When pollen makes your gut flare

Watering eyes? A streaming nose? The relentless urge to sneeze? It’s allergy season. But did you know that for many the season also brings bloating, cramping, diarrhoea, or constipation, that their GP may never have linked to pollen at all. 

The connection between seasonal allergies and irritable bowel syndrome (IBS) is often under-discussed. Research is still catching up, but early evidence suggests a possible link; what happens in your airways and sinuses during allergy season does not stay there. This connection isn't yet fully understood, or recognised in clinical guidelines, but emerging research points to shared underlying mechanisms 

The gut–immune axis 

To understand the link, it helps to understand just how central the gut is to the immune system. Around 70–80% of all immune cells in the body live in the gastrointestinal tract, housed within a specialised network called gut-associated lymphoid tissue (GALT). 

When allergy season arrives and your body encounters pollen, it mounts a classic IgE-mediated allergic response. Mast cells, immune cells found in large numbers in both the respiratory tract and the gut lining, release histamine and other inflammatory mediators. This is what causes the sneezing and the itchy eyes. But mast cells in the intestines react too, which may contribute to increased intestinal permeability. 

Mast cell activation 

Mast cells are the key players connecting both conditions. In classic allergic disease, they sit in the mucous membranes of the nose and eyes, primed to detect allergens. In people with IBS, studies consistently show elevated numbers of mast cells in the gut wall, and these cells appear to be activated even in the absence of a clear food trigger. During allergy season, the systemic inflammatory signal from airway mast cells may influence gut mast cells, potentially tipping them into a more reactive state. 

Histamine and gut motility 

Histamine does not simply cause itching. It acts on histamine receptors throughout the digestive system, speeding up the transit of contents through the intestines, increasing fluid secretion, and lowering the pain threshold of intestinal nerve fibres. This may help explain why some people report that their IBS flares shift from predominantly constipation-type to diarrhoea-type during high-pollen periods. 

The gut–brain axis 

The enteric nervous system, often called the "second brain", is exquisitely sensitive to systemic inflammation. Cytokines released during allergic reactions can cross from the bloodstream into the gut wall, modulating the signals travelling along the vagus nerve between gut and brain. This can heighten visceral hypersensitivity, meaning normal digestive activity may feel more painful to IBS sufferers. 

Allergy symptoms 

Practical steps to help  

Alongside any treatment your doctor recommends, a few lifestyle strategies may help reduce the impact of allergy season on your gut. 

During peak pollen season, consider reducing aged cheeses, fermented foods, and red wine, which add histamine load. 

A diverse, fibre-rich diet supports beneficial bacteria that help regulate immune responses in the gut. 

Keep a symptom diary correlating pollen counts with gut flares for your GP. 

If your hayfever is only partially controlled, discuss stepping up treatment as better allergy control may ease gut symptoms too.