What’s a FODMAP Diet?

The low FODMAP diet is a diet which has been rising steadily in popularity over the past few years. But what is it for? Who can benefit from it? And most importantly, should you attempt it yourself?

The acronym FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols. In very basic terms, these are sugars or sugar alcohols which are found in a large variety of foods, from dairy to vegetables to wholegrains and more. During digestion, we can easily digest these sugars as they pass through our gastrointestinal tract, using enzymes such as amylase to breakdown the longer chains into shorter ones. Most people can then absorb these smaller sugars easily, and either store it for later in our glycogen or fat stores or use it now to create ATP for energy. However, in some people, these sugars aren’t as easily absorbed. This means that these sugars remain in your intestines and they can wreak havoc, causing a lot of the symptoms of IBS (S. Magge, 2012).

These sugars are what is known as osmotic, and so they will draw water from your cells towards them. If they remain in your GI tract, they will therefore cause a net flow of water into the lumen (the inside) of your intestines, which can be difficult to absorb in large quantities. This can, therefore, cause diarrhoea and cramping, due to the gut trying to move this loose stool along the tract. For some people, this may only occur after eating large quantities of the food that they find difficult to absorb- however for others, even tiny amounts can cause very loose bowel movements.

As these sugars don’t get absorbed, they will travel through your GI tract to places where they shouldn’t usually go, such as your colon (large intestine). The bacteria here, which usually feed on the fibre in your food, will instead be able to feed on the sugars. These bacteria produce methane gas as a by-product of this fermentation, and this can cause incredibly painful gas and ultimately, embarrassing flatulence.

The idea of the low-FODMAP diet is to identify which of these sugars those with IBS are struggling to absorb. It involves a three-step approach, as follows (King’s College London, 2021):

  • Elimination- 4-8 weeks of eliminating all FODMAPs until symptoms have resolved.
  • Re-introduction- 3 step introduction trials of each high-FODMAP food to identify 1) which FODMAPs you are sensitive to, and 2) the quantity of those FODMAPs that trigger the symptoms
  • Personalisation- going back to your regular diet and only omitting the trigger foods which cause symptoms with even a small amount, and reducing the foods which cause symptoms up to the tolerable limit, as identified in step 2.

These three steps are key to understand when it comes to the FODMAP diet. Unfortunately, if people try to attempt this by themselves, they often only follow step one and do not do step two or three as they are scared that they will lose their symptom relief. This is very dangerous, as the elimination part of the low-FODMAP diet is not nutritionally complete and should not be attempted for longer than 4-8 weeks. This can mean you end up omitting key nutrients and can end up with various deficiencies. There may also be the case that, if this diet is attempted alone, you may not successfully eliminate all FODMAPs, as they can be very difficult to identify on pre-packaged foods, and therefore you may not see a benefit. Therefore, you may assume that your symptoms are not diet related and then potentially miss out on symptom relief, purely because you did not successfully eliminate all FODMAPs. Due to these reasons, it is recommended that you do NOT attempt the low-FODMAP diet without the supervision of a Registered Dietitian, whether private or NHS based, who is ideally accredited by Kings College London or Monash University as a FODMAP-registered dietitian (King’s College London, 2021).

So how do you know if you should contact an RD about trying the low-FODMAP diet?

Ultimately, there is going to be criteria that your RD will want to see that you have fulfilled prior to attempting this diet, as it’s always best practice to try the easier and less time-consuming things first before you try such a restrictive diet.

  1. It is important that you have ruled out all other possible conditions which could cause similar symptoms to IBS (National Institute for Health and Care Excellence, 2021). This will include going to your GP and being screened for any Inflammatory Bowel Diseases (such as ulcerative colitis or Crohn’s), coeliac disease, and in some cases, bowel or other gentio-urinary cancers. Once these have been ruled out, your GP will likely diagnose you with IBS as long as you fit the ROME IV criteria for IBS (ROME Foundation, 2016). This includes recurrent abdominal pain on average at least 1 day/week in the last 3 months (with symptom onset at least 6 months prior to diagnosis), associated with two or more of the following criteria:

    a) Related to defecation
    b)Associated with a change in frequency of stool
    c) Associated with a change in form (appearance) of stool
  2. Before you start the Low-FODMAP diet, your dietitian will often ask that you do the following to ensure that your symptoms are not caused by any more common factors. The British Dietetic association recommends (Y. McKenzie, 2019):

    a) Eating 3 regular meals per day
    b) Avoiding skipping meals and eating late at night
    c) Limiting alcohol intake to no more two units per day and aiming to have at least 2 alcohol-free days a week
    d) Reducing intake of caffeinated and/or carbonated beverages (including tea, coffee, and fizzy drinks)
    e) Ensuring adequate fluid intake, at least 8 cups (this includes all fluids that are non-alcoholic)
    f) Reducing intake of fatty or greasy foods, and trying to prepare food from scratch rather than relying on manufactured or ultra-processed foods
    g) Limit fresh fruit to no more than 3 portions per day (240g)
    h) Avoiding sugar-free sweets or gums that are sweetened with sugar alcohols such as xylitol.

Your dietitian may look at the types and amounts of fibre that is in your diet and may try to change this before you try the low-FODMAP diet. This should be done under the supervision of a dietitian as a sudden increase in fibre can worsen symptoms, and different types of fibre can have different effects on both bowel movements and gas/bloating.

Once all of these criteria have been met with no or little improvement, and if you fit the criteria required to undergo the low-FODMAP diet (loose stools, gas/bloating), your dietitian will then talk you through each of the stages of the low-FODMAP diet.

So what is the Diets Debunked verdict?

This diet has been shown to give an improvement in symptoms in ~79% of the people that undergo it with a registered dietitian and can therefore lead to a huge improvement in quality of life for those who are improved by it. This is a diet which should ONLY be undertaken with the support of an RD, however, due to its potential to be very deficient and dangerous long term if not done correctly.

Rating: ***** (5/5)

Special recommendations: Only to be done under the supervision of a dietitian to prevent deficiency or incorrect application of the diet.


King’s College London, 2021. FODMAPS. [Online]
Available at: https://www.kcl.ac.uk/lsm/schools/life-course-sciences/departments/nutritional-sciences/projects/fodmaps/faq
[Accessed 1 12 2021].

King’s College London, 2021. FODMAP-trained Dietitians. [Online]
Available at: https://www.kcl.ac.uk/lsm/schools/life-course-sciences/departments/nutritional-sciences/projects/fodmaps/files/list-of-kcl-fodmap-trained-dietitians-aug-2021.pdf
[Accessed 1 12 2021].

National Institute for Health and Care Excellence, 2021. Irritable bowel syndrome. [Online]
Available at: https://cks.nice.org.uk/topics/irritable-bowel-syndrome/
[Accessed 1 12 2021].

ROME Foundation, 2016. ROME IV Criteria. [Online]
Available at: https://theromefoundation.org/rome-iv/rome-iv-criteria/
[Accessed 1 12 2021].

S. Magge, A. L., 2012. Low-FODMAP Diet for Treatment of Irritable Bowel Syndrome. Gastroenterology & Hepatology, 8(11), pp. 739-745.

Y. McKenzie, L. R. M. W., 2019. Irritable Bowel Syndrome. [Online]
Available at: https://www.bda.uk.com/uploads/assets/5eded447-080a-4113-9b3cfc70e432fbd5/IBS-food-fact-sheet.pdf
[Accessed 1 12 2021].

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