Onion is one of the richest everyday sources of fructans, a type of carbohydrate that is poorly absorbed in the small intestine and fermented by bacteria in the large intestine, which makes gas. Many people eat onion daily without trouble. Some people, particularly some with irritable bowel syndrome (IBS), are more sensitive and can feel bloated or uncomfortable afterwards.
Whether onion matters for you cannot be read off a list. What makes onion hard to study in your own life is that it is usually cooked into the base of a dish and rarely written on the plate. This page covers what is known, why responses differ, and how to record onion so that a possible pattern can be checked rather than assumed.
What is in onion that might matter
Monash University’s FODMAP team lists onion, spring onion and garlic among the vegetables particularly rich in fructans. A 2024 laboratory analysis of 40 commonly eaten foods in Slovenia measured 1.97 g of fructans per 100 g of fresh onion, the highest of any food tested, ahead of wheat flour (0.75 g) and leek (0.24 g). The same study estimated that onions supplied about 0.72 g of the adults’ average 1.69 g daily fructan intake. The authors note that it analysed few samples and that the intake estimate is rough, and it reflects Slovenian foods, so figures elsewhere may differ.
The controlled evidence on symptoms comes from fructans themselves rather than onion. In a 2008 randomised, placebo-controlled trial, 25 people with IBS and fructose malabsorption who had already improved on a low fructose and fructan diet received drinks without knowing what they contained. 77% of those given fructans reported symptoms that were not adequately controlled, against 14% given glucose. The study was small and used purified fructans. It did not test onion, and it does not tell you how a typical onion-containing meal affects a typical person. The garlic page describes the fructan evidence in more detail.
Why the same onion affects people differently
Gas from fructans is a normal result of fermentation. In a 2017 MRI study of 29 people with IBS and 29 without, an inulin (fructan) drink increased colonic gas in both groups and the amounts did not differ significantly between them. 13 of the 29 people with IBS reached the study’s symptom threshold after inulin, compared with 6 of 29 after glucose, and people without IBS had lower symptom scores. The authors concluded that sensitivity to the bowel stretching, rather than making more gas, produced the symptoms.
Several practical things can differ between one onion meal and the next:
- Amount. A little onion in a large dish is not the same as a French onion soup or a handful of raw red onion.
- What it comes with. Onion usually arrives with garlic, wheat, beans or rich sauces, any of which could also matter.
- Where it goes. Monash notes that fructans are soluble in water, so taking pieces of onion out of a soup or sauce does not remove the fructans. Fructans do not dissolve in oil, so lifting onion out of a stir-fry has a different effect.
- Which part. Monash suggests the green tops of spring onion as a substitute for onion in recipes, while the vegetables listed as fructan-rich include spring onion generally. If you eat both, it helps to record which part you had.
- You. Monash’s own advice is that “everyone’s tolerance to foods is different”.
How to notice onion in your own meals
You do not need a separate onion diary. A few extra words in your usual entries are enough:
- Write onion down when you know it is there, with a rough amount in your own words: “salad, a few raw red onion rings”, “curry, onion-based sauce”, “burger, fried onions”.
- Note the form when it is obvious: raw, fried, in a soup or sauce, spring onion green or white part, onion powder.
- Mark it as unknown when you cannot tell. Takeaways, ready meals, soups, gravies, stock and spice mixes often contain onion without saying so. Monash suggests asking for dips, sauces and marinades separately or on the side if you are unsure what is in them. “Pasta bake, onion likely, not sure” is more honest than leaving it out or assuming.
- Check labels only when it helps you. An ingredient list that names onion is a clear note for your diary; “spices” or “flavouring” may or may not include it, so record that as unknown rather than guessing.
- Record meals with onion that went fine. A diary that mentions onion only on bad days will make it look more important than it is. The guide to why symptom-free meals matter explains why.
- Keep garlic and onion separate if you can. They often appear together and both contain fructans. Naming each lets you see later whether a possible pattern follows one, the other or only the combination.
The approach to recording a mixed meal applies here: name the parts you can identify and mark the rest as unknown. Sauces and side vegetables are easy to leave out, and onion often hides in exactly those.
What your diary can and cannot show
After a few weeks your notes may show that onion-heavy meals are often followed by bloating, sometimes followed by it, or no different from other meals. Each result is useful.
A repeated sequence is a possible pattern, not proof that onion causes your symptoms. Onion-heavy meals may also be larger, later, eaten out, or paired with alcohol or a stressful day. The guide to telling which meal caused bloating explains why one meal followed by symptoms is a weak clue and why several comparable entries matter more.
The NHS lists onions among foods that can be hard to digest and suggests keeping a food and symptom diary to look for foods that trigger IBS. A diary should not, however, talk you into cutting onion out on a hunch. Onion is in a very large share of savoury cooking, so avoiding it can make eating noticeably harder. The British Dietetic Association advises you to seek specialist advice if you are struggling or your diet becomes limited.
If a dietitian has suggested a low FODMAP diet
A low FODMAP diet is a structured, temporary process: high-FODMAP foods are reduced for a period and then reintroduced group by group to find your own tolerance. The BDA says a doctor may suggest it if symptoms persist and that you can ask for a dietitian referral. If you are going through reintroduction, a diary is where the results live: the food, the amount, the day and how you felt, including when you felt fine. The guide to preparing for a dietitian appointment covers what to take. Follow the plan your dietitian gives you for what to test and when, rather than designing your own.
When to get advice
The NHS advises seeing a GP if you feel bloated regularly, or you still feel bloated after changing your diet, or if bloating comes with unintentional weight loss or blood in your poo. It advises an urgent GP appointment or calling 111 if bloating comes with symptoms such as vomiting, diarrhoea or constipation, a stomach ache, a fever, a swelling or lump in your tummy, acid reflux, or being unable to pass urine, poo or wind. A diary can make an appointment more useful, but it does not replace one. This article was not professionally reviewed.
If you want onion, garlic and your symptoms on one timeline without a paper diary, you can get GutBloom on Google Play. It shows possible patterns with their evidence and limits, not verdicts.
Sources
- Monash FODMAP — Where FODMAPs are found in foods
- Monash FODMAP (2015) — Cooking with onion and garlic: myths and facts
- Frontiers in Nutrition (2024) — Fructan content in most commonly consumed Slovenian foods and estimation of daily fructan consumption
- Clinical Gastroenterology and Hepatology (2008) — Dietary triggers of abdominal symptoms in patients with irritable bowel syndrome: randomized placebo-controlled evidence
- Gastroenterology (2017) — Colon hypersensitivity to distension, rather than excessive gas production, produces carbohydrate-related symptoms in individuals with irritable bowel syndrome
- British Dietetic Association — Irritable bowel syndrome (IBS) and diet
- NHS — Irritable bowel syndrome: diet, lifestyle and medicines
- NHS — Bloating