Before we launch into the details of what FODMAPs are, and how they act in the gut, we first need to review how foods (particularly those rich in carbohydrates) are digested and absorbed in the body.


The process of digestion

The small intestine is responsible for the breakdown and absorption of nutrients from foods. The role of the large intestine is to absorb water and prepare undigested food for removal from the body (as faeces). As undigested food passes through the large intestine, it is fermented by the resident bacteria. This results in the production of a number of substances, including gases, such as hydrogen and methane.

Irritable Bowel Syndrome

For many years, patients have related their IBS (irritable bowl syndrome) symptoms to the ingestion of fatty foods, alcohol, gluten and caffeine. However, there is limited evidence to suggest that these food components play a role in triggering IBS symptoms.

IBS is a common functional gastrointestinal disorder (FGID) that affects one in seven adults. It is characterised by gastrointestinal symptoms that are not explained by other disorders. The most common symptoms of IBS include:

  • lower abdominal pain
  • bloating
  • excessive passage of wind
  • distension
  • altered bowel habit (diarrhoea, constipation, or a combination of both)

These symptoms overlap with several other serious gastrointestinal and gynaecological conditions, such as inflammatory bowel disease (IBD), coeliac disease, bowel cancer and endometriosis. To ensure that these conditions are rules out and IBS is properly diagnosed, IBS-like symptoms should be discussed with a medical practitioner.

Stress may also trigger IBS symptoms in some people. If stress is considered a major trigger of your IBS symptoms, other psychological-based therapies may be helpful, such as gut directed hypnotherapy and cognitive behavioural therapy (CBT).



Recently, research investigating diet and IBS symptoms has focused on a large group of dietary sugars, known as FODMAPs (Fermentable Oligo-saccharides, Di-saccharides, Mono-saccharides and Polyols).

These sugars can be poorly absorbed in the small intestine and fermented by bacteria in the large intestine to produce gas and there is now a large body of research which suggests that this group of sugars can trigger IBS symptoms.


What are FODMAPS?

FODMAPs are short-chain carbohydrates that are rapidly fermented and can be poorly absorbed.

Fructose in excess of glucose (found in certain fruits and honey and some high fructose corn syrups)

  • Fructose is a monosaccharide (one sugar unit). It is the smallest FODMAP which means that it has the largest effect on attracting water into the bowel (known as an osmotic effect).
  • The absorption of fructose in the small intestine relies on the activity of special ‘sugar transporters’ that are located in the wall of the small intestine.
  • Most people absorb fructose, but around 30 – 40% of all people (healthy people and people with IBS), malabsorb excess fructose.
  • Fructose is absorbed efficiently when it is in the presence of enough glucose (another single sugar). However, it is absorbed less efficiently when there is more fructose than glucose present (known as excess fructose). In high doses, excess fructose may be malabsorbed.

How does excess fructose cause symptoms?

Fructose attracts water into the bowel, so the symptoms most likely to occur following high intakes (in people with IBS) are diarrhoea, abdominal pain, and bloating. Research has shown us that fructose can have these effects regardless of whether it is malabsorbed.

Lactose (found in milk and milk products)

  • Lactose is a disaccharide sugar (two sugar units joined together).
  • To be absorbed, lactose must be split into two single sugar units by an enzyme known as lactase.
  • The enzyme, lactase is located in the wall of the small intestine. People who lack this enzyme will malabsorb lactose.

How does lactose cause symptoms?

Lactose that reaches the large intestine unabsorbed attracts water into the bowel. It is also fermented by gut bacteria, producing gas. These effects result in symptoms including diarrhoea, bloating and abdominal pain.

Polyols (eg. sorbitol, mannitol) (found in some fruits and vegetables and sometimes added as artificial sweeteners)

  • Sorbitol and mannitol are the most common polyols in the diet. In most people, they are incompletely absorbed in the small intestine
  • The majority of people with IBS may benefit from sorbitol restriction and some may benefit from mannitol restriction.

How do polyols cause symptoms?

Because polyols are small in size (like fructose), they attract water into the bowel. Common symptoms experienced after eating polyol-rich foods (in people with IBS) are diarrhoea and abdominal pain

Oligosaccharides – fructans (FOS) and galacto-oligosaccarides (GOS) (found wheat, rye, onions, garlic, legumes and lentils)

  • These are longer chains of sugars that humans do not have enzymes to digest. They are universally malabsorbed and fermented by bacteria in the large intestine.

How do fructans and GOS cause symptoms?

Fructans and GOS are malabsorbed and fermented in everyone, and are thought to have positive effects on the gut bacteria. However, in people with IBS, the gas produced through fermentation can lead to significant discomfort, and symptoms of abdominal pain, excessive wind, bloating and altered gut motility. This is because people with IBS have a highly sensitive gut.

If your symptoms allow, including small amounts of foods rich in oligosaccharides may be a good idea, due to the beneficial effects of these carbohydrates on the gut bacteria.

Who malabsorbs FODMAPs

  • As previously mentioned, oligosaccharides are malabsorbed in everyone, because humans lack the enzymes needed to break these carbohydrates down. People with IBS may experience gastrointestinal symptoms when larger doses of oligosaccharides are consumed due to their highly sensitive gut wall.
  • Fructose, lactose and polyols (sorbitol, mannitol) are only malabsorbed in some individuals. Currently, breath tests are used to detect fructose, lactose and sorbitol malabsorption. However, please read page 17 on hydrogen breath testing for more information.
  • Other reasons why FODMAPs can cause symptoms in people with IBS include
    • Gut hypersensitivity – people with IBS may produce more gas than those without IBS and their gut may be more sensitive to the gas produced.
    • Bacterial overgrowth in the small intestine. in some individuals, bacteria that are normally located in the large intestine, move up into the small intestine. This condition is referred to as ‘small intestinal bacterial overgrowth’. One theory is that when FODMAPs are fermented by bacteria in the small intestine, the gas produced bloats this narrow section of the bowel, causing abdominal discomfort, distension, bloating and abdominal pain.

What are the other types of carbohydrates?

Fibre and resistant starch

These are longer chain carbohydrates that resist digestion in the small intestine. They are important for stool formation and normal functioning of the bowel. Dietary fibre and resistant starch are an important part of the diet for everyone. See page 28 for more information about eating enough fibre on a low FODMAP diet.


Starches are long chain carbohydrates that are completely digested and absorbed in the small intestine. They are not likely to cause problems for people with IBS.


Basically it is adapting a lifestyle where you are aware of what high FODMAP foods and portions work for you as a unique being and being mindful of how food stacking or combinations impact you in terms of portioning.

A 3-step phase is recommended:

  1. Low FODMAP diet
    • Commenced under supervision of a dietitian/nutritionist
    • 2-6 weeks only
  2. Re-challenge phase
    • Food challenges completed under the guidance of a dietitian/nutritionist
    • 6-8 week process
  3. Adapted lifestyle
    • Interpret food challenge results with a dietitian/nutritionist
    • Repeat challenges with poorly tolerated foods
    • Reintroduce restricted food based on symptom response – only restrict foods and FODMAPs that trigger symptoms

Download the Monash university FODMAP App for super helpful and informative information with a food guide to help educate yourself.


Reference: Monash University, Department of Gastroenterology, FODMAP Diet

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