Low FODMAP: A Step-by-Step Guide

What exactly are FODMAPs?

The term FODMAP refers to a group of short-chain carbohydrates: Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols. What makes them problematic is their behaviour in the gut: poorly absorbed in the small intestine, they travel to the colon, where they draw in water through osmosis and are fermented by gut bacteria. The result is gas production (hydrogen and methane), distension of the colon wall, and activation of the visceral nerves — which are particularly hypersensitive in people with irritable bowel syndrome (IBS).

The mechanisms vary depending on the type of FODMAP:

• Lactose: not hydrolysed when lactase is deficient → direct fermentation.
• Fructose: absorbed via the GLUT5 transporter, which becomes insufficient when fructose intake exceeds that of glucose.
• Fructans and GOS: no human enzyme is capable of digesting them.
• Polyols (sorbitol, mannitol): passive absorption is too limited to be complete.

IBS affects 10 to 15% of the global population, with a prevalence of two women to every one man. Of those affected, 60 to 70% respond positively to the low FODMAP diet during the elimination phase.

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Phase 1: Elimination (4 to 6 weeks)

This is the most restrictive phase. The goal is straightforward: remove all high-FODMAP foods to calm symptoms and establish a neutral baseline.

Foods to set aside temporarily:

• Fructans: garlic, onion, wheat, barley, artichoke
• GOS: beans, lentils, chickpeas
• Lactose: cow's milk, standard yoghurt, soft fresh cheeses
• Excess fructose: apples, pears, honey, high-fructose corn syrup
• Polyols: apricots, mushrooms, sorbitol-containing sweets

Low FODMAP alternatives do exist and allow for a varied diet: rice, quinoa, tofu, strawberries, kiwi, ripe bananas, feta, aged cheddar, lactose-free milk, fresh herbs, and more.

In 70% of people, a noticeable improvement is felt within one to two weeks. A meta-analysis published in The Lancet Gastroenterology (Staudacher et al., 2022, n > 1,000) reported an odds ratio of 2.5 for symptom remission compared with a standard diet.

⚠️ This phase should not exceed six weeks. Guidance from a registered dietitian is strongly recommended to avoid deficiencies in fibre and calcium.

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Phase 2: Reintroduction (6 to 8 weeks)

This is the most important stage — and often the most overlooked. It involves reintroducing FODMAPs one at a time, in a methodical way, to identify which ones you can tolerate and at what dose.

The protocol recommended by Monash University (the creators of the diet, 2023 guidelines):

• Test one type of FODMAP per week
• Consume a test amount over three days (e.g. 12 g/day of lactose)
• Observe for three days with no reintroduction
• Record every reaction in a food diary

This phase allows you to build your own personal intolerance map. It is essential to avoid remaining unnecessarily locked into severe restriction.

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Phase 3: Personalisation (long term)

Once your tolerance thresholds have been identified, you enter the personalisation phase. The goal is no longer total elimination, but a tailored balance: reintegrating the FODMAPs you tolerate well and limiting the ones you don't.

The 2023 Monash guidelines suggest aiming for a diet that is approximately 75% low FODMAP, whilst progressively reintroducing tolerated foods to preserve microbiome diversity.

A cohort study published in Gut (Vujovic et al., 2024, n = 450) found that 75% of people maintained their improvements at 12 months thanks to this personalised reintroduction approach.

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Important cautions not to overlook

The low FODMAP diet is effective, but it carries risks if followed incorrectly:

• Loss of fibre: fructans are prebiotics. Eliminating them can reduce fibre intake by 20 to 30 g per day and contribute to constipation.
• Reduced microbial diversity: prolonged elimination beyond six weeks may decrease Bifidobacteria levels by 10 to 20% (Staudacher et al., 2022).
• Risk of excessive restriction: around 15% of people abandon the diet due to the demands it places on eating habits.

To compensate, consider:

• Including Bifidobacterium-based probiotics (encouraging data from 2022)
• Maintaining a fluid intake of 2 litres per day
• Eating slowly and in small portions to reduce the fermentable load
• Using a scientifically validated app to check the FODMAP content of foods

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What the science tells us, in summary

The low FODMAP diet is now recommended as a first-line non-pharmacological approach for IBS by the Rome IV guidelines (2022) and the American College of Gastroenterology (2021). It reduces symptoms in 50 to 75% of patients during the elimination phase, with lasting benefits when reintroduction is handled well.

It does not cure IBS, but it offers a powerful tool for regaining control of your digestive comfort — provided it is followed across all three phases, with a structured approach and professional support.