Intestinal Permeability

Intestinal permeability (colloquially “leaky gut”) refers to increased passage of molecules across the intestinal epithelial barrier that would normally be excluded. The intestinal lining is a single cell layer thick — the barrier between the gut lumen (which is technically outside the body) and the bloodstream.

The Barrier

Intestinal epithelial cells (enterocytes) are connected by tight junctions — protein complexes (claudins, occludin, zonulin-regulated) that seal the spaces between cells. In a healthy gut, only selectively transported molecules cross the barrier. In increased permeability, the tight junctions loosen, allowing larger molecules — partially digested food proteins, bacterial lipopolysaccharide (LPS), bacterial metabolites — to cross into the lamina propria (the tissue layer beneath the epithelium), where they encounter immune cells including Mast Cells.

The Mast Cell Feedback Loop

1. Mast Cells in the gut wall degranulate → Tryptase activates PAR-2 on epithelial cells → tight junctions loosen → permeability increases
2. Increased permeability → more foreign molecules cross the barrier → more immune activation → more mast cell triggering
3. More mast cell activation → more mediator release → more epithelial damage → more permeability

This is a self-perpetuating cycle. Mast cell activation both causes and is worsened by intestinal permeability.

Why It Matters

Increased permeability means:

• Dietary Histamine may be absorbed more readily (bypassing DAO in the epithelial layer)
• Bacterial products (LPS) entering the lamina propria activate mast cells via Toll-like receptors (see Non-IgE Activation Pathways)
• Food proteins that would normally be fully digested before absorption instead cross partially intact, potentially triggering immune responses
• DAO production is reduced because the enterocytes that produce it are damaged

This is why gut healing is described as foundational rather than optional in mast cell management. A permeable gut means more triggers entering the system continuously. See The Gut-Brain-Mast Cell Axis.

What Damages the Barrier

• Mast cell activation itself (the loop above)
• NSAIDs (directly damage the epithelial lining AND inhibit DAO)
• Alcohol
• Chronic stress (CRH increases permeability independently of mast cells)
• Dysbiosis (loss of protective microbial metabolites like butyrate)
• Infections
• Gluten (in susceptible individuals, via zonulin pathway)