Struggling with allergies? SIBO and Dysbiosis Amplify It

Struggling with Allergies? SIBO and Dysbiosis Can Amplify It

If you’re living with persistent allergies — seasonal sneezing, unexplained hives, food reactions, or chronic skin flares — and feel like your antihistamines are barely touching the surface, your gut may hold the missing piece of the puzzle. A growing body of research now links small intestinal bacterial overgrowth (SIBO) and gut dysbiosis to the worsening of allergy and histamine symptoms, through a pathway scientists are calling the “gut–allergy axis.”

What Is SIBO — And Why Should You Care If You Have Allergies?

Small intestinal bacterial overgrowth (SIBO) occurs when bacteria that normally reside in the colon migrate and proliferate in the small intestine, reaching concentrations of 10⁵ colony-forming units per millilitre or higher. The offending bacteria are often from the Enterobacteriaceae family — species such as Escherichia, Klebsiella, Proteus, Shigella, and Aeromonas — which the gut ordinarily keeps well away from the small bowel.

Common triggers of SIBO include impaired gut motility, low stomach acid, frequent antibiotic use, proton pump inhibitors, and chronic stress. Symptoms are often dismissed as “just IBS” — bloating, abdominal pain, altered bowel habits, diarrhoea, constipation, and fatigue — and yet mounting evidence suggests SIBO is also a potent driver of systemic immune dysfunction, including allergic disease.

A 2023 review published in the World Journal of Gastroenterology confirmed that SIBO’s prevalence is rising globally and that it represents an increasingly significant clinical concern far beyond the gastrointestinal tract (Efremova et al., 2023).

The Prevalence of SIBO in Patients with Allergies Is Striking

Several studies now report alarmingly high rates of SIBO among people with atopic (allergic) conditions:

• Bronchial asthma: 60–100% of patients tested positive for SIBO across multiple studies (Ivashkin et al., 2018; Ozimek et al., 2022; Peña-Vélez et al., 2019)
• Food allergies: 50–87.5% prevalence, with one adult cohort study finding positive breath tests in 85.7% of confirmed food allergy patients (Bartuzi-Lepczynska & Ukleja-Sokolowska, 2025)
• Mast cell activation syndrome (MCAS): around 30% of patients (Weinstock et al., 2020)
• Chronic spontaneous urticaria (CSU): approximately 27% (Campanati et al., 2013)

A 2025 review published in the International Journal of Molecular Sciences (Terlecki et al., 2026) synthesised this literature and concluded that SIBO prevalence across atopic populations is consistently higher than in the general population, and that treating SIBO in conditions like asthma and MCAS can lead to meaningful symptom improvement.

Importantly, a 2019 paediatric study by Peña-Vélez et al. found that children with chronic abdominal pain and allergic disease had an odds ratio of 5.45 for having SIBO compared to non-allergic controls — a striking association.

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How SIBO and Dysbiosis Trigger the Allergic Immune Response

So what is the biological mechanism linking gut bacteria to allergy symptoms? Researchers now describe two primary pathways.

1. The Mucosal (Direct) Pathway

When bacterial colonies overgrow in the small intestine, they produce excess lipopolysaccharide (LPS) — a component of the outer wall of gram-negative bacteria. This LPS activates a receptor called Toll-like receptor 4 (TLR4) found on the gut lining, triggering the release of immune alarm signals: IL-33, IL-25, and thymic stromal lymphopoietin (TSLP). These cytokines activate innate lymphoid cells type 2 (ILC2s), which in turn orchestrate a full Th2 immune response — the hallmark of allergic disease.

Th2 cells produce the cytokines IL-4, IL-5, and IL-13, which together:

• Drive IgE antibody production (the antibody central to classic allergic reactions)
• Recruit and mature eosinophils
• Promote goblet cell hyperplasia and mucus overproduction (goblet cells produce mucus)
• Cause smooth muscle hyperreactivity (relevant in asthma)

2. The Low-Grade Endotoxaemia (Indirect) Pathway

As SIBO damages the intestinal epithelium (the gut lining), it disrupts tight junctions between gut cells, increasing intestinal permeability — commonly referred to as “leaky gut.” This allows bacterial LPS to leak into the systemic circulation, causing persistent, low-grade endotoxaemia. Chronic TLR4 activation throughout the body further amplifies and sustains Th2 polarisation, creating a self-reinforcing cycle of immune dysregulation that makes allergic individuals increasingly reactive.

In essence: SIBO doesn’t just cause bloating — it trains your immune system into a state of chronic over-alertness that makes every allergenic encounter more severe.

The Histamine Connection: Where Gut Dysbiosis Makes Allergies Worse

One of the most clinically important — and underappreciated — links between gut dysbiosis and allergy is histamine metabolism. Histamine is a key chemical messenger released during immune responses, including allergic reactions. In healthy individuals, it is tightly regulated by two key enzymes:

• Diamine oxidase (DAO): the primary enzyme that breaks down dietary and gut-derived histamine in the intestinal lining
• Histamine-N-methyltransferase (HNMT): metabolises histamine in tissues (i.e not the gut but everywhere else)

The problem is that gut dysbiosis and SIBO directly undermine both of these systems.

How SIBO Raises Your Histamine Load

Certain bacteria produce histamine as a metabolic byproduct. When these histamine-producing bacteria overgrow in the small intestine — including species like Morganella morganii and Klebsiella pneumoniae — they flood the gut with excess histamine that the body struggles to clear. A 2022 study published in Nutrients (Sánchez-Pérez et al., 2022) confirmed that patients with histamine intolerance have a significantly altered gut microbiome, characterised by increased Proteobacteria, reduced microbial diversity, and lower levels of beneficial Bifidobacteriaceae — a pattern strikingly similar to what is seen in SIBO.

The same study found elevated stool zonulin levels in histamine-intolerant patients, a biomarker of intestinal permeability, supporting the leaky gut connection.

How SIBO Depletes Your Histamine-Clearing Capacity

SIBO doesn’t just increase histamine production — it simultaneously reduces your ability to break it down. The inflammation caused by bacterial overgrowth damages the intestinal epithelial cells responsible for producing DAO. As reviewed in a 2021 paper in Nutrients (Schnedl & Enko, 2021), histamine intolerance is fundamentally a gastrointestinal condition originating in the gut, and DAO deficiency is a central mechanism.

What’s more, SIBO leads to nutritional deficiencies in the very cofactors needed to activate DAO — including Vitamin C, copper, iron, Vitamin B6, and Vitamin B12. Without these micronutrients, DAO activity is further impaired, and the histamine bucket overflows.

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The result is a classic vicious cycle: SIBO → increased histamine production + reduced histamine clearance → histamine intolerance → systemic inflammation → worsened gut permeability → more SIBO.

Recognising Histamine Overload: Could SIBO Be Behind Your Symptoms?

When histamine accumulates faster than your body can clear it, symptoms can appear across multiple body systems simultaneously — which is why histamine intolerance can be so difficult to diagnose. Symptoms include:

Gut symptoms: bloating, abdominal pain, diarrhoea, nausea, constipation — often overlapping directly with SIBO

Skin reactions: hives (urticaria), itching, flushing, eczema flares, redness

Respiratory symptoms: nasal congestion, sneezing, worsened asthma, post-nasal drip

Neurological symptoms: headaches, migraines, brain fog, anxiety, sleep disturbances, fatigue

Cardiovascular effects: palpitations, low blood pressure, dizziness

Many people with this constellation of symptoms are misdiagnosed with multiple separate conditions — IBS, chronic rhinitis, atopic eczema, anxiety — when in reality a single underlying driver (gut dysbiosis or SIBO) may be amplifying all of them simultaneously.

The Short-Chain Fatty Acid (SCFA) Connection

Healthy gut bacteria produce short-chain fatty acids (SCFAs) — including butyrate, propionate, and acetate — through fermentation of dietary fibre. These molecules play a critical anti-inflammatory role: they reinforce the intestinal epithelial barrier, modulate immune tolerance, and support the production of regulatory T cells that suppress excessive allergic responses.

In dysbiosis, the balance of bacteria shifts away from SCFA-producers (primarily Firmicutes), meaning the gut loses a key brake on inflammation. A 2022 study by Ozimek et al. found that treating SIBO with rifaximin plus probiotics significantly improved the fecal SCFA profile in asthmatic patients, with increased acetic and butyric acid levels and improved mucosal integrity — suggesting that restoring the microbiome genuinely improves the immune environment.

What Does Treatment Look Like?

Research on treating SIBO in the context of allergies is still evolving, but several promising approaches are emerging.

Antibiotic therapy: Rifaximin (a non-absorbable antibiotic) is the most studied treatment for SIBO. In patients with asthma, rifaximin therapy led to significant reductions in IgE levels and improved lung function scores (FEV1), along with a two-to-three-fold reduction in hospitalisation rates (Ivashkin et al., 2018).

Rifaximin + probiotics/prebiotics: Studies suggest that combining rifaximin with targeted probiotic therapy (including strains such as Bifidobacterium bifidum, B. longum, Lactobacillus rhamnosus) or/and the prebiotic PHGG  produces better outcomes than antibiotics alone. Importantly, in histamine intolerance, probiotic selection matters — strains such as L. bulgaricus and L. casei can actually increase histamine production and worsen symptoms. Strains producing minimal histamine should be prioritised.

Herbal Antimicrobials: Many, understandably, don’t want to take antibiotics and choose more natural options such as oregano oil, garlic or neem. These can also be combined with pro- and prebiotics (taken 2+ hours away from each other) to improve successful outcomes.

• Recommended probiotic: Probiota Histamin X by Seeking Health
• Recommended gut lining support: Rezcue by Thera Nordic (l-glutamine and zinc carnosine)
• An Ultimate Gut Health Test could be considered to rule in/out infections, leaky gut and specific microbiome imbalances.

Dietary modification: A low-histamine diet reduces the overall histamine load while gut healing is underway. A low-FODMAP diet is often helpful in parallel, as many FODMAP foods ferment rapidly and feed bacterial overgrowth. A diet rich in soluble and insoluble fibre from whole foods (fruits, vegetables, legumes, whole grains) supports SCFA-producing bacteria and promotes gut barrier integrity. This is why we need to be cautious about using any restrictive diet for too long, or without support around it (probiotics, high intake of polyphenol rich foods, exercise, prebiotic supplements and food).

DAO enzyme supplementation: Taking supplemental diamine oxidase before meals can help break down dietary histamine during the gut healing process, though this addresses symptoms rather than root cause.

• Recommended product: DAO by Seeking Health

Lifestyle factors: Chronic stress activates the nervous system in ways that impair gut motility and worsen dysbiosis. Stress management strategies — including mindfulness, sleep optimisation, and regular exercise — are important adjuncts to gut-directed treatment.

Important Caveats and What Research Still Needs to Answer

It is important to note that the relationship between SIBO and allergic disease is not yet fully established as causal. Most existing studies are cross-sectional and observational, with small sample sizes and heterogeneous populations. The direction of the relationship may also be bidirectional: while SIBO can worsen allergies, atopic inflammation itself may reduce gut motility (via smooth muscle effects of IL-13) and promote antibiotic use — both of which can predispose to SIBO.

Larger, prospective, randomised controlled trials with validated diagnostic criteria are urgently needed to establish causation, identify which subgroups of allergic patients benefit most from SIBO screening, and determine the optimal treatment protocols.

Key Takeaways: Struggling with Allergies?

If you are struggling with allergies, histamine reactions, or atopic symptoms that don’t respond well to standard treatments, consider discussing the following with your doctor or specialist:

• A breath test for SIBO, particularly if you also experience bloating, abdominal pain, or altered bowel habits
• Testing for diamine oxidase (DAO) levels if histamine intolerance is suspected
• A gut-focused assessment including dietary history, microbiome influences, and relevant medications (antibiotic history, proton pump inhibitor use)
• A trial low-histamine or low-FODMAP diet under clinical supervision

The gut–allergy axis is no longer a fringe concept — it represents one of the most exciting and clinically relevant frontiers in allergy medicine. By addressing SIBO and gut dysbiosis, it may be possible not just to relieve gastrointestinal symptoms, but to genuinely reset the immune environment and reduce the burden of allergies.

References and Further Reading

1. Bartuzi-Lepczynska M, Ukleja-Sokolowska N. Small intestine bacterial overgrowth in patients diagnosed with allergic disease: preliminary findings. Adv Dermatol Allergol 2025; 42(4): 407–413.
2. Terlecki M et al. Possible Crosstalk Between Small Intestinal Bacterial Overgrowth (SIBO) and Atopic Manifestations — A Short Overview. Int J Mol Sci 2026; 27: 1865.
3. Schnedl WJ, Enko D. Histamine intolerance originates in the gut. Nutrients 2021; 13(4): 1262. PMID: 33921522.
4. Sánchez-Pérez S et al. Intestinal dysbiosis in patients with histamine intolerance. Nutrients 2022; 14(9): 1774. PMID: 35565742.
5. Ivashkin V et al. A correction of a gut microflora composition for the allergic bronchial asthma complex therapy. Ital J Med 2018; 12: 260–264.
6. Peña-Vélez R et al. Small intestinal bacterial overgrowth: could it be associated with chronic abdominal pain in children with allergic diseases? Rev Esp Enferm Dig 2019; 111: 927–930.
7. Weinstock LB et al. Small intestinal bacterial overgrowth is common in mast cell activation syndrome. J Med Clin Res Rev 2020; 4: 1–3.
8. Hassan M et al. Histamine intolerance and the association with chronic idiopathic urticaria in patients with small intestinal bacterial overgrowth. Ameri J Clin Med Re 2025; 5(6).
9. Ozimek M et al. A metabolic activity recovery of the intestinal microbiota in patients with bronchial asthma. Pulm Med 2022; 2022: 9902438.
10. Efremova I et al. Epidemiology of small intestinal bacterial overgrowth. World J Gastroenterol 2023; 29(22): 3400–3421. PMID: 37389240.

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This article is for informational purposes only and does not constitute medical advice. Please consult a qualified healthcare professional before making changes to your diet or treatment plan.