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Irritable Bowel Syndrome and the Gut Microbiome

Updated: Oct 1, 2020

Recently the description of irritable bowel syndrome (IBS) was changed from ‘functional gut disorder’ to ‘disorder of the gut-brain interaction’. This is important for it acknowledges the work that has been done on the link between the gut bacteria and IBS.

We now know that IBS is multi factorial. Involved in what causes the abdominal pain, bloating, changes in stools and flatulence are:

· Visceral sensitivity (the gut is very sensitive), so enteric nerves are affected.

· Mucosal lining of the gut is thinner, making it a less effective barrier against pathogens

· Stress, anxiety and depression worsen symptoms (gut-brain interaction)

· Microbiota seem to be disturbed - there are fewer of some of the key beneficial bacteria present in the gut, and less diversity in gut bacteria overall.

It is not clear whether IBS develops due to the altered gut microbiome, or whether the foods which typically irritate the gut are avoided by people with IBS, leading to lower diet diversity and possibly affecting the make up of the microbiome. Certainly in one study, those with the biggest alterations in their microbiome (versus healthy individuals) were those with the most severe IBS symptoms.

First line strategies to manage symptoms are stress reduction (for example yoga is quite effective), removing gut irritants such as fatty foods, coffee, other caffeinated drinks, smoking and alcohol and checking that adequate water, fruit and vegetables and fibre overall are present in the diet.

If those strategies do not bring about enough symptoms improvement to make life comfortable, then a low fodmap diet might be considered.

Fodmaps are Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols. In short, small carbohydrates which are fermentable in the large intestine. It is the fermentation process with attendant gas production which seems to exacerbate symptoms for people with IBS.

The low fodmap diet has three stages.

In stage one, all fodmaps are restricted. This should ease symptoms considerably.

In stage two a regime of testing individual fodmap types one by one occurs, to see which ones affect the individual, and which are tolerable, and at what level of intake.

Stage three is when all tolerated fodmaps are added back to the diet to form an ongoing diet which aims to maximise variety and fibre in the diet without causing symptoms at unacceptable levels.

All three stages are overseen by an experienced dietitian or nutritionist, as there are many pitfalls and other factors which can derail the process.

A low fodmap diet is effective in about 70% of people with IBS.

So, does the low fodmap diet cause changes in the microbiome, and if so, are they deleterious to health?

Over as short a period as four weeks, the microbiome does change in response to a low fodmap diet. This occurs because fermentable carbohydrates are an important food source for bacteria in the gut (they are prebiotics) and the low fodmap diet severely restricts intake of these carbohydrates.

In five out of seven studies looking at microbiome changes after changing to a low fodmap diet, beneficial bifidobacteria were reduced on the low fodmap diet. No-one knows, however, what the long term effects of this change are on overall health, or whether the changes endure over time. It may be a trade off between controlling symptoms of IBS and eating an ideal diet for health.

One area of interest which is currently being studied is the possibility of using probiotics in tandem with a low fodmap diet, to maintain levels of bifidobacteria in the microbiome. In one study this was effective, however it is too early to recommend this across the board.

Another fascinating possibility is the ability to predict who will benefit from a low fodmap diet most, based on their microbial signature. Early studies suggest that those with the most altered microbiomes (versus healthy individuals) are also those who get the best results from a low fodmap diet. It is a diet which requires a high level of effort and motivation, therefore being able to predict whether it may or may not work for you could be valuable.

Science has moved quickly in this area in the last five years and I believe we will see increasingly that the links between brain, diet, microbiome and IBS are complex, but that they can hold the key to relieving symptoms for most people if we delve deep enough into their personal experience of the disease and personal mic

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