FODMAP diet: what to eat, what to avoid and how long it lasts

What is the FODMAP diet and what does the word mean?

FODMAP stands for fermentable oligosaccharides, disaccharides and monosaccharides, and polyols. They are short-chain sugars that the small intestine does not fully absorb.

These sugars draw in water and reach the large intestine undigested. There, bacteria ferment them and produce gas [1]. The gas and water stretch the wall of the intestine. Most people do not notice it. People with irritable bowel syndrome do, because their gut responds in an exaggerated way to that distension.

That is why this diet is reserved for an already diagnosed irritable bowel syndrome. The British Society of Gastroenterology describes it as a disorder of gut-brain interaction [2].

Which foods contain FODMAPs and which do not?

The lists above have the detail. This is the map by group, which is easier to remember than two hundred separate foods.

• Fructans. Wheat, rye, onion, garlic, leek, artichoke and asparagus.
• Galactans. Pulses and whole soybeans.
• Lactose. Milk, yoghurt, cream and fresh cheeses.
• Fructose, when it is well above glucose. Honey, fructose syrup, apple, pear and mango. If this sounds familiar, see the guide to fructose intolerance.
• Polyols. Anything ending in -ol, such as sorbitol, mannitol, xylitol or maltitol, and stone fruits such as peach, nectarine, plum or cherry. They are in chewing gum and in “sugar-free” products.

The amount of FODMAP you consume depends on the portion. Monash University, which designed this diet, classifies each food as green, amber or red according to the amount [1]. Half a cup may be green and a whole cup red. That is why the lists in circulation contradict each other. It is also why some of the foods above come “in a small portion”, below about 40 g.

Meat, fish, eggs and oil contain no FODMAPs. If any of these disagrees with you, the cause is something else.

How long does a low-FODMAP diet last?

The elimination phase lasts two to six weeks [1]. What comes next decides the result, and it is what almost nobody gets round to doing.

1. Elimination. You swap high-FODMAP foods for low ones for that period. If you have noticed nothing by the end, your symptoms do not depend on FODMAPs. In that case you stop and try another alternative [3].
2. Reintroduction. You reintroduce one group at a time. You eat a food containing only that FODMAP for three days in a row, such as milk for lactose, and note what happens [1]. It takes several weeks, and it is the part that teaches you the most.
3. Personalisation. You set aside only what disagrees with you, and only in the amount that disagrees with you. That list is yours and is usually much shorter than the one from the first phase.

The American College of Gastroenterology recommends a limited trial of this diet [4]. Keeping it going with no end date reduces the variety of what you eat, and with it that of your gut microbiota. From then on it no longer does what it was designed for.

Does the FODMAP diet work?

Yes. It is the dietary intervention with the most evidence in irritable bowel syndrome, according to the American Gastroenterological Association [3]. An analysis of 13 trials with 944 people ranked it first against the usual diet and against other dietary advice [5]. It came first for symptoms overall, pain and bloating. It is recommended when general advice on eating and lifestyle has not been enough [5].

It does not work for everyone. According to Monash University, about three in four people with irritable bowel syndrome improve, and one in four do not [1]. If you are in that quarter, extending the elimination does not fix it and takes food away from you.

The evidence also has a limit. Almost all of these trials were carried out in specialist clinics and hospitals, and none studied reintroduction or personalisation [5]. The first phase is well tested, and less is known about the other two.

A twelve-month follow-up of eighteen people who completed all three phases found that two in three were still doing better [6]. Their bifidobacteria had not fallen compared with the start [6]. Eighteen people is not many, so take it as a pointer.

Is the FODMAP diet good for losing weight?

It is not designed for that. The American College of Gastroenterology recommends it for irritable bowel syndrome symptoms [4]. If you lose weight while following it, it is usually because you have removed half the pantry. You are eating less variety and less quantity, the opposite of what matters here.

There is also a serious reason not to use it that way. Restrictive diets are not suitable for anyone with an eating disorder, who already eats very little variety or who is at risk of malnutrition [3]. That is why this should be assessed before starting. If your relationship with food worries you, talk to your doctor and seek the support of a psychology professional.

Is it the same as eating gluten-free?

No. In a low-FODMAP diet, wheat, barley and rye are reduced because of the fructans they contain. Gluten is something else. Fructans reach the large intestine undigested, where bacteria ferment them.

The difference shows on the plate. Some long-fermentation breads and small portions of wheat fit. In contrast, onion and garlic, which contain no gluten, are among the first things removed. Removing gluten only makes sense if there is coeliac disease or if your doctor has advised it. If you suspect coeliac disease, do not remove gluten before testing.

Observational studies find that many people with irritable bowel syndrome improve without gluten, but randomised trials give mixed results [3].

If you eat vegetarian

It can be done, although it is harder. Pulses are the main protein of a vegetarian diet and at the same time a major source of galactans and fructans.

What remains is quite a lot: firm tofu, tempeh, eggs if you eat them, quinoa, polenta and cornflour, buckwheat, walnuts and seeds. The less variety you start with, the more it is worth doing this with a registered dietitian-nutritionist at your side [3].

If you have been bloated for weeks without knowing why, start with the guide to bloating and abdominal distension. If you have been diagnosed with gastritis, the approach is different.

What mistakes do we see in consultations?

Some mistakes come up almost every week in consultations. None of them is the fault of the person making it. They almost always come from lists that do not explain how this diet is used.

• Starting without a diagnosis. Bloating and pain have many causes. If nobody has diagnosed irritable bowel syndrome, removing FODMAPs can mask something else without resolving it.
• Staying in the first phase. People arrive who have spent months eating twelve foods and are more afraid than at the start. Elimination was meant for a few weeks.
• Not writing anything down. Without noting what you eat and how you feel, reintroduction is done from memory, and memory misleads.
• Trusting “gluten-free” and “sugar-free”. A gluten-free bread may contain pulse flour or inulin, and a “sugar-free” sweet usually contains polyols.