FODMAPs are a group of carbohydrates that can cause issues in the gut that may not be digested and/or absorbed very well across the gut wall and are thus retained in the gut.

The effects FODMAPs have on the gut have previously been simply explained as being malabsorbed  in the gut resulting in extra water being dragged into the gut (this is called the OSMOTIC effect of FODMAPs) This is followed by overflow  of the malabsorbed FODMAPs into the large intestine where very rapid fermentation occurs producing excess gas. With the presence of visceral sensitivity (over sensitive nerve endings in the gut), the result is bloating, pain and either diarrhoea or constipation (or both).

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The truth is a little bit more complex and  potentially involve dysbiosis (dysbiosis is when the colonies of bacteria that live in the large intestine are not in a healthy state), and production of bioactive chemicals such as lipopolysaccharide in the gut  which disrupts the gut barrier.

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If you want a little bit more information on the why FODMAPS seem to exacerbate IBS symptoms, keep reading:

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• FODMAPs may potentially be a problem in some people with gut issues who have pre-existing dysbiosis. 

• For these people, when FODMAPs are rapidly fermented in the large intestine there is an increase increase in a chemical called LPS or lipopolysaccharide. LPS  is a prominent part of gram-negative bacteria (potentially in the gut in larger amounts as a result of the dysbiosis). ​

• The increase in LPS activates mast cells receptors  which result in the mast cells releasing  active molecules such as PGE2, histamine and trypsin. These molecules are responsible for  disrupting the gut barrier so there is a 'leaky gut'.

• a LOW FODMAP Diet diet reverses these gut mucosal changes

• There is also evidence that points to heightened visceral sensitivity being apart of the above sequelae.

 

A 2022 study  by Verver K et al has shown that the microbiome for IBS- D (diarrhoea dominant) subjects responded positively to the LOW FODMAP diet had a shift in their microbiome from a state of dysbiosis to that similar to normal controls.

While it is likely that much more information will be revealed with ongoing research, at this stage it appears that the increased levels of  LPS  levels in the large intestine  are powered by a high FODMAP diet and the increased LPS causes mast cell activation which disrupts the gut barrier and drives the gut issues found in many people with Irritable Bowel Syndrome.​​

 

causes mast cell activation via LPS, which in turn leads to colonic barrier loss, and a LFM diet reverses these pathophysiologic mucosal changes

Potentially, the underlying issue is a dysbiosis of the gut and the predominance of less desirable bacteria and their metabolites which are being fuelled by the FODMAPs.

 

When we take away the FODMAP fuel, then the metabolites are being produced in significant less quantities thus less symptoms (or resolution of symptoms).

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REFERENCES: 

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Singh P, Grabauskas G, Zhou SY, Gao J, Zhang Y, Owyang C. High FODMAP diet causes barrier loss via lipopolysaccharide-mediated mast cell activation. JCI Insight. 2021 Nov 22;6(22):e146529. doi: 10.1172/jci.insight.146529. PMID: 34618688; PMCID: PMC8663790.

 

Vervier K, Moss S, Kumar N, Adoum A, Barne M, Browne H, Kaser A, Kiely CJ, Neville BA, Powell N, Raine T, Stares MD, Zhu A, De La Revilla Negro J, Lawley TD, Parkes M. Two microbiota subtypes identified in irritable bowel syndrome with distinct responses to the low FODMAP diet. Gut. 2022 Sep;71(9):1821-1830. doi: 10.1136/gutjnl-2021-325177. Epub 2021 Nov 22. PMID: 34810234; PMCID: PMC9380505.