With irritable bowel syndrome (IBS) being a highly prevalent condition that affects 10-15% of the North American population, the low FODMAP diet is becoming increasingly well-known as a potential treatment approach. As a Naturopathic Doctor, I am concerned that many people misunderstand the low FODMAP diet to be a sustainable, long-term approach to nutrition, when it is actually a short-term clinical intervention. In this article I am going to share the important information that I believe everyone should know before deciding to dive into the low FODMAP diet.

1. What is IBS? 

IBS is a gastrointestinal condition characterized by recurrent abdominal pain that is associated with bowel movements or changes in stool frequency or form. IBS is a functional condition, in which there are no structural changes to the digestive tract (which makes it very different from celiac disease or inflammatory bowel disease (IBD)). 

2. What causes IBS? 

Unfortunately, we don’t have this answer yet. Our best understanding is that IBS results from multiple factors, and that each case of IBS arises from a unique combination of one or several of those factors. Potential causative factors include:

  • Gastrointestinal motility: Changes in the frequency and regularity of smooth muscle contractions along the small intestine and colon. 
    • e.g. prolonged transit time (the amount of time it takes food to travel through the digestive tract) in constipation-predominant IBS
    • e.g. rapid transit time in diarrhea-predominant IBS
  • Visceral hypersensitivity: Heightened sensitivity (awareness and pain) to normal sensations (intestinal distension and gas) within the digestive tract.
  • Psychosocial: Our understanding of the gut-brain axis is ever-evolving, but it is clear that this bidirectional axis (in which signalling from the gut impacts our brain and signalling from the brain impacts our gut) has hugely important implications for digestive function and IBS. Stressful events, sleep problems, anxiety, and depression can all influence IBS (and IBS can influence all of them in return). 
  • Carbohydrate malabsorption: Impaired absorption of carbohydrates, leading to their fermentation by intestinal microflora.
  • Inflammation: Activation of specific immune cells and proinflammatory signalling molecules (cytokines) within the digestive tract.
  • Postinfection: Studies have identified an elevated risk of IBS following an episode of acute infectious gastroenteritis (“stomach flu”).
  • Intestinal microflora: Changes in the balance of species making up the intestinal microbiome.
  • Bacterial overgrowth: Possible associations have been identified between small intestinal bacterial overgrowth (SIBO) and IBS.
  • Genetics: Some patients with IBS may have a genetic susceptibility for the condition.

3. What does FODMAP stand for? 

FODMAP is an acronym for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. FODMAPs are carbohydrates that are often poorly absorbed (by those with and without IBS). Poor absorption means that rather than being absorbed into the bloodstream, they remain in the intestines where they draw water into the bowels (which may lead to bloating and alterations in bowel movements) and are fermented (which favors the survival of certain species of microflora and leads to gas production). People with IBS may be more sensitive to the effects of these fermentable carbohydrates within the digestive tract.

Specific names of the FODMAPs:

  • Oligosaccharides: fructans, galactans
  • Disaccharides: lactose
  • Monosaccharides: fructose
  • Polyols: xylitol, sorbitol, mannitol, maltitol, lactitol, isomalt

4. What is the low FODMAP diet? 

The low FODMAP diet is a short-term clinical intervention that can be separated into two phases: (1) elimination and (2) reintroduction.

  1. Elimination phase: Foods that contain a high amount of FODMAPs are temporarily removed from the diet for 2-6 weeks. 
    • Purpose: 
      • To provide symptomatic relief
      • To increase confidence that IBS is the correct diagnosis (if you experience symptomatic relief in this phase, we are more confident that you have IBS)
  1. Reintroduction phase: Stepwise testing/reintroduction of the fermentable carbohydrates (FODMAPs) to determine which of them do not cause you any symptoms (and thus can be consumed freely), and which of them only cause you symptoms at particular doses (and thus can be consumed below those doses). 
    • Purpose:
      • To reintroduce as many FODMAP-containing foods as possible, while remaining symptom-free.

This is one of the most important messages I hope you take away from this article:
Most people with IBS are sensitive to some FODMAPs, but not all of them. Additionally, sensitivity to a FODMAP is often dose-dependent, meaning that people can often consume some amount of it without any symptoms at all. It is important to understand this, as the ultimate goal should always be as much food variety and freedom as possible.

5. Why is the low FODMAP diet helpful in IBS?

An individual with IBS may respond negatively to FODMAPs for a variety of reasons: 

  • They could have visceral hypersensitivity, meaning that normal amounts of gas produced by the fermentation of FODMAPs causes them pain, while someone without IBS may experience no pain from the same amount of gas. 
  • They could have carbohydrate malabsorption, meaning they absorb even less and ferment even more of these FODMAPs than someone without IBS. 
  • They could have changes in their intestinal microflora with a preponderance of high gas-producing species, while the same amount of FODMAP may produce little gas in someone with a different microflora. 

The benefit of the low FODMAP diet is that it provides a systematic approach to identifying which specific FODMAPs cause symptoms, and at what doses. It is a means to alleviating symptoms while working towards the goal of maximal freedom within the diet.

6. What expectations do we have when using the low FODMAP diet?

  • If the low FODMAP diet is an appropriate therapy for you, we expect your symptoms (abdominal pain, bloating, gas, stool frequency, stool form) to improve within the elimination phase of the low FODMAP diet.
  • If there is no symptom improvement within the elimination phase, this is not the correct therapy for you. This does not necessarily mean that you do not have IBS. 
  • The low FODMAP diet tends to offer greater benefit for those with diarrhea-predominant IBS, rather than those with constipation-predominant IBS.

7. What risks are associated with the low FODMAP diet?

  • Following the elimination phase for 2-6 weeks and then moving into the reintroduction phase is a very safe clinical intervention that carries minimal risks.
  • Risks accumulate when the elimination phase extends beyond these parameters (months or years in duration).
  • Potential risks include:
    • Detrimental changes to intestinal microflora: FODMAPs are fuel for good bacteria too (not just for bad bacteria!) and are fermented into short chain fatty acids (e.g. butyrate) which are beneficial for intestinal health. 
    • Social isolation and household stress due to dietary restrictions
    • Nutrient deficiencies in the long term
    • Disordered eating 


The low FODMAP diet is a very helpful clinical intervention in certain cases of IBS but is not a sustainable, long-term approach to nutrition. It may offer symptom relief and allow the individual to identify which of the fermentable carbohydrates are problematic for them, and at what doses. Patients should follow the elimination phase for no more than 2-6 weeks before moving into the reintroduction phase, and they should be guided and supported by a healthcare provider throughout the whole process.

Photos by Jimmy Dean, Nick Fewings, and Calum Lewis on Unsplash