The Low-FODMAP Diet Explained: What to Eat and Avoid

The low-FODMAP diet has the strongest evidence base of any dietary approach for IBS. Multiple randomised controlled trials show symptom improvement in 50–80% of people with IBS who follow it correctly.

But it’s also one of the most frequently misunderstood diets. Many people hear “low-FODMAP” and commit to avoiding all FODMAP foods permanently — which is the wrong approach and can actually harm gut health over time by reducing microbiome diversity.

Here’s how it actually works.

What are FODMAPs?

FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols — a group of short-chain carbohydrates that are:

1. Poorly absorbed in the small intestine
2. Quickly fermented by colonic bacteria (producing gas)
3. Osmotically active (drawing water into the gut)

For people with IBS, this combination produces bloating, cramping, loose stools, constipation, or all of the above.

The three phases

Phase 1: Elimination (2–6 weeks)

Remove all high-FODMAP foods to establish a symptom-free baseline. The goal isn’t to eat this way forever — it’s to confirm that FODMAPs are actually driving your symptoms (they aren’t in everyone).

Low-FODMAP foods you CAN eat:

Vegetables: carrots, courgette, capsicum, cucumber, lettuce, spinach, aubergine, tomatoes, potatoes, green beans

Fruits: bananas (unripe), blueberries, strawberries, grapes, kiwi, orange, cantaloupe

Grains: rice, oats, quinoa, gluten-free pasta, sourdough spelt bread, corn tortillas

Protein: meat, fish, eggs, firm tofu, tempeh

Dairy alternatives: lactose-free milk, almond milk, oat milk (check for inulin), hard cheeses, lactose-free yoghurt

Fats: olive oil, butter, most nuts (except cashews/pistachios) in small amounts

High-FODMAP foods to AVOID:

• Onions and garlic (very high — one of the most common triggers)
• Wheat-based foods in regular amounts
• Most legumes (beans, lentils, chickpeas)
• Soft cheeses, regular milk, ice cream
• Apples, pears, mangoes, stone fruits (peaches, plums, cherries)
• Mushrooms, cauliflower, avocado (in large amounts)
• Honey, agave, high-fructose corn syrup
• Sugar-free products containing xylitol, sorbitol, mannitol

Phase 2: Reintroduction (6–8 weeks)

This is the most important phase — and the one most often skipped. You reintroduce one FODMAP category at a time (not individual foods), while keeping the rest of your diet low-FODMAP.

The categories:

1. Fructans (test with wheat, then onion/garlic separately)
2. Lactose (test with milk)
3. Fructose (test with honey or mango)
4. GOS (test with lentils)
5. Sorbitol (test with avocado or peaches)
6. Mannitol (test with mushrooms or cauliflower)

Each test takes 3 days: day 1 (moderate amount), day 2 (larger amount), day 3 (rest, no FODMAP foods). Record symptoms carefully.

Phase 3: Personalisation

Based on reintroduction results, you create a personalised, as-liberal-as-possible diet that manages your specific sensitivities. Most people discover they only need to avoid 2–3 FODMAP categories, not all of them.

Tips for success

Cook from scratch where possible — processed foods often contain hidden FODMAPs (onion powder, garlic powder, inulin, sorbitol).

Watch portion sizes — many high-FODMAP foods are dose-dependent. Small amounts of avocado or chickpeas may be fine; large amounts may not be.

Use garlic-infused oil — FODMAPs don’t pass into oils, so garlic-flavoured olive oil gives you the taste without the fructans.

Track every symptom — the reintroduction phase only works if you’re accurately recording symptoms against specific tests. A symptom journal (or an app like Belly Well) makes this far more reliable than memory alone.

Work with a dietitian — really. FODMAP-trained dietitians improve outcomes significantly. Many offer remote consultations.

The low-FODMAP diet done well is a diagnostic tool, not a permanent lifestyle. The goal is to eat as freely as possible once you understand your specific sensitivities.