What Are FODMAPs?
FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols.
They are a group of small carbohydrates and sugar alcohols that are:
- Poorly absorbed in the small intestine (fully or partly), and
- Rapidly fermented by colonic bacteria, producing gas and short-chain organic acids.
In people with visceral hypersensitivity or altered gut motility, common in irritable bowel syndrome (IBS), that combination can produce bloating, distension, flatulence, pain, and diarrhoea or constipation. The effect is dose-dependent: the same food may be tolerated in a small portion but problematic in a larger one.
The low-FODMAP approach was developed at Monash University (Melbourne) by researchers including Peter Gibson and Sue Shepherd, building on earlier work on fructose and fructan malabsorption. It is now widely used as a short-term elimination and reintroduction protocol, not as a lifelong restriction of all fermentable carbohydrates.
High vs low FODMAP servings
Monash University measures FODMAP content per serving, not per food name alone. The same ingredient can be green (low), amber (moderate), or red (high) depending on portion size.
| Level | Practical rule |
|---|---|
| High FODMAP | Avoid during the strict elimination phase (or avoid that serving size entirely if it reliably triggers symptoms). |
| Low FODMAP | Allowed in moderation, but stacking matters: several low-FODMAP foods eaten together in one meal, or repeated across the same day, can add up to a high total load. |
| Moderate | Often treated like low: small portions may be fine; track cumulative intake. |
Stacking is one of the most common reasons elimination “doesn’t work.” Example: a meal with modest amounts of onion-derived flavouring, legumes, lactose-containing cheese, and a polyol-sweetened drink may stay under individual thresholds but exceed tolerance in combination.
The Monash University Low FODMAP Diet App uses traffic-light ratings based on tested portions. When in doubt, use tested serving sizes rather than generic food lists.
The four FODMAP groups
1. Oligosaccharides
What they are chemically
- Fructans: chains of fructose molecules, usually with a terminal glucose unit (includes inulin-type fructans and some fructo-oligosaccharides).
- GOS (galacto-oligosaccharides): short chains of galactose units attached to glucose, includes raffinose and stachyose in legumes.
Humans lack the intestinal enzymes to break these bonds, so they pass to the colon intact, classic prebiotic substrates for bacterial fermentation.
Why the chemistry matters
Fructans and GOS are among the most fermentable FODMAP classes. They increase colonic gas production and can have an osmotic effect when present in high concentration. They also selectively feed bacteria such as Bifidobacterium, which is relevant when considering long-term restriction.
Commonly high-FODMAP sources (typical problem foods)
| Sub-group | Examples |
|---|---|
| Fructans | Wheat, rye, barley (in larger servings), onion, garlic, leek, shallots, artichoke, inulin-added foods |
| GOS | Legumes (chickpeas, lentils, black beans), cashews, pistachios |
Low-FODMAP notes
Small servings of some legumes or wheat products may be green-rated in the Monash app; always check portion size.
2. Disaccharides (lactose)
What they are chemically
Lactose = one glucose + one galactose, linked by a β-1,4 bond.
Absorption requires lactase (β-galactosidase) at the brush border. When lactase activity is low, lactose remains osmotically active and reaches the colon for fermentation.
Why the chemistry matters
Symptoms correlate with dose and degree of lactase deficiency. Fermentation of lactose can reduce colonic pH and alter bacterial populations, including Bifidobacterium, which can metabolise lactose.
Commonly high-FODMAP sources
| Examples |
|---|
| Milk (cow, goat), soft cheeses, ice cream, yoghurt (portion-dependent), cream, milk powder |
Low-FODMAP notes
Hard cheeses (e.g. cheddar, parmesan) are often low in lactose. Lactose-free milk is widely available. Many people tolerate yoghurt in small tested portions.
3. Monosaccharides (excess fructose)
What they are chemically
Fructose is a single sugar (monosaccharide), absorbed via GLUT5 and facilitated by glucose through GLUT2.
The issue is not fructose alone but excess fructose relative to glucose in a food: when fructose concentration exceeds glucose, absorption capacity can be overwhelmed, leaving fructose for colonic fermentation.
Why the chemistry matters
Fructose is osmotically active. Malabsorbed fructose is fermented rapidly. Foods with balanced fructose:glucose ratios (e.g. sucrose, which is glucose + fructose 1:1) are often better tolerated than free fructose-heavy sources.
Commonly high-FODMAP sources
| Examples |
|---|
| Honey, high-fructose corn syrup (HFCS), apples, pears, mangoes, watermelon, cherries, agave syrup, fruit juice concentrates |
Low-FODMAP notes
Citrus fruits, grapes, and strawberries are often lower in excess fructose at standard servings (verify in Monash app).
4. Polyols (sugar alcohols)
What they are chemically
Polyols are low-molecular-weight sugar alcohols with multiple hydroxyl groups. Common dietary polyols include sorbitol, mannitol, maltitol, xylitol, and isomalt.
They are poorly absorbed in the small intestine and draw water osmotically; unabsorbed polyols are fermented in the colon.
Why the chemistry matters
Different polyols are absorbed at different rates (e.g. sorbitol vs mannitol). This is why Monash tests foods individually, mushrooms (mannitol) and apples (sorbitol) are separate problems.
Commonly high-FODMAP sources
| Polyol | Examples |
|---|---|
| Sorbitol | Apples, pears, stone fruit, “sugar-free” gum and mints |
| Mannitol | Mushrooms, cauliflower (portion-dependent), snow peas |
| Maltitol / xylitol / isomalt | Sugar-free confectionery, some protein bars and lozenges |
Low-FODMAP notes
Check ingredient labels for -ol sweeteners. Polyols are a frequent hidden FODMAP source.
How FODMAPs cause symptoms (brief mechanism)
Two parallel effects:
- Osmotic load, poorly absorbed molecules draw water into the small intestine and colon, increasing stool water and urgency.
- Fermentation, colonic bacteria metabolise FODMAPs, producing hydrogen, methane, and carbon dioxide, plus short-chain fatty acids.
Mechanism depth: Osmotic load and diarrhoea, Luminal environment.
In IBS, visceral hypersensitivity and altered motility mean normal fermentation can be perceived as pain and bloating. FODMAP restriction reduces substrate load; it does not fix underlying hypersensitivity.
The three phases of a low-FODMAP diet
Monash and major clinical guidelines describe three phases:
- Elimination (typically 2–6 weeks): reduce high-FODMAP foods to establish symptom baseline.
- Reintroduction: challenge individual FODMAP groups or foods systematically to identify personal triggers.
- Personalisation: long-term diet that minimises symptoms while liberalising as much fermentable intake as tolerated.
Strict elimination without reintroduction is discouraged in clinical practice.
Why strict low-FODMAP should be temporary
FODMAPs are not only symptom triggers, many are prebiotic fibres that support colonic bacteria.
Evidence from Monash-led and related trials shows that short-term low-FODMAP restriction can:
- Reduce absolute abundance of bifidobacteria and other fermentative taxa
- Alter colonic luminal environment (pH, fermentation products)
- Reduce intake of fermentable substrates that feed butyrate-producing communities
Key studies (see reading list) conclude that liberalising FODMAP restriction to the level needed for symptom control is preferable to indefinite strict avoidance. The goal is personalised tolerance, not permanent elimination of all fermentable carbohydrates.
Longer-term personalised protocols, with structured reintroduction, may maintain symptom relief while avoiding sustained depletion of beneficial bacteria, though effects on short-chain fatty acids and full microbiome recovery remain active research areas.
Helpful resources
Primary reference (Monash University)
- Monash FODMAP, official site
- Monash University Gastroenterology, FODMAP research
- Monash University Low FODMAP Diet App (traffic-light serving ratings; the practical reference for portion sizes)
Clinical and patient education
- International Foundation for Gastrointestinal Disorders (IFFGD), IBS and diet
- British Dietetic Association, IBS and dietetic management (UK dietitians publish evidence-based IBS guidance)
- ACG Clinical Guideline: Management of Irritable Bowel Syndrome (includes dietary therapy context; check current edition)
Related concepts on this site
- Bloating, lists FODMAP sensitivity among non-microbiome and microbiome-overlap causes
- High-fiber diet, many high-fiber foods are high in FODMAPs; timing and type matter
- The dietary fiber paradox, why fiber helps microbiomes but can worsen sensitive guts