If you have been diagnosed with IBS, one of the first questions your doctor or dietitian will ask you to consider is your diet. The connection between food and IBS symptoms is now so well established that dietary management — specifically the Low FODMAP approach — is the first-line recommendation from both the NHS and the British Dietetic Association before medication is considered.
But knowing you should change your diet and knowing how to actually do it are two very different things. This guide gives you a practical, week by week framework for starting a Low FODMAP diet in the UK — what to eat, what to avoid, how to structure your meals and how to get enough protein throughout the process.
What Is the Low FODMAP Diet and Why Does It Work for IBS
FODMAP is an acronym developed by researchers at Monash University in Australia. It stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols — a group of short-chain carbohydrates that are poorly absorbed in the small intestine and rapidly fermented by gut bacteria in the large intestine. This fermentation process produces gas and draws water into the bowel, triggering the bloating, cramping, diarrhoea and constipation that characterise IBS.
The Low FODMAP diet works by temporarily removing these fermentable carbohydrates, allowing your gut to settle, then systematically reintroducing them to identify which specific FODMAPs trigger your individual symptoms. Research from Monash University and multiple independent clinical trials consistently shows that around 75 percent of people with IBS experience significant symptom reduction when following the diet correctly.
The Three Phases You Need to Understand
Before starting a Low FODMAP diet, it is essential to understand that it operates in three distinct phases. Skipping any phase — particularly the reintroduction phase — significantly reduces the diet's effectiveness and can lead to unnecessary long-term restriction.
Phase One is the elimination phase, lasting two to six weeks. You remove all high FODMAP foods from your diet completely. This is not a permanent way of eating. It is a diagnostic tool designed to give your gut time to settle so you can accurately assess your baseline.
Phase Two is the reintroduction phase, lasting six to eight weeks. You systematically reintroduce foods from each FODMAP category one at a time, in controlled amounts, while monitoring your symptoms. This is the most important phase and the one most people skip — which is a mistake. Without reintroduction you have no idea which FODMAPs are actually your triggers.
Phase Three is the personalisation phase, which is ongoing. Based on what you learn in phase two, you build a long-term eating pattern that avoids only your specific triggers. Most people find they can tolerate the majority of foods without issue.
Week by Week — Your IBS Diet Plan
Week One — Clear Out and Stock Up
Before you begin the elimination phase, spend the first week preparing. Remove the high FODMAP staples from your kitchen: onion, garlic, wheat bread and pasta, regular milk, apples, pears, honey, baked beans and any protein powders containing inulin, chicory root or artificial sweeteners.
Stock up on Low FODMAP alternatives: oats, rice, quinoa, firm tofu, almond milk in 250ml portions, lactose-free milk, hard cheeses, eggs, strawberries, blueberries, kiwi fruit, courgette, carrots, green beans, spinach, potatoes and a certified Low FODMAP plant protein powder for reliable protein without digestive risk.
Download the Monash University FODMAP app. It is the most accurate and up to date database of FODMAP ratings for hundreds of foods, with traffic light ratings for specific portion sizes. It costs a few pounds and is worth every penny.
Week Two and Three — The Elimination Phase Begins
Your goal during weeks two and three is strict adherence to the Low FODMAP food list. Every meal should be built from verified Low FODMAP ingredients in verified Low FODMAP portions.
A sample day at this stage might look like this. Breakfast is protein porridge made with 52g of oats, almond milk and a serving of That Protein Blissful Raw Cacao — certified Low FODMAP and made with just two organic ingredients. Lunch is a salad of firm tofu, cucumber, cherry tomatoes, spinach, pumpkin seeds and olive oil with a rice cake. A mid afternoon snack is a small handful of walnuts and a few strawberries. Dinner is grilled salmon with roasted courgette, carrots and potatoes, dressed with garlic-infused oil.
The key things to watch during these weeks are hidden sources of FODMAPs. Onion and garlic powder in spice blends, inulin in protein bars and supplements, fructose in sauces and dressings, and lactose in protein shakes and coffee creamers are among the most common hidden triggers that undermine the elimination phase.
Week Four — Assess and Stabilise
By the end of week four, most people following the elimination phase correctly are experiencing meaningful symptom improvement. Use this week to consolidate. Keep a symptom diary — note what you eat, when you eat it and how you feel in the hours afterwards.
If symptoms have not improved significantly by week four, speak to your GP or a FODMAP-trained dietitian. Lack of improvement at this stage usually indicates either a hidden FODMAP source in your diet that you have not identified, or that a condition other than IBS may be contributing to your symptoms.
Week Five and Six — Begin Reintroduction
The reintroduction phase is systematic and structured. You test one FODMAP group at a time, over three days, then allow three days of Low FODMAP eating before testing the next group. You should be eating your baseline Low FODMAP diet throughout, with only the test food added.
The FODMAP groups to test, in the recommended order, are fructose (test with honey or mango), lactose (test with regular milk), fructans from wheat (test with two slices of wheat bread), fructans from garlic (test with a small amount of garlic), GOS (test with canned chickpeas, rinsed), and polyols (test with mushrooms for mannitol and avocado for sorbitol).
Record your symptoms carefully after each test. A reaction means that FODMAP group is a trigger for you. No reaction means you can tolerate it in normal amounts.
Getting Enough Protein on a Low FODMAP Diet
Protein is one of the most common nutritional concerns for people starting a Low FODMAP diet, particularly those eating plant-based. Many of the most convenient protein sources are high FODMAP — legumes contain high levels of GOS, most protein powders contain inulin or artificial sweeteners, and many plant protein bars are full of FODMAP triggers.
Reliable Low FODMAP protein sources include eggs, firm tofu in portions up to 170g, tempeh up to 100g, canned lentils rinsed thoroughly in portions up to 46g, and a certified Low FODMAP plant protein powder. That Protein is the only protein brand in the UK holding FODMAP Friendly certification — independently laboratory tested and confirmed safe for IBS sufferers at a full serving. It can be added to porridge, smoothies, overnight oats or simply mixed with almond milk for a reliable daily protein source throughout all phases of the diet.
Working With a Dietitian
The Low FODMAP diet is complex enough that doing it without professional guidance significantly increases the risk of errors, missed triggers or unnecessary long-term restriction. The British Dietetic Association strongly recommends working with a registered dietitian trained in FODMAP methodology.
Your GP can refer you on the NHS, or you can find a FODMAP-trained dietitian through the British Dietetic Association's Find a Dietitian tool at bda.uk.com.
Explore That Protein's certified Low FODMAP range at thatprotein.com — the UK's only certified Low FODMAP vegan protein powder, free UK delivery on orders over 40 pounds.
Disclaimer: This article is for informational purposes only and does not constitute medical or dietetic advice. If you have IBS or any digestive condition, please consult your GP or a registered dietitian before making significant dietary changes.