Onions, FODMAPs, and IBS

Table of Contents

1. What FODMAPs Are
2. Why Fructans Cause Symptoms
3. Why Onion Specifically
4. There Is No Safe Small Portion of Onion
5. The Cooking-and-Removing Myth
6. Onion-Infused Oil: Why It Works
7. Where Onion Hides
8. What to Use Instead
9. The Three Phases of a Low-FODMAP Diet
10. Reintroducing Onion — and Why It Matters That You Try
11. FODMAPs Are Not the Whole Story
12. What the Evidence Actually Supports
13. Key Research Papers
14. Connections
15. Featured Videos

What FODMAPs Are

FODMAP is an acronym coined by the Monash University group in Melbourne for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols. It is a functional grouping, not a chemical family — these compounds have little in common structurally. What they share is behaviour: they are poorly absorbed in the small intestine, they are osmotically active, and they are rapidly fermented by colonic bacteria.

1. Oligosaccharides — fructans (onion, garlic, wheat, rye) and galacto-oligosaccharides (legumes). Nobody absorbs these; we all lack the enzymes.
2. Disaccharides — lactose, in people with low lactase activity.
3. Monosaccharides — fructose in excess of glucose, as in honey, apples and mango.
4. Polyols — sorbitol, mannitol, xylitol, maltitol; stone fruit, mushrooms, cauliflower and sugar-free products.

Gibson and Shepherd set out the framework and its rationale in 2010, and their 2012 paper made the broader case for food choice as a management strategy for functional gut symptoms. Onion sits in the first group, and within that group it is close to the top.

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Why Fructans Cause Symptoms

Nothing about the mechanism requires anything to be wrong with you. Two ordinary physical processes are involved, and in a sensitive gut they produce pain.

Osmosis. Fructans are small, soluble and unabsorbed, so they stay in the gut lumen and hold water there. That increases the volume of liquid moving through the small intestine, distending it, and contributing to loose stools in the diarrhoea-predominant pattern. Halmos and colleagues measured this directly in a controlled feeding study.

Fermentation. When the fructans reach the colon, the bacteria ferment them, producing gas — hydrogen, carbon dioxide, and methane in some people. Gas takes up space, the bowel distends, and distension produces pain and bloating.

The crucial point is what happens next, because it is what separates IBS from ordinary digestion. Everybody produces gas from fructans. Most people do not notice. People with IBS have visceral hypersensitivity — the nerves of the gut wall report normal distension as painful, and the brain's processing of those signals is altered too. The low-FODMAP diet does not fix that sensitivity. It reduces the stimulus reaching an over-reactive system.

This framing matters emotionally as well as practically. You are not damaging your gut by eating onions and you are not allergic to them. Your gut is producing a normal amount of gas that your nervous system is reporting as pain. That is a real, physiological condition, and it also means the goal is finding your threshold rather than eliminating a poison.

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Why Onion Specifically

Three things combine to make onion the standout trigger.

It is exceptionally high in fructans. Muir and colleagues' HPLC analysis of Australian vegetables and fruits — the analytical work underpinning essentially all FODMAP food data — puts onion at the top of the vegetable range for fructan content.

It is eaten constantly. A food can be very high in FODMAPs and rarely matter if you eat it once a month. Onion is the base layer of most savoury cooking in most cuisines. The exposure is daily, often at every meal.

It is frequently invisible. Diced onion in a stew is obvious. Onion powder in a stock cube, a spice blend, a crisp seasoning or a restaurant sauce is not. Böhn and colleagues surveyed self-reported food-related symptoms in IBS and found that food-related symptoms are extremely common and associated with more severe overall symptoms; onion appears repeatedly in such surveys near the top of the list of implicated foods.

Put together, onion becomes the classic case of a food that people react to and cannot identify, because the correlation between the meal and the symptoms is blurred by an unseen exposure. Many people arrive at a low-FODMAP diet having concluded they react to "everything", when in fact they react to something present in nearly everything.

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There Is No Safe Small Portion of Onion

Most of the low-FODMAP approach works on portion size rather than prohibition. A small serve of avocado is fine and a large one is not; a quarter of a ripe banana is fine and a whole ripe one is not. Almost every food has a threshold serving below which it is considered low FODMAP.

Onion bulb is one of the few foods where no useful low-FODMAP serving size has been established. The fructan content is so high, and so evenly distributed through the bulb, that even the small quantities people think of as negligible — a tablespoon of diced onion, the onion in a spoonful of a sauce — can exceed the threshold. This is why the standard dietetic advice during the restriction phase is total avoidance of the bulb rather than portion control, and it is a genuine difference from how the rest of the diet works.

Two consequences worth internalising:

1. "I only had a little bit" is not a defence with onion. With most foods it would be.
2. All onion bulbs count. Brown, white, red, sweet, shallots, and the white bulb portions of spring onions and leeks are all high. Sweet onions are not lower in fructans just because they are lower in pungency — pungency tracks sulfur compounds, not carbohydrate, and the two are unrelated.

What is low: the green tops of spring onions and the green leaves of leeks, and chives. Those parts of the plant do not accumulate the storage fructans that the bulb does, because the bulb is the storage organ and the leaves are not. This is a real and useful loophole and it is the basis of most low-FODMAP allium cooking.

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The Cooking-and-Removing Myth

This is the single most useful correction on the page, so it is worth being precise.

The advice circulates in several forms: "cook the onion whole and lift it out", "sauté the onion for flavour then fish out the pieces", "push the onion to the side of your plate", "use onion in the stock and strain it". The reasoning behind it sounds sensible: if the fructans are in the onion, removing the onion removes the fructans.

It does not work that way, because fructans are water-soluble.

The moment onion is cooked in a liquid, its fructans dissolve out of the tissue and into the liquid — the soup, the stock, the sauce, the braising liquid, the curry. Lifting the onion pieces out afterwards removes the fibre and the pulp, and leaves the fructans behind in exactly the part you are about to eat. A stock made by simmering onion and then straining it is, from a FODMAP perspective, close to onion water. Straining removes the visible evidence and keeps the cause.

The same applies on a plate: pushing sautéed onion to one side after it has been cooked in a sauce does very little, because the sauce has already taken on what it is going to take on.

Two nuances keep this honest rather than absolute:

1. Discarded cooking water is genuinely discarded. If you boil onion and throw the water away, the fructans that leached into the water go with it. That is the reverse of the usual situation, where the cooking liquid is the meal. It is not a practical technique for onion — you would be left with washed-out onion tissue that still contains fructans — but it is why boiled-and-drained vegetables generally test lower than the same vegetables in a soup.
2. A very brief contact in dry heat transfers less than a long simmer in liquid. An onion half seared for two minutes in a dry pan and removed is not the same exposure as an onion simmered for an hour in a stew. But "less" is not "low", and for a food with no established safe threshold this is not a reliable strategy.

Nothing here means the advice-givers are being dishonest. It is a genuinely counter-intuitive point: the thing you can see is not the thing that causes the symptom.

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Onion-Infused Oil: Why It Works

The same chemistry that defeats the cook-and-remove approach is what makes onion-infused oil valid, and it comes down to one property.

Fructans are polar carbohydrates. They dissolve in water. They do not dissolve in oil.

Onion's aroma and flavour, by contrast, come largely from volatile sulfur compounds that are soluble in fat. So when you heat onion in oil and then remove every piece of the onion, the fat carries away the flavour and leaves the fructans behind in the discarded solids. You get the taste without the trigger. This is the standard, dietitian-endorsed technique on a low-FODMAP diet, and it is not a workaround of marginal validity — it follows directly from the solubility difference. The same logic applies to garlic-infused oil, which is if anything more widely used.

Making it, with the safety point that matters:

1. Heat a neutral oil or olive oil gently with sliced onion (or crushed garlic) until fragrant. Do not let it brown to bitterness.
2. Strain out every solid piece. Any onion left in the oil is onion you will eat.
3. Refrigerate it and use it within a few days. This is the important part: onion and garlic are low-acid vegetables, and storing them submerged in oil at room temperature creates the oxygen-free, low-acid conditions in which Clostridium botulinum can grow. Home-made garlic-in-oil has caused documented botulism outbreaks. Make small quantities, keep them cold, and do not keep a bottle of home-infused oil on the counter.
4. Commercially produced infused oils are formulated and acidified for shelf stability — a bought bottle is not the same product as a home-made one and should be stored as its label says.

Two things infused oil does not do: it does not give you the sweetness and body that cooked onion contributes to a dish, and it does not replace onion's bulk. It gives you the aromatic top notes, which is most of what you miss.

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Where Onion Hides

People fail elimination diets on hidden onion more than on any other single ingredient. The list to check:

1. Stock cubes, bouillon powder, liquid stock and gravy granules — almost universally contain onion, and often onion powder, which is concentrated.
2. "Natural flavouring", "vegetable powder", "spice blend", "seasoning" on an ingredient list — frequently includes onion or garlic powder without naming it separately.
3. Crisps, savoury snacks and crackers — onion powder is a default seasoning ingredient far beyond the flavours that name it.
4. Sausages, burgers, meatballs, pâté and most processed meat.
5. Curry pastes, tomato pasta sauces, salsa, chutney, relish, ketchup and most bottled sauces.
6. Salad dressings, mayonnaise-based sauces and marinades.
7. Soups — nearly all of them, tinned, packet or fresh.
8. Restaurant food, where the base of almost every savoury preparation is a sweated onion. Asking for a dish "without onion" usually gets the visible pieces removed and not the base it was built on.

The reliable move during a restriction phase is to cook from whole ingredients for a few weeks. That is more work, and it is also the only way to know what you have actually eaten.

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What to Use Instead

The goal is savoury depth without the bulb, and it is achievable.

1. Green tops of spring onions — the closest direct substitute. Buy them for the greens, use the whites for something else or discard them.
2. Green parts of leeks — the same principle; excellent softened in oil as a base.
3. Chives — low FODMAP, fresh onion note, best added late.
4. Onion- and garlic-infused oil — as above, made safely.
5. Asafoetida (hing) — a resin used in Indian cooking specifically as an allium substitute, in Jain and some Hindu cooking traditions that exclude onion and garlic entirely. A pinch bloomed in hot oil gives a remarkably onion-like savouriness. Check the label: it is commonly cut with wheat flour, which matters if you also avoid gluten.
6. Fennel bulb, celery and carrot — for the sweetness and body a sweated onion contributes. A classic Italian soffritto minus the onion still works.
7. Cumin, coriander seed, smoked paprika, mustard, fresh ginger and black pepper — for aromatic complexity in place of allium pungency.
8. Real umami sources — tomato paste cooked until it darkens, aged hard cheese, anchovy, dried seaweed, and slow-browned meat. Much of what onion contributes to a finished dish is savoury depth, and there is more than one route to it.

Indian cooking without onion or garlic is a fully developed cuisine, not an improvisation. It is worth looking at for anyone who thinks a no-onion kitchen is a diminished one.

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The Three Phases of a Low-FODMAP Diet

The low-FODMAP diet is a structured, time-limited diagnostic process. It is not a way of eating, and the most common way it goes wrong is that people stop after phase one and stay there for years. Whelan and colleagues' review sets out the full three-phase structure and the evidence for each part.

1. Restriction — typically two to six weeks. All high-FODMAP foods are removed. This phase answers one question only: do FODMAPs matter for this person? If symptoms do not improve substantially in this window, FODMAPs are not the driver and the diet should be stopped rather than tightened.
2. Reintroduction (challenge) — each FODMAP subgroup is reintroduced one at a time, in increasing amounts, with a washout between challenges. Tuck and Barrett's paper on the challenge phase is the practical reference for how this is done. Fructans are usually challenged with a defined food, and onion is often used because it is the purest fructan challenge available.
3. Personalisation — a long-term diet that restricts only the specific subgroups, at the specific doses, that actually caused problems. For most people this is far less restrictive than phase one.

Two things about the process:

1. Doing it with a dietitian meaningfully improves the outcome. The British Dietetic Association's evidence-based guidelines for the dietary management of IBS, published by McKenzie and colleagues, set out why: the diet is complex, easy to get wrong, and nutritionally risky if extended.
2. Staying in phase one is a real harm, not a theoretical one. Staudacher and colleagues showed that fermentable carbohydrate restriction reduces luminal bifidobacteria, and Halmos and colleagues showed it alters the colonic luminal microenvironment. You are starving the bacteria described on the prebiotic fructans page. Long-term unnecessary restriction also narrows the diet, risks nutritional gaps, and for some people slides into a disordered relationship with food.

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Reintroducing Onion — and Why It Matters That You Try

Onion is the challenge people most dread and most often skip. It is worth doing anyway, for three reasons.

You may tolerate more than you think. Sensitivity is a spectrum with a threshold, not a switch. Some people who react badly to a whole sautéed onion in a dish tolerate a small amount of onion in a shared sauce without trouble. Knowing your own threshold is worth several years of unnecessary avoidance.

Thresholds change. IBS symptom severity fluctuates with stress, sleep, illness, hormonal cycle and the state of the gut after an infection. A challenge that fails during a bad period may pass six months later. Retesting periodically is normal practice, not stubbornness.

The alternative is a permanently narrower life. Avoiding onion forever means avoiding most restaurants, most convenience food, and most cooking done by other people. If the truthful answer is that you cannot tolerate it, that is worth knowing precisely so you can plan around it — and if you can tolerate some, that is worth even more.

How the challenge is usually structured: a small measured amount on day one, a larger amount on day two, a larger amount again on day three, with symptoms recorded; then a washout of several days on a low-FODMAP baseline before the next subgroup. Do not challenge two subgroups at once, and do not run a challenge during a week that is unusually stressful — the result will not be interpretable.

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FODMAPs Are Not the Whole Story

Two important qualifications keep this in proportion.

Fructans may be responsible for symptoms people attribute to gluten. Skodje and colleagues ran a double-blind crossover challenge in people with self-reported non-coeliac gluten sensitivity, giving them fructan, gluten, or placebo. Fructan produced significantly more symptoms than placebo; gluten did not. Since wheat is a major fructan source as well as the gluten source, this suggests that a proportion of people who believe they react to gluten are in fact reacting to fructans — the same molecules that are in onions. If you avoid wheat and still have symptoms, onion is a reasonable next suspect.

IBS has drivers other than diet. Visceral hypersensitivity, gut–brain axis signalling, post-infectious changes, altered motility, bile acid handling, stress and sleep all contribute. The American College of Gastroenterology's 2021 clinical guideline for IBS, by Lacy and colleagues, places a low-FODMAP diet among the management options with a conditional recommendation based on evidence of limited quality — a fair reflection of the state of the field. Diet is one lever among several, and for some people it is not the main one.

If restriction gives no improvement after a properly executed restriction phase, the answer is to stop and look elsewhere, not to restrict further.

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What the Evidence Actually Supports

The evidence for the low-FODMAP diet is good by the standards of dietary research and modest by the standards of drug trials, and both halves of that sentence are true.

In favour. Halmos and colleagues' 2014 controlled feeding trial is the foundational study: participants with IBS ate a low-FODMAP diet or a typical Australian diet, all food provided, and reported substantially lower overall gastrointestinal symptom scores on the low-FODMAP arm. Because the food was supplied rather than self-selected, adherence was not a confounder.

In perspective. Böhn and colleagues randomised IBS patients to a low-FODMAP diet or to traditional IBS dietary advice — regular meals, avoiding large portions and known irritants — and found that both improved symptoms, with no significant difference between the two. This is an important result and it is often omitted. It suggests that structured dietary attention helps, and that the FODMAP framework is one effective way of delivering it rather than a uniquely effective one.

The methodological problem. You cannot blind someone to what they are eating. Diet trials carry expectation effects that drug trials control for, and the low-FODMAP diet has been the subject of considerable enthusiasm, which raises expectation. This does not invalidate the findings; it caps how confident anyone should be about effect size.

The fair summary. A properly conducted low-FODMAP process helps a majority of people with IBS to a meaningful degree, fructans are among the most commonly implicated subgroups, and onion is the highest-exposure fructan food in most diets. Restriction is a diagnostic step, not a destination, and the goal is to end up eating as widely as your symptoms allow — which for many people includes some onion.

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Key Research Papers

1. Halmos EP, Power VA, Shepherd SJ, Gibson PR, Muir JG. A diet low in FODMAPs reduces symptoms of irritable bowel syndrome. Gastroenterology. 2014;146(1):67-75.e5. — doi:10.1053/j.gastro.2013.09.046
2. Böhn L, Störsrud S, Liljebo T, et al. Diet low in FODMAPs reduces symptoms of irritable bowel syndrome as well as traditional dietary advice: a randomized controlled trial. Gastroenterology. 2015;149(6):1399-1407.e2. — doi:10.1053/j.gastro.2015.07.054
3. Muir JG, Rose R, Rosella O, et al. Measurement of short-chain carbohydrates in common Australian vegetables and fruits by high-performance liquid chromatography (HPLC). Journal of Agricultural and Food Chemistry. 2009;57(2):554-565. — doi:10.1021/jf802700e
4. Gibson PR, Shepherd SJ. Evidence-based dietary management of functional gastrointestinal symptoms: the FODMAP approach. Journal of Gastroenterology and Hepatology. 2010;25(2):252-258. — doi:10.1111/j.1440-1746.2009.06149.x
5. Gibson PR, Shepherd SJ. Food choice as a key management strategy for functional gastrointestinal symptoms. American Journal of Gastroenterology. 2012;107(5):657-666. — doi:10.1038/ajg.2012.49
6. Whelan K, Martin LD, Staudacher HM, Lomer MCE. The low FODMAP diet in the management of irritable bowel syndrome: an evidence-based review of FODMAP restriction, reintroduction and personalisation in clinical practice. Journal of Human Nutrition and Dietetics. 2018;31(2):239-255. — doi:10.1111/jhn.12530
7. Tuck C, Barrett J. Re-challenging FODMAPs: the low FODMAP diet phase two. Journal of Gastroenterology and Hepatology. 2017;32(Suppl 1):11-15. — doi:10.1111/jgh.13687
8. Böhn L, Störsrud S, Törnblom H, Bengtsson U, Simrén M. Self-reported food-related gastrointestinal symptoms in IBS are common and associated with more severe symptoms and reduced quality of life. American Journal of Gastroenterology. 2013;108(5):634-641. — doi:10.1038/ajg.2013.105
9. Staudacher HM, Lomer MCE, Anderson JL, et al. Fermentable carbohydrate restriction reduces luminal bifidobacteria and gastrointestinal symptoms in patients with irritable bowel syndrome. The Journal of Nutrition. 2012;142(8):1510-1518. — doi:10.3945/jn.112.159285
10. Halmos EP, Christophersen CT, Bird AR, et al. Diets that differ in their FODMAP content alter the colonic luminal microenvironment. Gut. 2015;64(1):93-100. — doi:10.1136/gutjnl-2014-307264
11. Skodje GI, Sarna VK, Minelle IH, et al. Fructan, rather than gluten, induces symptoms in patients with self-reported non-celiac gluten sensitivity. Gastroenterology. 2018;154(3):529-539.e2. — doi:10.1053/j.gastro.2017.10.040
12. Lacy BE, Pimentel M, Brenner DM, et al. ACG clinical guideline: management of irritable bowel syndrome. American Journal of Gastroenterology. 2021;116(1):17-44. — doi:10.14309/ajg.0000000000001036
13. McKenzie YA, Bowyer RK, Leach H, et al. British Dietetic Association systematic review and evidence-based practice guidelines for the dietary management of irritable bowel syndrome in adults (2016 update). Journal of Human Nutrition and Dietetics. 2016;29(5):549-575. — doi:10.1111/jhn.12385
14. Biesiekierski JR, Rosella O, Rose R, et al. Quantification of fructans, galacto-oligosaccharides and other short-chain carbohydrates in processed grains and cereals. Journal of Human Nutrition and Dietetics. 2011;24(2):154-176. — doi:10.1111/j.1365-277X.2010.01139.x

External Resources and Live PubMed Searches

1. Monash University FODMAP program — the group that developed the diet; food composition data and the app
2. PubMed: low FODMAP diet randomised trials
3. PubMed: fructan challenge in IBS
4. PubMed: FODMAP reintroduction and long-term use

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Connections

• Onions — the main topic page
• Onions — Benefits Deep Dive
• Onions: History and Origins
• Onion Prebiotic Fructans and Gut Bacteria — the benefit side of the same molecule
• Onion Quercetin and Heart Health
• Onion Organosulfur Compounds and the Tear Factor
• Irritable Bowel Syndrome (IBS)
• SIBO
• Crohn's Disease
• Gastroenterology
• Gut Health
• Garlic
• Leeks
• All Food Topics

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