsifiers, and food additives disrupt the mucus layer and microbiome composition that protect the epithelium.
Non-steroidal anti-inflammatory drugs increase gut permeability through prostaglandin inhibition — a well-documented effect even with short-term use. Antibiotics disrupt the microbiome that supports barrier integrity, with permeability effects that can persist long after the course is complete.
Alcohol and its metabolite acetaldehyde directly damage tight junction proteins and the intestinal mucosa. Regular alcohol consumption — even at moderate levels — is associated with measurably increased gut permeability and elevated LPS in circulation.
Zinc, vitamin D, and glutamine are essential for tight junction maintenance and epithelial repair. Deficiencies — common in the context of poor diet, stress, and chronic inflammation — impair the gut's capacity to maintain and restore barrier integrity.
How Intestinal Permeability is Tested in the UK
Assessment of intestinal permeability in UK functional practice uses two complementary approaches — a standalone permeability panel and inferential markers within the GI-MAP stool test. Together they provide both direct measurement and contextual gut immune data.
LPS is a particularly significant marker — circulating LPS drives systemic inflammation, insulin resistance, neuroinflammation, and immune dysregulation through TLR4 activation. Its inclusion in this panel, absent from older IP tests, makes it a more complete picture of barrier breach consequence than Zonulin alone.
The GI-MAP stool test provides additional permeability-relevant markers within the context of the full microbiome and pathogen picture:
| Marker | What it Indicates | Source |
|---|---|---|
|
Secretory IgA (sIgA)
|
Mucosal immune defence — low sIgA indicates depleted gut immune capacity; high sIgA indicates active immune challenge at the mucosal surface | GI-MAP |
|
Calprotectin
|
Neutrophil-derived inflammatory marker — elevated in active intestinal inflammation; used to monitor IBD and distinguish inflammatory from functional gut disorders | GI-MAP |
|
Zonulin (stool)
|
Tight junction regulator — elevated stool zonulin indicates increased intestinal permeability at the gut mucosal level | GI-MAP |
|
Occult Blood
|
Presence of blood in stool — may indicate mucosal damage requiring investigation; contextualises other inflammatory findings | GI-MAP |
Addressing Leaky Gut — The Clinical Approach
Restoring gut barrier integrity requires addressing both the drivers that are maintaining permeability and the conditions needed for repair. This is not a supplement protocol — it is a multi-system intervention guided by what the test data shows rather than what is assumed.
Test the Barrier — Not Just the Symptoms
Intestinal permeability assessment sits alongside GI-MAP stool analysis, food sensitivity testing, DUTCH hormone panel, organic acids, and blood chemistry in the TDG Five-Test System. The clinical picture explains why the barrier is compromised, what it is driving, and what is needed to restore it.