Onion
Onions are a widely used allium vegetable providing savoury flavour in cooked dishes, sauces, and stocks. They are one of the most concentrated dietary sources of fructans, making onion one of the most common high-FODMAP triggers for people managing IBS.
Onion (Allium cepa) contains fructans, chains of fructose that humans cannot break down in the small intestine, so they pass largely intact to the large intestine.
There, gut bacteria ferment fructans, producing gas and drawing water into the bowel — implicated in bloating and cramping for people with IBS. Monash University consistently identifies onion as a high-fructan food, and even small amounts (such as onion powder) can trigger symptoms.
Who this affects
Avoid
- IBS & Low-FODMAP: Very high in fructans.
Frequently asked questions
Yes. Monash University rates onion as high FODMAP because it is high in fructans, a type of oligosaccharide. Even small amounts can be enough to trigger symptoms in sensitive people, so onion is typically avoided during the strict elimination phase of a low-FODMAP diet. This is general guidance, not a certified serving assessment for a specific product.
Onion is high in fructans, a group of FODMAP sugars not fully digested or absorbed in the small intestine. In people with IBS and FODMAP sensitivity these ferment in the gut, which can lead to bloating, cramping, wind or diarrhoea. Sensitivity varies from person to person, so this is general information rather than medical advice.
No — cooking with a whole onion and then removing the pieces does not work, because fructans are water-soluble and leach into the surrounding food or liquid. Onion-infused oil can be a low-FODMAP way to add flavour, because fructans are not fat-soluble and stay out of the oil. Cooking onion does not reduce its fructan content.
Yes. Onion is a vegetable with no animal-derived ingredients, so it is suitable for a vegan diet, which excludes all products derived from animals.
Yes. Fresh onion is a naturally gluten-free vegetable, as gluten is found only in wheat, barley and rye. Take care with processed forms such as onion gravy, battered onion rings or some seasoning blends, which may contain wheat or other gluten sources — check the label.
No. Onion is not one of the 14 allergens that EU law requires to be declared and emphasised on food labels. It can still be a common trigger for people with IBS, but that is a food-intolerance issue rather than a regulated allergen. This is general information, not medical advice.