The Low FODMAP Diet, Phase by Phase
Restriction, reintroduction and personalisation — how the diet is actually meant to be done, and why the phase most people skip is the one that matters.
Part of the Complete IBS Guide.
Key fact: The low FODMAP diet is a three-phase diagnostic process, not a permanent way of eating. Strict restriction lasts two to six weeks; the reintroduction phase that follows is what tells you which foods are actually your triggers. Stopping after phase one is the most common mistake, and it leaves people needlessly restricted for years.
What FODMAPs actually are
FODMAP is an acronym for fermentable oligosaccharides, disaccharides, monosaccharides and polyols — a group of short-chain carbohydrates that share three properties. They are poorly absorbed in the small intestine, they draw water into the bowel by osmosis, and they are rapidly fermented by bacteria in the colon, producing gas. In a gut of normal sensitivity that produces nothing worse than wind. In a gut with the visceral hypersensitivity that characterises IBS, the same volume registers as pain and distension.
They are not unhealthy foods and they are not an allergy. Onions, garlic, wheat, apples, pears, milk, legumes, cashews and sugar-free mints are all high-FODMAP, and most are foods anyone would be glad to eat. The point of the diet is not to remove them permanently — it is to find out, systematically, which subgroups your gut actually objects to and at what dose.
Do the simple things first
The low FODMAP diet is a second-line dietary intervention, not a starting point. NICE and the NHS both put general dietary and lifestyle advice first, and a meaningful share of people improve enough on that alone to never need the harder version.
- Eat regular meals and do not skip them or eat very late
- Drink 8 to 10 cups of fluid a day, mostly water
- No more than 3 caffeinated drinks a day, and limit alcohol and fizzy drinks
- Limit fresh fruit to 3 portions a day, spread out rather than together
- Reduce fatty, spicy and heavily processed foods
- For wind and bloating, add oats and linseed; for diarrhoea, cut sorbitol and reduce insoluble fibre
- Try a probiotic for four weeks at the dose on the packet, and stop it if nothing has changed
Give it four weeks. If regular meals, controlled caffeine, less alcohol and steadier fibre have not moved your symptoms after a month of genuine consistency, that is the point at which a structured low FODMAP trial becomes worth the effort.
Phase 1: restriction (2–6 weeks)
You swap high-FODMAP foods for low-FODMAP alternatives across the whole diet. This phase is strict but deliberately short — Monash University, which developed the diet, specifies two to six weeks, and there is no benefit in extending it.
| Group | Common high-FODMAP sources | Low-FODMAP swaps |
|---|---|---|
| Fructans | Wheat, rye, onion, garlic, leek | Oats, rice, sourdough spelt, garlic-infused oil, spring onion tops |
| GOS | Chickpeas, lentils, kidney beans, cashews | Firm tofu, small portions of tinned rinsed lentils, peanuts |
| Lactose | Milk, soft cheese, yoghurt, ice cream | Lactose-free milk and yoghurt, hard cheese, oat or almond milk |
| Fructose | Apples, pears, mango, honey, high-fructose syrup | Berries, oranges, kiwi, unripe banana, maple syrup |
| Polyols | Stone fruit, mushrooms, cauliflower, sorbitol, xylitol, mannitol | Grapes, cucumber, courgette, carrot, sugar rather than sugar-free |
Two practical points decide whether this phase works. First, portion size matters as much as the food itself — many foods are low-FODMAP in one serving and high in two. Second, read labels: onion and garlic powder are in most stocks, sauces and crisps, and sorbitol is in most sugar-free products.
If nothing changes in six weeks, stop. A properly executed restriction phase that produces no improvement is a useful result — it tells you FODMAPs are not your problem, and continuing is all cost and no benefit. Go back to your clinician rather than restricting further.
Phase 2: reintroduction (6–10 weeks)
This is the phase people skip, and skipping it is what turns a diagnostic tool into a permanently narrowed diet. You stay on the low-FODMAP baseline and challenge one FODMAP group at a time with a food that contains only that group — typically eaten daily for three days, with symptoms recorded, then a washout before the next challenge.
The purpose is to establish two things for each group: whether it triggers you at all, and if so, how much you tolerate. Most people find they react to one or two subgroups and tolerate the rest at normal portions, which is a far better outcome than avoiding everything.
1
Pick one group
Choose a test food containing a single FODMAP — mango for fructose, milk for lactose, bread for fructans.
2
Challenge for three days
Increase the portion across the three days while keeping the rest of the diet low-FODMAP, and record symptoms daily.
3
Wash out
Return to strict low-FODMAP until symptoms settle before starting the next challenge, so results are not contaminated.
4
Record the threshold
Note not just whether a group triggered you, but at what portion. Tolerance is usually a dose, not a yes or no.
Phase 3: personalisation
The final phase is the whole point: you rebuild the widest diet your gut tolerates, restricting only the specific groups and portions that reliably cause trouble. Tolerance is not fixed either — it often improves over time, so groups that failed a challenge are worth retesting every few months rather than being written off permanently.
A good outcome looks like a normal diet with two or three known limits, not a life of restriction. If you have reached phase 3 and your diet still feels narrow, that is a reason to go back to a dietitian rather than to accept it.
The risks worth taking seriously
Nutritional gaps
Long restriction reduces calcium, iron and fibre intake, particularly where dairy and wholegrains are both cut.
Microbiome effects
FODMAPs are prebiotic. Prolonged restriction reduces the bacteria that feed on them, which is one reason the strict phase is time-limited.
Disordered eating
The diet demands vigilance about food. In anyone with a history of restrictive eating that vigilance can become the bigger problem.
Masking a diagnosis
Cutting wheat before coeliac testing can make the test falsely normal — get tested first, while still eating gluten.
These are the reasons Monash and UK dietetic practice both say the diet should be done with a dietitian experienced in IBS rather than from an app alone, and why it should never be started before IBS has actually been diagnosed and the red flags excluded. Our IBS page covers that diagnostic step, and the bloating page covers what else produces these symptoms.
What to run alongside it
Diet is one lever among several, and there is no reason to use it alone. Antispasmodics remain first-line for cramping — see mebeverine vs Buscopan vs alverine — and enteric-coated peppermint oil has reasonable evidence for bloating and pain specifically. Which subtype you have shapes the rest: see IBS-D, IBS-C and IBS-M.
Frequently Asked Questions
How long should I stay on the low FODMAP diet?
The strict restriction phase should last two to six weeks and no longer. It is a diagnostic phase, not a treatment. If symptoms improve you move on to systematic reintroduction; if they have not improved after six weeks of doing it properly, FODMAPs are not your trigger and continuing will not help.
Do I need a dietitian to do it?
It is strongly recommended. Monash University, which developed the diet, advises it be followed under the guidance of a dietitian with expertise in IBS. The restriction phase is nutritionally demanding and the reintroduction phase is where most self-directed attempts go wrong — people stay restricted because they never systematically test what they can tolerate.
What is the difference between the low FODMAP diet and going gluten-free?
They overlap but are not the same. Wheat is high in fructans, so cutting wheat removes a FODMAP source, which is why some people feel better gluten-free without having coeliac disease. The low FODMAP diet targets the carbohydrate, not the protein, and it is structured to be temporary. If you suspect coeliac disease, get tested before cutting gluten — the test needs you to be eating it.
Can I do the low FODMAP diet if I am vegetarian or vegan?
Yes, but it needs planning, because legumes and pulses are both staple protein sources and high in GOS. Firm tofu, tempeh, eggs where eaten, peanuts, and small rinsed portions of tinned lentils are the usual routes through. This is one of the situations where dietetic input matters most.
Why did the diet work at first and then stop working?
Most often because the diet has drifted — high-FODMAP ingredients such as onion and garlic powder creep back in through sauces and processed foods. It can also mean that FODMAPs were only ever part of the picture, and that stress, sleep or an untreated constipation problem is now driving symptoms. Re-checking with a food and symptom diary usually separates the two.
Is the low FODMAP diet safe long term?
Strict restriction is not intended to be long term. It reduces intake of prebiotic carbohydrates that feed beneficial gut bacteria, and it makes adequate calcium, iron and fibre harder to achieve. The end point is a personalised diet that restricts only your specific triggers at the portions that actually cause symptoms.
References
- National Institute for Health and Care Excellence. Irritable bowel syndrome in adults: diagnosis and management (CG61). nice.org.uk
- National Institute for Health and Care Excellence. Suspected cancer: recognition and referral (NG12). nice.org.uk
- NHS. Irritable bowel syndrome (IBS): diet, lifestyle and medicines. nhs.uk
- Monash University. Starting the low FODMAP diet. monashfodmap.com
- National Institute for Health and Care Excellence. Coeliac disease: recognition, assessment and management (NG20). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. IBS should be diagnosed by a clinician who has excluded the conditions that mimic it. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.