SIBO and MCAS: Understanding the Gut - Immune Connection

Digestive symptoms rarely occur in isolation. Alongside bloating, abdominal discomfort, diarrhoea, and constipation, some people also report things such as flushing, itching, headaches, heart palpitations, breathlessness, dizziness, skin reactions, sinus issues, and food-related symptom flares. These patterns have led to growing interest in how the gut interacts with the immune system - particularly mast cells and histamine signalling.

Before exploring this further, one important distinction is worth making is that mast cell involvement in gut symptoms is not the same as having Mast Cell Activation Syndrome (MCAS). Much of the strongest current evidence relates to IBS and mucosal immune activation more broadly. MCAS is a specific diagnosis with defined criteria (though diagnosis is challenging), and it shouldn’t be assumed from food sensitivities, flushing, or chronic digestive symptoms alone.

What Are Mast Cells?

Mast cells are specialised immune cells found throughout the body, particularly within tissues that interact closely with the external environment, such as the gut lining, skin, and airways.

They play an important role in immune defence, releasing chemical messengers including pro-inflammatory cytokines, histamine, tryptase, prostaglandins, and leukotrienes. These mediators can influence gut barrier function, nerve sensitivity, motility, and local inflammation.

Under normal circumstances, mast cells help protect the body. However, in some individuals they may become overly reactive or dysregulated, contributing to a wide range of symptoms.

Potential mast-cell related symptoms may include:

• Flushing or skin redness

• Itching or hives

• Reflux or nausea

• Bloating and diarrhoea

• Headaches or migraines

• Heart palpitations

• Dizziness or light-headedness

• Breathlessness

• Anxiety

• Nasal congestion

• Food sensitivities

Importantly, mast-cell activation can exist on a spectrum, and these symptoms also overlap with many other conditions.

Mast Cells, MCAS and SIBO: Where the Evidence Is Strongest

There is a subset of IBS that appears to involve mucosal immune activation, with mast cells interacting with microbial and dietary factors. Research suggests that bacterial components such as lipopolysaccharides (LPS) can activate mast cells, contributing to gut barrier dysfunction and symptom generation. Studies also show that mast-cell mediators can excite pain-sensing nerves, playing a role in the visceral hypersensitivity seen in many IBS patients. 

A particularly interesting line of research has found that certain IBS-associated microbiota profiles produce higher amounts of histamine, which may activate mast cells and contribute to increased gut sensitivity without those individuals necessarily having MCAS.

What is MCAS?

Mast Cell Activation Syndrome (MCAS) is a complex condition involving inappropriate or excessive mast cell mediator release across multiple body systems.

Symptoms can be broad and fluctuating, potentially affecting the gastrointestinal, dermatological, neurological, cardiovascular, and respiratory systems. They can also vary considerably between individuals but because symptoms can overlap so significantly with other conditions (such as IBS, allergies, anxiety disorders, and other inflammatory conditions), appropriate medical evaluation and investigation is important.

Where SIBO May Fit In

SIBO occurs when excessive numbers of bacteria accumulate in the small intestine, where bacterial levels are normally low. As these bacteria ferment carbohydrates prematurely, symptoms including bloating, flatulence, abdominal discomfort, and altered bowel habits can develop.

Research does suggest that bacterial overgrowth and dysbiosis may influence mast-cell activity through several mechanisms:

• Increased intestinal permeability.

• Inflammatory immune signalling.

• Histamine production by certain bacterial species.

• Impaired histamine breakdown.

• Altered short-chain fatty acid production.

Some bacteria are capable of producing histamine directly, whilst others may impair histamine breakdown within the gut environment. In susceptible individuals, this may contribute to symptoms triggered or worsened by histamine-rich foods.

Current evidence suggests an association between gut dysbiosis, SIBO, and mast cell activation, although the relationship is likely complex and bidirectional rather than purely causal.

Histamine Intolerance vs MCAS

Histamine intolerance and MCAS are often discussed together, but they are not the same.

Histamine intolerance is generally thought to involve an imbalance between histamine load and the body's capacity to break it down. Contributing factors may include reduced DAO (diamine oxidase) enzyme activity, genetic variants, gut inflammation, SIBO or dysbiosis, certain medications, and nutrient deficiencies.

MCAS, on the other hand, involves dysregulated mast cell activation with the release of multiple inflammatory mediators (not histamine alone). Symptoms may also be broader and more systemic, and may involve cardiovascular, neurological, respiratory, gastrointestinal and dermatological systems.

Potential Contributing Factors

Mast cell activation and histamine-related symptoms are unlikely to arise from a single cause. In many cases, multiple overlapping factors may be involved, including:

• Gut dysbiosis and SIBO

• Gastrointestinal infections

• Chronic stress and HPA-axis dysregulation

• Hormonal fluctuations

• Impaired gut motility

• Nutrient deficiencies

• Environmental triggers

• Genetic variants

• Underlying inflammatory conditions

Management should always be individualised and functional support is often aimed at identifying and addressing potential underlying drivers. Depending on the individual, this may involve addressing bacterial overgrowth, supporting gut motility and barrier integrity, nervous system and stress support, sleep optimisation, targeted dietary strategies, and addressing nutrient status.

Whilst low-histamine or low-fermentable diets may help reduce symptoms in some individuals, overly restrictive long term diets are not generally recommended, as they may negatively affect nutrient diversity and microbiome health over time.

The relationship between the gut microbiome, immune system, and mast-cell activity is genuinely complex and still evolving. For individuals with persistent IBS-like symptoms, food sensitivities, or unexplained multisystem presentations, exploring the gut - immune connection as part of a broader picture may offer valuable insights.

How We Can Help

At The Autoimmune Clinic, we specialise in helping people with complex chronic health conditions including SIBO, IBS, histamine intolerance, MCAS, autoimmune disease, chronic fatigue, mould-related illness and Long COVID.

Our approach focuses on understanding the underlying factors that may be contributing to symptoms, including gut health, immune dysfunction, infections, environmental exposures, nutrition and nervous system regulation.

If you're struggling with persistent digestive symptoms, food sensitivities, unexplained reactions or a combination of symptoms that don't seem to fit neatly into a single diagnosis, we'd be happy to help. To learn more about how we work and whether we may be the right fit for you, book a discovery call with one of our practitioners.

Book a Discovery Call