FODMAPs explained: what they are and why they cause symptoms

FODMAPs are types of short-chain carbohydrates found in a wide variety of everyday foods. In some people, especially those with irritable bowel syndrome (IBS), they can bring on bloating, wind, tummy pain and changes in bowel habits. The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) notes that a doctor may suggest a special eating plan, the low FODMAP diet, for IBS.

Below: what each letter stands for, why FODMAPs cause symptoms in some people, how the low FODMAP approach works, why it is not meant to be followed for ever, and what fructose malabsorption is.

What does FODMAP stand for?

FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols. A 2022 review in the Journal of Neurogastroenterology and Motility, written by dietitians and gastroenterologists, explains each part:

• Fermentable. When these carbohydrates reach the large intestine, bacteria there ferment them quickly and release gas.
• Oligosaccharides are short chains of sugars. The two main kinds are fructans, found in wheat, onion and garlic, and galacto-oligosaccharides, found in pulses such as lentils and chickpeas. The human small intestine has no suitable enzymes to digest them.
• Disaccharides are double sugars. The one that matters here is lactose, the sugar in milk, which some people digest poorly because their bodies make little lactase. Our guide to lactose intolerance explains why.
• Monosaccharides are single sugars. Here, that means fructose in foods that contain more fructose than glucose, such as honey and mango. It is absorbed slowly in the small intestine.
• Polyols are sugar alcohols. NIDDK points to sweeteners ending in "-ol", such as sorbitol, mannitol, xylitol and maltitol, in sweets and gum. Polyols are also found naturally in some fruit and vegetables.

The review lists common sources of each group in Middle Eastern diets:

These are everyday, nutritious foods, not foods to avoid. The same review advises against labelling foods "good" or "bad", because that kind of language can feed fear of food. It also lists low FODMAP staples in Middle Eastern diets, such as rice, tomato, cucumber, spinach, red pepper, olives and olive oil, sesame, pine nuts, fish, meat and chicken.

Why do FODMAPs cause symptoms in some people?

The review describes two effects:

1. Water. Because the smaller FODMAP molecules are slowly absorbed or not digested, they draw water into the small intestine.
2. Gas. The FODMAPs that reach the large intestine are rapidly fermented by bacteria, releasing gas. NIDDK explains that this fermentation of carbohydrates not digested earlier is one of the two main sources of gas in the gut; our guide to gas and bloating covers the other.

The extra water and gas stretch the gut, and that stretching produces the symptoms. Not everyone reacts the same way: NIDDK notes that some people with IBS feel pain from a normal amount of gas in the gut, and a 2016 review of fructose malabsorption adds that gut sensitivity and pain perception may vary a great deal from person to person.

How does the low FODMAP approach work?

It is not a diet for life. The review describes three phases, usually guided by a dietitian:

1. Restriction. For a short time, high FODMAP foods are swapped for low FODMAP alternatives, to see whether symptoms improve. The review describes a follow-up 2 to 8 weeks after the first appointment, at the end of this phase.
2. Reintroduction. FODMAP groups are brought back one at a time, to find out which ones trigger symptoms, and in what amounts.
3. Personalisation. A long-term diet that is as varied as possible, limiting only the FODMAPs that cause trouble. The aim is to keep symptoms under control while eating as many FODMAPs as you tolerate. Tolerance can change over time, so the review suggests trying poorly tolerated foods again now and then.

NIDDK describes the same idea: try the low FODMAP diet for a few weeks and, if symptoms improve, slowly add FODMAP foods back. You may be able to eat some of them without symptoms. If the diet does not help, the review says it should be stopped, and other approaches considered.

Why a dietitian? Almost all the published trials were dietitian-led. In one evaluation cited by the review, people who followed the diet on their own ate more than double the FODMAPs during the restriction phase, and stuck to each stage less well, than people supported by a dietitian. The British Dietetic Association (BDA) suggests asking your doctor to refer you to a dietitian, who may suggest trying this approach if general IBS advice has not helped.

Why not cut foods out long term without advice?

Because FODMAPs are found in so many foods, strict long-term avoidance can leave gaps in your diet. The review reports that, in a study of over 3,000 patients, those who followed the restriction phase most strictly took in significantly less calcium, magnesium, vitamin C, folate and riboflavin. Fibre intake can also fall in the early phase.

A 2024 review in JGH Open adds that staying on the restrictive phase for a long time may affect both nutrition and the gut's bacteria. It describes the approach as unsuitable for some people, including those with a current or past eating disorder, people who are underweight or losing weight without explanation, frail older adults and young children.

One more reason to wait: wheat is a FODMAP food, and the NHS says you should keep eating gluten until you have been tested for coeliac disease, or the tests may not be accurate. Our guide to gluten and coeliac disease explains more. The BDA stresses that it is important to have an IBS diagnosis confirmed, and conditions such as coeliac disease and inflammatory bowel disease ruled out.

What is fructose malabsorption?

Fructose is a simple sugar found mainly in fruit. In fructose malabsorption, according to MedlinePlus Genetics, part of the US National Library of Medicine, the intestine cannot absorb fructose normally, which leads to bloating, diarrhoea or constipation, wind and stomach pain. It is thought to affect about 40% of people in the Western hemisphere, and its cause is unknown.

The 2016 review, in Molecular and Cellular Pediatrics, explains the mechanism. Fructose that is not absorbed reaches the large intestine, where bacteria ferment it and produce gases such as hydrogen; the extra sugar may also draw in water and cause diarrhoea. Everyone's capacity to absorb fructose is limited: a dose of 50g, about the amount in seven medium apples, is more than most adults can absorb. Doctors usually diagnose it with a hydrogen breath test, although the right test dose is still debated. Fructose is one of the sugars limited in the low FODMAP approach.

Fructose malabsorption is not the same as hereditary fructose intolerance, a rare inherited condition that MedlinePlus Genetics estimates at 1 in 20,000 to 30,000 people. It usually shows in infancy, once fruit and juice are added to the diet, and can damage the liver and kidneys. It is a different, and much more serious, condition.

When to see a doctor

See your doctor before starting a low FODMAP diet: the 2022 review stresses having a clear diagnosis before any diet therapy begins. The NHS advises seeing a GP if you think you might have IBS and have had symptoms for over 4 weeks.

Ask your doctor or dietitian for help if, as the 2022 review lists, you are stuck in the restriction phase, you are following several restrictive diets at once, you have lost weight (5% of your body weight in a month, or 10% in six months), or you have signs of disordered eating.

Get medical advice urgently, the same day, the NHS advises, if:

• you have lost a lot of weight for no reason;
• you are bleeding from your bottom or have bloody diarrhoea;
• you find a hard lump or swelling in your tummy;
• you have shortness of breath, noticeable heartbeats (palpitations) and skin that is paler than usual.