Low FODMAP diet: how to do it properly

By the DieticiansWorldwide.com editorial team · 17 October 2026 · Clinical nutrition

What FODMAPs are and why they matter in IBS

FODMAP stands for fermentable oligosaccharides, disaccharides, monosaccharides and polyols. Monash University, whose team developed the diet, describes them as "a collection of short-chain carbohydrates (sugars) that aren't absorbed properly in the gut" (Monash University FODMAP). In everyday terms they are:

• Fructans and galacto-oligosaccharides (GOS): in wheat, rye, onion, garlic, beans and lentils.
• Lactose: the sugar in milk, yoghurt and soft cheese.
• Excess fructose: in apples, pears, mango, honey and high-fructose corn syrup.
• Polyols: sorbitol, mannitol, xylitol and maltitol, found in some fruit, mushrooms and sugar-free sweets.

Because these carbohydrates are poorly absorbed, they draw water into the small intestine and are fermented by bacteria in the colon, producing gas. In most people that causes nothing more than a little wind. In people with IBS, whose guts tend to be more sensitive to stretching, the same amount can trigger pain, bloating, diarrhoea or constipation.

That distinction matters. FODMAPs are not harmful, and many FODMAP-rich foods, such as onions, pulses and whole grains, are good for the rest of the population. The diet is a tool for finding your personal triggers, not a list of "bad" foods.

Does the low FODMAP diet work? The evidence

The low FODMAP diet has one of the stronger evidence bases of any diet used for a gut condition, though it is still built mostly on small, short trials.

• The early controlled trial. In a 2014 cross-over study, 30 people with IBS ate almost entirely provided meals for 21 days on a low FODMAP diet and 21 days on a typical Australian diet. Overall symptom scores were roughly halved on the low FODMAP diet, at 22.8 mm against 44.9 mm on a 100 mm scale (Halmos and colleagues, Gastroenterology 2014).
• A placebo-controlled test. Because people expect to feel better on any new diet, a UK trial compared it with a "sham" diet that restricted a similar number of foods. In the per-protocol analysis, 61% on the low FODMAP diet reported adequate relief against 39% on the sham diet, although the intention-to-treat result (57% against 38%) narrowly missed statistical significance (Staudacher and colleagues, Gastroenterology 2017).
• The pooled picture. A 2022 network meta-analysis of 13 randomised trials with 944 patients found the low FODMAP diet ranked first for global symptoms, abdominal pain, bloating and bowel habit. Against people's habitual diet, the relative risk of symptoms not improving was 0.67 (Black, Staudacher and Ford, Gut 2022).

Monash reports that, in its research, IBS symptoms improve in 3 out of 4 people who follow the diet (Monash University FODMAP). Independent trials tend to show smaller differences once a convincing placebo diet is used, so it is fairer to say that most people improve and a substantial minority do not.

The American College of Gastroenterology's first IBS guideline recommends "a limited trial" of the diet to improve global symptoms (Lacy and colleagues, ACG 2021). Note the word limited. The authors of the 2022 meta-analysis also flagged the main gap: most trials took place in hospital clinics and did not study the reintroduction and personalisation phases, which are how the diet is actually meant to be used.

Before you start: rule other things out

IBS is common. In the Rome Foundation's 33-country survey of more than 73,000 adults, 4.1% of internet respondents met the strict Rome IV criteria for IBS (Sperber and colleagues, Gastroenterology 2021). The British Dietetic Association puts it at "around one in five adults" using broader definitions (BDA Food Fact Sheet).

Because it is so common, other conditions are easily mislabelled as IBS. NICE recommends antibody testing for coeliac disease in people with possible IBS, and referral to secondary care if any "red flag" indicators are present (NICE CG61). The BDA lists these as unexplained weight loss, rectal bleeding, a family history of bowel or ovarian cancer, and looser or more frequent stools for more than six weeks after the age of 60. This has a practical consequence: coeliac blood tests only work while you are still eating gluten. If you cut out wheat for a FODMAP trial first, you can make the test falsely negative. Our guide to the coeliac diet explains how the two conditions differ.

Start with first-line advice

The low FODMAP diet is not where NICE begins. Its guideline, last updated in April 2017, first recommends general habits (NICE CG61):

• regular meals, without skipping meals or leaving long gaps;
• at least 8 cups of fluid a day, mainly water or non-caffeinated drinks (our water intake calculator gives a personal estimate);
• tea and coffee limited to 3 cups a day, and less alcohol and fizzy drinks;
• fresh fruit limited to 3 portions a day, about 80 g each;
• less insoluble fibre such as bran, with oats and up to 1 tablespoon of linseeds a day for wind and bloating;
• avoiding sorbitol if you have diarrhoea.

The NHS gives similar advice and adds avoiding fatty, spicy or processed foods and not eating too quickly (NHS). Only "if a person's IBS symptoms persist" does NICE move to exclusion diets such as low FODMAP, and it says that advice should "only be given by a healthcare professional with expertise in dietary management".

If you are not sure your basic diet is balanced to begin with, our healthy eating check is a useful starting point.

The three phases, step by step

Monash describes the diet as three distinct steps (Monash University, 3 steps of the FODMAP diet). Most problems come from doing step one and never moving on.

Phase 1: restriction

Monash describes this as "a substitution diet" rather than an elimination diet: you "switch your daily apple for an orange, or swap an onion for chives". The aim is to keep the diet nutritionally complete while lowering the total FODMAP load enough to see whether your symptoms settle. If they have not improved after six weeks, low FODMAP is probably not the answer for you, and you should stop and review other options.

Phase 2: reintroduction

This is the diagnostic part. Each FODMAP subgroup is tested separately with a "challenge" food, for example mango for excess fructose, over a few days of increasing portions, with a break of a few days between challenges to avoid crossover effects. A dietitian helps you choose the order, the amounts and how to read your response. It takes most people six to eight weeks.

Phase 3: personalisation

You now rebuild a long-term diet that limits only what you have proven troublesome, and only to the amount that causes symptoms. Monash notes that "FODMAP tolerance can change over time", so foods that failed a challenge are worth retesting a few months later.

The long-term picture is encouraging when people complete all three phases. In a London study of 103 patients after dietitian-led education, satisfactory symptom relief rose from 12% at baseline to 57% at long-term follow-up after reintroduction, and nutritional adequacy was not compromised (O'Keeffe and colleagues, Neurogastroenterology and Motility 2018).

What you can and cannot eat: the food lists

The rule that surprises most people is that serving size matters. Many foods are low FODMAP in a small portion and high in a large one. That is why Monash warns that FODMAP lists found online "are often inaccurate", and why its laboratory-tested app uses a traffic-light system by portion (Monash, high and low FODMAP foods).

The examples below come from Monash's sample list. They are a guide to swaps, not a complete diet.

Three points from the Monash guidance are often missed:

• Dairy is not banned. Lactose is the issue, and butter and hard cheeses are naturally low in it.
• Plain meat, fish and eggs are FODMAP-free, but sausages, marinades, stock cubes and gravies often hide garlic and onion.
• Plant milks should be calcium-fortified; Monash suggests choosing ones with more than 120 mg calcium per 100 ml.

The US National Institute of Diabetes and Digestive and Kidney Diseases publishes a similar list of high FODMAP foods, including sweeteners ending in "-ol" such as sorbitol, mannitol, xylitol and maltitol (NIDDK).

How strict does it really need to be?

This is the most common debate, and the honest answer is: strict enough to test the idea, for a short time only.

During phase 1, accuracy matters because you are running an experiment. If you keep eating garlic-heavy sauces or large portions of "safe" foods, you will not know whether the diet failed or was never really tried. But perfection is not required. Trials delivered the diet through counselling, not locked wards, and still found benefit.

After phase 1, strictness becomes the enemy. Several concerns apply to staying low FODMAP indefinitely:

• Gut bacteria. In the 2017 UK trial, four weeks of restriction lowered levels of Bifidobacterium, a group of bacteria generally regarded as beneficial, compared with the sham diet (Staudacher and colleagues, 2017). FODMAPs such as fructans are prebiotics that feed these bacteria.
• Fibre. Cutting wheat, rye, pulses and many fruits and vegetables can drop fibre intake sharply unless you replace them deliberately. Our fibre calculator helps you track it.
• Cost and social life. In the long-term London study, people who kept an adapted FODMAP diet said it cost significantly more and affected social eating (O'Keeffe and colleagues, 2018).

The aim is the least restrictive diet that controls your symptoms.

Generic diet sheet or a dietitian?

You can download a FODMAP list in seconds, so why pay for help? Because the evidence for the diet was largely produced with dietitians delivering it, and because the hard parts are the ones a sheet cannot do.

A generic sheet gives you a single long "avoid" list. A trained dietitian:

• checks that coeliac disease and other conditions have been considered first;
• tailors the restriction to the FODMAPs you actually eat a lot of, which is sometimes only one or two groups;
• plans reintroduction challenges, portions and order, and interprets ambiguous reactions;
• protects your fibre, calcium and energy intake throughout;
• watches for signs that food restriction is becoming a problem in itself.

That last point is serious. Restrictive diets can feed into food fear and disordered eating, particularly in people with a history of an eating disorder. Our guide to ARFID explains how avoidant and restrictive food intake differs from ordinary pickiness. If you have ever had an eating disorder, talk to a professional before starting any exclusion diet.

In the UK, "dietitian" is a protected title regulated by the Health and Care Professions Council, while "nutritionist" is not (our explainer on the difference between a dietitian and a nutritionist shows what to look for); see our United Kingdom country guide for how to check a registration. Monash also runs FODMAP training courses for dietitians. If you would like an individual plan, you can book a session with a registered dietitian.

Book a session with a dietitian

Who should not do it alone

Talk to a doctor or dietitian before starting a low FODMAP diet if you:

• are pregnant or breastfeeding, or the diet is for a child;
• have diabetes, especially if you take insulin or sulfonylureas, because changing carbohydrate sources can affect blood glucose;
• have kidney disease, where swapping foods can change potassium and phosphate intake (see our renal diet guide);
• are underweight, losing weight without trying, or already eat a narrow range of foods;
• also want to lose weight, where combining two diets at once needs planning (see our guide to working with a dietitian for weight loss);
• have a current or past eating disorder;
• already follow another restricted diet, such as vegan or gluten-free.

The diet does not cure IBS, as Monash itself says. It is a way of managing symptoms, alongside other options such as soluble fibre, medicines and gut-directed psychological therapies, which the ACG guideline also recommends.