Are SIBO and MCAS Connected? The Gut Mast Cell Axis, Explained
Both diagnoses often show up in the same person. What the overlap between bacterial overgrowth and mast cell reactivity explains, and where it stops.
Photo: Sue Winston / Unsplash
Why both diagnoses land on the same person
They are connected, though not in the way that word usually gets used. There is no good evidence that either condition causes the other. What exists instead is a strong pattern of co-occurrence, plus several plausible mechanisms for why the two keep turning up in the same body. Small intestinal bacterial overgrowth (SIBO) means too many bacteria living in a stretch of gut meant to hold relatively few. That changes the environment gut mast cells live in. Mast cells that are already reactive, in turn, change the conditions that let bacteria overgrow. Neither of those is proof of cause.
The co-occurrence half is the better documented one. A structured review published in 2026 gathered SIBO prevalence across a set of allergic and mast cell conditions. In mast cell activation syndrome (MCAS, where mast cells release their contents too readily and too often), SIBO was present in 30.9 percent of patients. That is lower than the rates reported in asthma and food allergy.
Electronic health records give a second angle. Researchers searched a US database covering more than 53 million patients over more than 20 years. People with irritable bowel syndrome were at least four times more likely than the general population to carry a mast cell disorder. The strongest single association was between IBS and MCAS, at an odds ratio of 16.3 (95 percent confidence interval 13.1 to 20.3). That analysis looked at IBS, which isn't the same diagnosis as SIBO. It remains the largest available signal that gut symptom disorders and mast cell disorders travel together.
Mast cells in the gut wall, and why location changes things
Mast cells live inside tissue rather than circulating in blood. Each one is packed with granules holding histamine, tryptase, prostaglandins, and cytokines. In the gut they concentrate in the mucosa, which is the lining layer, within reach of sensory nerve endings.
That proximity is what lets them change how the gut feels. A 2022 review of mast cells in IBS describes them positioned close to sensory nerves, releasing bioactive substances in response to infection, stress, and neuroendocrine signals. Histamine and proteases are among those products, and both are implicated in heightened gut pain sensitivity and in defects of the epithelial barrier. If the barrier gets leakier, then more of what is in the gut lumen reaches the immune cells underneath it.
Location gets left out of almost every explanation of this overlap. Mast cells don't behave identically everywhere in the body. In some people the mast cells in the airway stay relatively calm while the ones in the gut react much more strongly. That is why a substance can be fine inhaled and cause trouble when it is eaten. It is a clinical pattern rather than a trial result, and it is one reason a full ingredient list often tells you more than the name of the active compound.
Bacteria that make their own histamine
Some bacteria produce histamine themselves. In a 2022 study in Science Translational Medicine, researchers colonized germ-free mice with stool microbiota from people with IBS. Mice that received microbiota from patients with high urinary histamine developed heightened gut pain sensitivity and mast cell activation. Mice given microbiota from patients with low urinary histamine did not. Feeding the affected mice a diet lower in fermentable carbohydrates reduced both the sensitivity and the build-up of mast cells in the colon. The team identified a gut bacterium carrying a variant of the histidine decarboxylase gene, which codes for the enzyme that makes histamine, as a major producer.
This was a mouse study using human donor material, not a trial in people. What it establishes is that a bacterial source of histamine exists and can act on mast cells. It doesn't establish that this is what is happening in any particular person carrying both diagnoses.
Disposal is the other half. Diamine oxidase (DAO) is the enzyme that breaks histamine down in the gut. A 2021 review makes the case that histamine intolerance originates there rather than systemically, and that conditions involving the gut mucosa reduce DAO activity locally. If overgrowth inflames the same lining that produces the enzyme, then the capacity to clear histamine falls while the load is rising. There is more on what a low DAO blood test can and cannot tell you.
The loop, and the limits of what it proves
Both halves of the loop have been described in the small intestine specifically. A 2022 paper in BMC Medicine notes that SIBO and increased populations of small intestinal mast cells are each frequently associated with IBS, and that altered small intestinal barrier function has been shown in the diarrhea-predominant subtype.
The proposed loop runs in five steps. Overgrowth raises bacterial products in the small intestine and, in some people, bacterial histamine. Those reach mucosal mast cells through a barrier that inflammation has already made more permeable. Activated mast cells release mediators that increase pain signaling and loosen the barrier further. Slowed motility follows, which leaves more material sitting in the small intestine. That is the condition permitting overgrowth in the first place.
Every arrow there is an association with a plausible mechanism behind it. None is a demonstrated cause in humans.
The strongest reason to hold the loop loosely is a 2025 Mayo Clinic case series. Researchers reviewed 42 patients with more than 20 mast cells per high-power field on gastrointestinal biopsy, having excluded systemic mastocytosis. SIBO was present in 78 percent of those tested, which fits the overlap story neatly. What came next does not. Of the 21 patients with enough follow-up to judge, only 5, or 24 percent, reported partial or complete improvement. The authors concluded that elevated mucosal mast cells, absent a systemic mast cell disorder, may not represent a distinct disease entity.
Sequencing is where the overlap changes something
The practical consequence is about order. If someone's gut immune environment is already highly reactive, then a push aimed only at lowering bacterial counts can leave them feeling worse. A rapid drop in bacterial load can provoke symptoms on its own, and so can the fillers and binders in the products used to get there.
The order I work in is to calm the system before rebuilding anything. Depending on the case, that can include antihistamine or mast cell stabilizing medication where the prescribing clinician judges it appropriate, alongside changes that lower overall immune provocation. Stability is the goal, so the gut isn't in constant defense mode. Reducing bacterial load becomes the sensible target after that, and even a partial reduction can ease symptom pressure without full eradication. Long-standing SIBO also tends to leave nutrient deficiencies behind, and correcting those supports the repair work. Food tolerance usually widens later, once inflammation is lower, rather than at the start.
That is why a plan addressing only one side tends to plateau. Treating the bacteria while ignoring the reactivity leaves a person too reactive to tolerate the treatment. If the mast cells are settled and the overgrowth is left alone, the trigger stays where it is. The bucket model of mast cell load is one way to hold both at once, and the motility side has its own story in why SIBO keeps coming back.
None of this is a self-directed protocol, and none of it substitutes for care. Both conditions need a practitioner who can order the right testing and manage any medication involved. If you are starting from the beginning, there is broader background on mast cell activation syndrome and on SIBO and gut motility.
If you're living with MCAS and want a structured starting point, MCAS: What You Need to Know First walks through what the diagnosis actually means, where to begin, and what to do when diet alone isn't enough.
FAQ
Can SIBO cause histamine intolerance?
Not in a way anyone has demonstrated, though there are two plausible routes. Some gut bacteria produce histamine directly, and conditions involving the gut lining lower the activity of diamine oxidase, the enzyme that clears histamine there. Both would raise the histamine load without either one proving that overgrowth caused an intolerance. If your symptoms started or worsened alongside a SIBO diagnosis, then that sequence is something to describe to whoever is managing your care.
Does treating SIBO improve MCAS symptoms?
In the 2026 review that reported a 30.9 percent SIBO rate in MCAS, treating the overgrowth improved symptoms in the MCAS group and in asthma, though not in chronic spontaneous urticaria. That is a signal rather than a guarantee. A Mayo Clinic series of 42 patients with high gut mast cell counts found that only 24 percent of those followed improved on the treatments they were given, so what happens in practice varies a great deal.
Are there gut bacteria that make histamine?
Yes. Researchers working with germ-free mice colonized with stool microbiota from people with IBS identified a gut bacterium carrying a variant of the histidine decarboxylase gene, which codes for the enzyme that makes histamine. Mice colonized from high-histamine donors developed gut pain sensitivity and mast cell activation; mice colonized from low-histamine donors did not. That work was done in mice, so it shows the mechanism is possible rather than that it explains any given case.
Does a biopsy showing high mast cell counts mean I have MCAS?
Not on its own. The Mayo Clinic authors who reviewed 42 such biopsies concluded that elevated mucosal mast cells, without a systemic mast cell disorder, may not represent a distinct disease entity, and that routine counting on biopsies isn't supported by their findings. Their advice was to rule out other inflammatory, allergic, infectious, and motility disorders, as well as SIBO.
Which should be treated first, the overgrowth or the mast cells?
In my practice the mast cell side comes first, because a highly reactive gut often cannot tolerate an aggressive antimicrobial phase. Once things are steadier, reducing bacterial load becomes workable, and even a partial reduction can lower symptom pressure. This is a sequencing decision that belongs with the clinician managing your case, and not a protocol to run alone.
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