The Low-FODMAP Diet, Explained

The low-FODMAP diet is a structured, three-phase dietary approach developed by Monash University that may help support digestive comfort in people living with irritable bowel syndrome (IBS). It works by temporarily limiting fermentable carbohydrates that can draw water into the bowel and be rapidly fermented by gut bacteria, then systematically reintroducing them to identify personal triggers.

What is the low-FODMAP diet?

The low-FODMAP diet is a structured, evidence-informed dietary protocol designed to identify and reduce the fermentable carbohydrates that may contribute to digestive discomfort in people living with irritable bowel syndrome and similar functional gut conditions. Rather than being a single restrictive eating plan, it is a three-phase process that begins with a period of temporary restriction before methodically reintroducing foods to pinpoint individual triggers.

The concept was first published in 2005 by Professor Peter Gibson and Dr Sue Shepherd at Monash University in Melbourne, Australia — the institution that remains the globally recognised research authority on the approach. Since then, the Monash FODMAP App (launched 2012) has been downloaded by more than 1.75 million people across 100-plus countries, and the approach is referenced in clinical guidelines from organisations including the American College of Gastroenterology and, closer to home, the National Institute for Health and Care Excellence (NICE).

In the UK, NICE guideline CG61 (updated 2015) lists the low-FODMAP diet as a second-line dietary option for people living with IBS whose symptoms have not improved sufficiently with first-line lifestyle and dietary changes. NICE specifies that this guidance “should only be given by a healthcare professional with expertise in dietary management” — a point we return to throughout this article.

What does FODMAP stand for?

The acronym FODMAP refers to four distinct groups of fermentable short-chain carbohydrates that the small intestine absorbs poorly in many people.

• Fermentable Oligosaccharides — this group includes fructans (found in wheat, rye, onions, garlic, and leek) and galacto-oligosaccharides (GOS, found in legumes such as chickpeas, lentils, and kidney beans). These carbohydrates pass largely undigested into the large intestine, where gut bacteria ferment them rapidly.
• Disaccharides — the primary disaccharide of concern is lactose, the sugar found in cow’s milk, soft cheeses such as ricotta and cottage cheese, most yoghurts, and ice cream. People with low levels of the enzyme lactase may find lactose is not fully broken down before it reaches the large intestine.
• Monosaccharides — this refers specifically to excess fructose, where the ratio of fructose to glucose in a food is high. Apples, pears, mango, honey, and high-fructose corn syrup are well-known examples. The issue is not fructose itself, but an excess of it relative to glucose, which affects absorption.
• And Polyols — sugar alcohols including sorbitol (found in stone fruits such as peaches, plums, and apricots, and in many “sugar-free” products) and mannitol (found in mushrooms and cauliflower). Polyols are not fully absorbed in the small intestine and move on to the large intestine, where they are fermented.

In people with a sensitive gut, this combination of poor absorption, osmotic water-drawing, and rapid bacterial fermentation may be associated with bloating, cramping, excess wind, and changes in bowel habit — the hallmark symptoms of IBS support consultations.

How does the low-FODMAP diet work? The three phases

The low-FODMAP diet is not a single, permanent eating plan — it is a structured three-phase protocol that typically takes between three and six months to complete in full.

Phase 1: Restriction (2–6 weeks)

During the restriction phase, high-FODMAP foods are swapped for low-FODMAP alternatives to reduce the overall FODMAP load on the gut. This is a substitution diet, not a starvation approach — there are plenty of nutritious, satisfying foods to eat throughout this phase. The purpose is to create a low-symptom baseline, providing a clearer picture of how your gut responds when fermentable carbohydrates are temporarily reduced.

Monash University and the British Dietetic Association (BDA) are clear that this phase should last no longer than 2–6 weeks. It is a diagnostic window, not a long-term diet. Prolonged restriction is associated with reduced levels of bifidobacteria — beneficial gut bacteria that feed on the prebiotic fibre found in fructans and GOS — as well as potential gaps in calcium, iron, dietary fibre, and other key nutrients.

Phase 2: Systematic Reintroduction (approximately 6–8 weeks)

The reintroduction phase is where the real detective work happens. One FODMAP subgroup at a time is reintroduced in increasing amounts over several days, with a washout period between each challenge to avoid crossover effects and allow the gut to return to baseline.

The aim is not to find a diet that avoids all FODMAPs — the aim is to identify your personal triggers. Not everyone reacts to all five FODMAP subgroups. Many people find they tolerate one or two groups perfectly well. This phase requires careful, systematic testing and benefit greatly from professional interpretation of results.

Phase 3: Personalisation (ongoing)

Once reintroduction testing is complete, the goal is to build the most varied, nutritionally complete long-term diet possible — restricting only the specific FODMAP subgroups shown to trigger your symptoms. Monash research published in 2021 found that bifidobacteria levels, which can fall during the restriction phase, recover during personalisation as prebiotic-rich foods are reintroduced.

The personalisation phase is the intended destination of the entire process. A long-term diet that unnecessarily excludes entire food groups risks nutritional gaps, reduced gut microbiome diversity, and a diminished relationship with food. Working with a suitably qualified practitioner helps ensure that the final eating pattern is both appropriate for your gut and nutritionally robust.

High-FODMAP and low-FODMAP foods: a practical guide

During Phase 1, the focus is on identifying lower-FODMAP alternatives for each food category — not on eliminating entire food groups permanently. The table below gives practical examples drawn from the Monash University FODMAP food list, which is the most rigorously tested reference available and is continuously updated as new foods are analysed.

Source: Monash University FODMAP food list. Portion sizes matter — some foods are low FODMAP in small amounts but high FODMAP in larger servings. Always refer to current Monash guidance or work with a qualified practitioner for specific portion information.

It is worth noting that portion size is critical on this diet. A food that is low FODMAP in a standard portion may become high FODMAP in a larger one. This is one reason why the Monash FODMAP App — which includes tested portion data for hundreds of foods — is considered such a useful tool, and why working with a practitioner who can help you interpret the data is strongly recommended, particularly if you also have concerns about food intolerance support.

Is the low-FODMAP diet the same as going gluten-free?

No — and this is an important distinction that is frequently misunderstood. Wheat is high in FODMAPs because it contains fructans, a type of fermentable oligosaccharide. Wheat does also contain gluten, but the low-FODMAP diet is not a gluten-free protocol and it does not address gluten as a separate concern.

Many commercially available gluten-free products still contain high-FODMAP ingredients — for example, apple juice used as a sweetener, or legume flours. Conversely, traditionally fermented sourdough breads made from wheat or spelt may be lower in FODMAPs because the long fermentation process breaks down a significant proportion of the fructans.

If you have been tested for and diagnosed with coeliac disease, a strict gluten-free diet remains medically essential regardless of FODMAP content. If you are wondering whether gluten or FODMAPs (or both) are affecting your gut, a practitioner assessment is the appropriate starting point — self-diagnosis and self-directed elimination diets can lead to unnecessarily restricted eating patterns and make accurate testing more difficult.

What does the evidence say about the low-FODMAP diet?

Monash University notes that, in their research, around three in four people following the low-FODMAP diet with professional support reported improvements in gut symptoms — though individual results vary. Gloucestershire Hospitals NHS Trust references a similar figure of around 70% of patients who try it with the support of a FODMAP-trained dietitian.

It is important, however, to hold these figures with appropriate context. NICE, when it updated guideline CG61 in 2015, noted that the underlying evidence base at that time was of low quality and called for further research. For this reason, practitioners trained in evidence-based nutritional therapy will always frame the diet in conditional language: it may be associated with meaningful improvements in digestive comfort for many people living with IBS, rather than guaranteeing a particular outcome.

What is consistently clear across the research is that professional support significantly affects outcomes. This is not a diet to navigate from a blog article alone. The restriction phase is complex, the reintroduction testing requires methodical execution, and the interpretation of results benefits from an experienced clinical eye.

Why you shouldn’t follow the low-FODMAP diet without professional support

The NHS, the BDA, and Monash University all consistently advise against attempting the low-FODMAP diet without guidance from a suitably qualified practitioner. There are several reasons for this.

• Nutritional adequacy: Without careful planning, the restriction phase can reduce intake of calcium (from dairy), iron and B vitamins (from wheat-containing foods), dietary fibre (from fructans and GOS-rich legumes), and other key nutrients. A qualified practitioner will help ensure adequate substitutions are in place.
• Risk of unnecessary long-term restriction: Without systematic reintroduction testing, people often continue avoiding all high-FODMAP foods indefinitely — which is not the intention of the diet and carries real risks for gut microbiome diversity and nutritional status.
• Complexity of the reintroduction phase: Testing one FODMAP subgroup at a time, with appropriate washout periods and challenge doses, requires careful structure. Incorrect implementation makes it very difficult to interpret which foods are genuinely problematic.
• Ruling out other causes: IBS-like symptoms can sometimes have other underlying causes that warrant medical assessment. Your GP should be the first port of call for new or changing digestive symptoms, and a referral for investigation may be appropriate before beginning any dietary protocol.

A FODMAP-trained registered nutritional therapist can work alongside your GP and medical team to guide you through each phase of the diet, supporting nutritional adequacy and helping you interpret your results. If your GP considers it appropriate, they may also refer you to an NHS dietitian for further support.

This article is for general education and is not medical advice. Nutritional therapy supports general health and wellbeing; it is not a substitute for medical care, and we do not diagnose, treat, cure or prevent any disease. Always consult your GP about any medical concern.