FODMAP stands for fermentable oligosaccharides, disaccharides, monosaccharides and polyols. They are a family of small carbohydrates, found in everyday foods from onions and wheat to apples and milk, that the small intestine absorbs poorly. Whatever is not absorbed carries on through the gut, pulling water along with it, and gut bacteria then ferment it into gas. In a sensitive gut, that combination of water and gas produces bloating, wind, cramps and changes in bowel habit in the hours after eating. This guide covers what FODMAPs are, what happens when you eat them, which foods carry the most, and why they trouble some people and leave others untouched.
What does FODMAP stand for?
The acronym gathers five groups of carbohydrates into one word. Fermentable describes what happens to all of them, and the rest names them. Oligosaccharides are short chains of sugars, mainly fructans from wheat, onions and garlic, and galacto-oligosaccharides from beans and lentils. The disaccharide is lactose, the sugar in milk. The monosaccharide is fructose, counted when a food contains more of it than the glucose alongside it. Polyols are sugar alcohols such as sorbitol and mannitol, present naturally in some fruits and vegetables and added to sugar-free sweets and chewing gum.
The groups behave differently in the small intestine. A 2019 review sets out that nobody digests fructans or galacto-oligosaccharides, because humans do not make the enzymes that break them down, while lactose, fructose and polyols are absorbed well by some people and poorly by others. In everyone, at least some of what these foods carry reaches the gut bacteria intact.
Fermentable carbohydrates are one of several distinct mechanisms that can sit behind a food intolerance, alongside enzyme shortfalls and sensitivities to food chemicals, and they are the mechanism with the clearest evidence trail.
What happens when you eat a FODMAP?
The effects come in two stages. In the small intestine, an unabsorbed carbohydrate holds on to water, so more fluid sits inside the bowel than usual. Further along, in the large intestine, bacteria ferment the carbohydrate and release hydrogen and other gases. Water stretches the bowel from the inside, gas stretches it further, and stretch is what the gut's nerves respond to, which is why bloating after eating is the symptom most often traced back to FODMAPs.
Both stages have been imaged directly. In a 2014 MRI study, 16 healthy volunteers drank sugar solutions while their guts were scanned hourly. Fructose roughly doubled the water in the small intestine within an hour, while inulin, a fructan, left the small intestine unchanged and instead filled the large intestine with gas over several hours. Different FODMAPs load different parts of the gut on different timescales, which is part of why symptoms vary with the food. The wait for fermentation also explains the delay. FODMAP symptoms tend to build hours after a meal rather than during it.
A 2026 review adds that the products of this fermentation also influence how quickly the gut moves and how its nerves signal, which connects FODMAPs to pain and urgency as well as to bloating and wind.
Why do FODMAPs cause symptoms in some people and not others?
The physical response to FODMAPs is much the same in everyone. In a 2017 UK trial, 29 people with IBS and 29 people without drank fructose and inulin on separate days while researchers tracked their guts by MRI. The water shifts, gas volumes and breath hydrogen readings were similar in both groups. What differed was the experience. Far more of the IBS group reported bloating and pain, and in those who did, the intensity tracked the amount of gas in the large intestine. The researchers concluded that symptoms come from a gut that is more sensitive to being stretched, not from producing more gas than other people.
That sensitivity to stretch, called visceral hypersensitivity, is common in IBS, and it is why the same bowl of lentils can leave one person comfortable and another in pain. The carbohydrates behave identically. The nerves reporting on the result do not.
In people without a sensitive gut, even a fairly high FODMAP intake seems to cause little trouble. A small 2025 feeding study gave 25 healthy adults three weeks of low FODMAP meals and three weeks of a moderate intake, and found no difference in gut symptoms, stools or mood between the two, although the mix of their gut bacteria shifted measurably. This is early research in a small group, so it cannot settle the question on its own, but it fits the wider picture. FODMAPs are a normal part of eating, and a gut that is not oversensitive handles them without complaint.
Which foods are high in FODMAPs?
FODMAPs sit in ordinary, mostly healthy foods rather than in any single food group. A 2015 review from the research group that developed the concept names onions, garlic, shallots, wheat-based foods and pulses as the biggest everyday sources. By group, the main carriers are these.
- Fructans. Onions, garlic, shallots, leeks, wheat and rye products such as bread and pasta, and inulin added to some high-fibre foods.
- Galacto-oligosaccharides. Beans, lentils, chickpeas and other pulses.
- Lactose. Milk, soft cheese, ice cream and other fresh dairy, in people whose lactase levels are low.
- Excess fructose. Apples, pears, mango, honey and fruit juice.
- Polyols. Some stone fruits and mushrooms, and the sweeteners sorbitol and xylitol in sugar-free gum and mints.
Two entries on that list need a note. Wheat appears for its fructans, not its gluten, so it can cause trouble in people who have tested negative for coeliac disease. And lactose only counts as a FODMAP for you if your own lactase level is low. In Britain most adults keep the enzyme into adulthood and digest milk without difficulty, so this branch of the FODMAP family mainly matters where lactose intolerance is present.
The amount matters as much as the food. Most of these foods cause no trouble in modest portions even in sensitive guts, and a large serving of a moderate source can deliver more fermentable carbohydrate than a small serving of a notorious one.
How does the low FODMAP diet fit in?
Restricting FODMAPs is the best studied dietary approach to IBS, and it grew directly out of the mechanism above. Less fermentable carbohydrate means less water shift and less gas, so a sensitive gut has less to react to. It is a structured, short-term process with three phases, restriction, reintroduction and personalisation, rather than a permanent way of eating, and the British Society of Gastroenterology guideline recommends it be delivered with support from a dietitian, because unguided restriction risks an unbalanced diet as well as unhelpful changes to your gut bacteria.
It is also not the first step. NHS advice for IBS starts with regular meals, enough fluid, and adjustments to caffeine, alcohol, fat and fruit before any formal exclusion diet. Where those basics have not helped, a dietitian will usually work through FODMAP restriction as one option within wider diet management for IBS, alongside fibre changes and trigger food identification, and a GP can refer you.
What can a food intolerance test show?
A FODMAP reaction is not an immune reaction. It is carbohydrates fermenting, there is no antibody involved, and no consumer test can measure it, so working out whether FODMAPs are behind your symptoms rests on the pattern of your own reactions, ideally mapped with a dietitian through structured reintroduction.
Where testing earns its place is when FODMAP logic has not explained everything. If symptoms keep following a food that is low in FODMAPs, or follow one specific food while its FODMAP cousins cause nothing, the question becomes whether your immune system is reacting to that food itself rather than fermenting its carbohydrates. Our IgG food intolerance test measures IgG antibodies to 270 individual foods and drinks, and an elevated result may indicate a food sensitivity to that specific food. Our clinical team reads results with exactly this distinction in mind, a sensitivity to the food on one side, a fermentable carbohydrate load on the other, because the two point to different foods and different next steps.
Are FODMAPs bad for your gut?
No. For most people they are a feature of a healthy diet, not a flaw in it. The same fermentation that produces gas also feeds the gut's resident bacteria, and fructans and galacto-oligosaccharides in particular feed the species generally counted as beneficial.
The clearest evidence for that comes from what happens when FODMAPs are removed. In a UK trial of 104 people with IBS, four weeks of a low FODMAP diet eased symptoms, but it also lowered levels of bifidobacteria, one of the most consistently beneficial groups of gut bacteria, compared with a sham diet. Quieting fermentation starves the bacteria that live on it. That trade-off is one reason the restrictive phase of any FODMAP change is kept short, and it means someone without gut symptoms has nothing to gain from avoiding these foods. If your gut is comfortable, the onions, beans and apples on the list are doing it good.
When should you see a GP?
Bloating, wind and changeable bowels that follow what you eat fit the FODMAP pattern. Some symptoms do not fit it and need a GP appointment first, whatever a food diary suggests. See your GP before experimenting with food if you have blood in your stool, unexplained weight loss, a change in bowel habit that has lasted more than three weeks, bloating present most days for three weeks or more, difficulty swallowing, persistent vomiting, signs of iron-deficiency anaemia such as constant tiredness, new gut symptoms over the age of 50, or a family history of bowel or ovarian cancer.
IBS itself is a diagnosis a GP makes after other conditions have been ruled out, and coeliac testing only works while gluten is still being eaten. If wheat is on your suspect list, keep eating it until your GP has arranged that test.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms.
Sources
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- Major G, Pritchard S, Murray K, et al. (2017). Colon Hypersensitivity to Distension, Rather Than Excessive Gas Production, Produces Carbohydrate-Related Symptoms in Individuals With Irritable Bowel Syndrome. Gastroenterology. PubMed | DOI
- Chen B, Zhang Y, Du L, Jin J, Dai N (2026). Advances in the mechanism of low FODMAP diet in the treatment of irritable bowel syndrome: a review. Frontiers in Nutrition. PubMed | DOI
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