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Your first weeks on low FODMAP:
the elimination phase explained
Starting the low FODMAP diet? Here is what the elimination phase is, why it lasts only a few weeks, the swaps people make most, and what comes next.
By the Belly Buddy team · Updated October 2026 · Reviewed against Monash University's public research
You have read about FODMAPs, you have a plan, and now the first week is in front of you. The low FODMAP elimination phase is the part of the diet most people hear about. It is also the part most often misunderstood.
This guide explains what the phase is for, what changes on your plate and in your shopping basket, and when it is meant to end. If you need a refresher on the carbohydrates themselves, read what FODMAPs are.
What the elimination phase is, and what it is not
The low FODMAP diet was developed by researchers at Monash University as a three-step process. Step 1 is elimination. Step 2 is reintroduction. Step 3 is personalisation.
During elimination, you replace high FODMAP foods with low FODMAP alternatives. The aim is to see whether your symptoms settle when your overall FODMAP intake is low. Monash describes it as a substitution diet: you swap one food for another, rather than removing whole meals.
Monash recommends following Step 1 for 2 to 6 weeks. That makes it a short test with a clear question. It is not a lifestyle, and it is not a list of foods to avoid for good.
Two things should come before it:
- A diagnosis from a doctor. Several conditions can cause symptoms that look like IBS and need different care. Monash describes the diet for people with medically diagnosed IBS.
- A FODMAP-trained dietitian, ideally. Monash advises that the diet is followed under the guidance of a dietitian with specialist skills in IBS. A dietitian can adapt the plan to your usual meals and check that your diet stays nutritionally varied.
Why it only lasts a few weeks
FODMAPs are not just a source of symptoms. Fructans and GOS are also prebiotics. They feed some of the bacteria that live in your large intestine.
When you restrict them, the bacteria lose part of their food supply. Monash reports that levels of some beneficial bacteria fall during strict restriction. Its research also suggests that people who complete all three steps, and reintroduce tolerated FODMAPs, can keep their levels of bifidobacteria in the long term.
Nutrition is the second reason. Monash notes that over-restriction may reduce your intake of calcium, iron, dietary fibre, vitamins and protein, especially without guidance from a dietitian.
Elimination also cannot tell you which FODMAP groups affect you. It only tells you whether lowering all of them together helps. The answers come later.
What changes in practice: the common swaps
Most of your plate stays the same. In the first week of a low FODMAP diet, the changes usually concern a few ingredients that people eat every day.
Bread, pasta and cereals. Regular wheat bread contains fructans in amounts that matter at normal servings. People often change to oats, rice, quinoa, corn or rice pasta, or breads made without wheat, rye or barley. Some sourdough breads are also tested as low FODMAP, because the long fermentation can reduce their fructan content. Check the specific type, because not every sourdough is made the same way.
Milk and yoghurt. Regular milk contains lactose. Lactose-free milk is usually ordinary milk with the enzyme lactase added, so the lactose is already broken down. Hard cheeses are naturally low in lactose. For the full picture, read lactose-free vs low FODMAP.
Onion and garlic. These are often the hardest change, because they appear in so many recipes. Fructans dissolve in water but not in oil. That is why garlic-infused oil, with the solid pieces removed, can flavour a dish without the fructans. The green tops of spring onions and chives are other common options. Our garlic and onion guide explains the details.
Fruit. Apples and pears contain more fructose than glucose, and polyols too. Monash lists fruit such as orange, mandarin, kiwifruit, pineapple and blueberries among its low FODMAP alternatives.
Legumes. Beans, lentils and chickpeas contain GOS, but they do not need to disappear. GOS are water soluble, so some of them move into the liquid of canned legumes. Monash testing has found low FODMAP servings for some canned and rinsed legumes, such as lentils and chickpeas. Small, tested servings keep a useful source of fibre and protein on the plate.
Serving size and stacking
A food is rarely just "low" or "high". Its FODMAP content depends on how much of it you eat. Many foods have a low FODMAP serving and a larger serving that is moderate or high.
Several small servings of moderate FODMAP foods in one meal can also add up. This is often called FODMAP stacking. Our serving sizes guide explains both ideas, and why you do not need to weigh everything forever.
What to expect
Monash reports that about three in four people with IBS see their symptoms improve on the diet. That leaves about one in four who do not. Both results are useful information.
If your symptoms improve, the elimination phase has done its job. It is time to find out which groups matter for you.
If nothing improves after 2 to 6 weeks, do not make the diet stricter. Talk to your dietitian or doctor. Monash suggests that your symptoms may not be sensitive to FODMAPs, and that other approaches can help. Examples include stress management, gut-directed hypnotherapy, fibre supplements or medication. The low FODMAP diet is one tool, and it is not the right tool for everyone.
The kitchen and shopping side
In your own kitchen, the swaps above cover most meals. The supermarket is where the first weeks often get difficult. Packaged foods can contain onion powder, garlic, wheat, honey, inulin or sweeteners such as sorbitol. The ingredient list tells you what is in a product. It does not tell you how much.
Some terms, such as "natural flavours" or "spices", may need more checking. Labelling rules vary between countries, so a vague term does not automatically mean a FODMAP source is present.
To learn which product types often contain these ingredients, browse our FODMAP aisle guides. Each one covers a section of the supermarket, from sauces to snacks.
What comes next
When the elimination phase ends, you move to reintroduction. You test one FODMAP group at a time, in increasing servings, while the rest of your diet stays low FODMAP. This is where you learn which groups affect you, and at what amount.
The goal is the most varied diet you personally tolerate. Our reintroduction guide explains how each challenge works.
Let Belly Buddy help
Label reading is the part of the elimination phase that takes the most time. Belly Buddy helps you identify ingredients that may affect a product's FODMAP profile. Scan a barcode or photograph the ingredient list, and the app flags declared ingredients such as onion, garlic, wheat or lactose. It cannot see quantities or ingredients the manufacturer does not disclose. To learn how the app reaches a rating, read how we rate ingredients.
Belly Buddy is not affiliated with or endorsed by Monash University. Our FODMAP information is informed by publicly available scientific sources, including research from Monash University.
Sources
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