What Are FODMAPs?
FODMAP is an acronym developed by researchers at Monash University in Melbourne, Australia: Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols. The term describes a collection of short-chain carbohydrates that share two important properties: they are poorly absorbed in the small intestine, and they are rapidly fermented by colonic bacteria.
The FODMAP framework was developed in the early 2000s by gastroenterologist Peter Gibson and dietitian Susan Shepherd as a tool for understanding dietary triggers of irritable bowel syndrome (IBS). It has since become one of the most evidence-supported dietary interventions in gastroenterology, with randomised controlled trials demonstrating symptom improvement in 50–80% of IBS patients who follow a low-FODMAP diet.
The Components
Oligosaccharides include fructans (chains of fructose molecules) and galactooligosaccharides (GOS). Fructans are found in wheat, rye, onion, garlic, and leek — foods that are difficult to avoid in most Western diets. GOS is found primarily in legumes. Humans lack the enzymes to digest either, so they pass intact to the colon.
Disaccharides in the FODMAP context refers primarily to lactose, the sugar in dairy products. Lactose requires the enzyme lactase for digestion. Lactase production typically declines after weaning in most of the world's population, making lactose malabsorption the norm rather than the exception globally.
Monosaccharides refers to excess fructose — specifically, fructose in amounts that exceed the intestine's absorption capacity. The small intestine absorbs fructose via a transporter (GLUT5) that can be saturated. Fructose absorbed alongside glucose uses a different, more efficient transporter (GLUT2), which is why foods with equal fructose and glucose (like table sugar) are generally better tolerated than foods with excess fructose (like apples, honey, and agave syrup).
Polyols are sugar alcohols: sorbitol (found in stone fruits and used as a sweetener), mannitol (found in mushrooms, celery, and sweet potato), xylitol and maltitol (common sweetener additives). These are absorbed slowly and incompletely, with the unabsorbed portion drawing water into the bowel osmotically and providing fermentation substrate in the colon.
The Mechanism of Symptoms
When FODMAPs reach the colon, they are fermented by bacteria within hours, producing hydrogen (and sometimes methane) gas. The rapid gas production causes distension of the colonic wall, triggering the stretch receptors that produce the sensation of bloating and cramping. In people with IBS, visceral hypersensitivity means these sensations are experienced more acutely than in people without the condition.
The osmotic effect of polyols and excess fructose draws water into the colon simultaneously, which can cause looser stools or diarrhoea. Conversely, methane gas (produced in approximately one-third of the population) slows intestinal transit and is associated with constipation-predominant IBS.
High and Low FODMAP Foods
High-FODMAP foods that commonly trigger symptoms include wheat and rye products; onion and garlic (even in small amounts — among the highest fructan foods per gram); apples, pears, and stone fruits (sorbitol, excess fructose); legumes (GOS); milk, soft cheese, and ice cream (lactose); and mushrooms and cauliflower (mannitol).
Lower-FODMAP alternatives that are generally better tolerated include rice, oats, and gluten-free products; spring onion greens (but not the white bulb); bananas, berries, citrus, and grapes; firm tofu and canned legumes (rinsed — some GOS leaches into the canning water); hard aged cheeses and lactose-free dairy; and zucchini, capsicum, and eggplant.
The Three-Phase Protocol
A low-FODMAP diet is implemented in three phases. The elimination phase (typically two to six weeks) involves removing all high-FODMAP foods to establish a symptom baseline. If symptoms improve, this confirms FODMAP sensitivity and the diet is working as a diagnostic tool.
The reintroduction phase involves systematically testing each FODMAP subgroup — one at a time, with washout periods between tests — to identify which specific types trigger symptoms. This is critical because most people are not sensitive to all FODMAPs; the goal is to identify individual triggers, not to permanently avoid all fermentable carbohydrates.
The personalisation phase involves building a long-term diet that excludes only confirmed personal triggers while maximising dietary diversity and prebiotic fiber intake. A permanently restrictive low-FODMAP diet is not recommended because fermentable carbohydrates are important for gut microbiome health.
FODMAPs and the Gut Microbiome
It is worth noting that the foods most likely to cause gas — legumes, whole grains, vegetables, fruits — are also the foods most important for a healthy gut microbiome. FODMAPs are, by definition, fermentable — and fermentation by colonic bacteria is what produces the short-chain fatty acids that nourish colonic cells, regulate inflammation, and maintain the gut barrier.
A permanent low-FODMAP diet reduces microbiome diversity and lowers populations of Bifidobacterium — a bacterial genus associated with multiple health benefits. The goal for most people should be to identify and manage specific triggers, not to eliminate all fermentable carbohydrates indefinitely.