Research update: Dietary fibres in IBS

Our recently-published review [1] looks at a different approach to how dietary fibres can be used to benefit individuals with IBS. A high-level summary is presented here.

What are dietary fibres and why are we interested in them?

Broadly, ‘dietary fibre’ is an umbrella term that describes any type of indigestible carbohydrate.

There are many different types of fibre, each with distinct physical and chemical properties (e.g., cellulose vs psyllium; Figure 1 below) which dictates their actions along the gastrointestinal tract. These actions (or functional characteristics), in turn, induce specific, physiological responses such as promoting regularity, improving satiety and producing gases via fermentation. 

A better understanding of the functional characteristics of different fibres will, therefore, allow us to work out their therapeutic value in IBS, possibly enabling personalised fibre therapy to achieve specific therapeutic responses in patients.

Figure 1 

Looking at fibres from a functional perspective

The main functional characteristics of fibre along the gastrointestinal tract are

1. Bulking, which describes how a fibre contributes to regularity, via:

• Increasing stool output.
• Normalising transit time through the colon (slowing down transit if too quick, hastening transit if too slow).
• Normalising stool form (firming up loose stools, softening firm stools). 

2. Viscosity, which describes whether a fibre thickens when mixed with water and forms gels.

There are many ways of looking at fermentability - the angle of most relevance in IBS is the rate of fermentation:

• Fibres that are fermented more quickly (e.g., oligosaccharides in FODMAPs) may exacerbate symptoms via gas production.
• Fibres that resist fermentation (e.g., karaya gum) or a slowly-fermented (e.g., psyllium) are less likely to induce symptoms via accumulation of gases. 

Figure 2 

Overview of dietary fibres and their overlapping functional characteristics. Abbreviations: GOS, galacto-oligosaccharides; PHGG, partially hydrolysed guar gum; RS, resistant starch; XOS, xylo-oligosaccharides.

How will this change applications of dietary fibres in IBS?

Current clinical guidelines for fibre supplementation in IBS looks at fibres through the perspective of solubility: ‘soluble’ fibres are recommended for reducing symptoms whereas ‘insoluble’ fibres are recommended to be avoided [2, 3]. Issues with such advice are as follows:

• Whether a fibre dissolves in water has little bearing on their functional characteristics. 
• 'Soluble' fibres are not interchangeable. For example, both psyllium and inulin readily dissolve in water, but the latter is rapidly fermented and likely to exacerbate symptoms.
• These recommendations are for the use of fibres to reduce symptoms. Give their functional characteristics, fibres may offer more value in providing specific effects (e.g., karaya gum for improving regularity) rather than improving overall symptoms.

This is not an overt criticism of these guidelines given much of the research of fibres in IBS are based on the principle of ‘solubility’ with a ‘one-size-fits-all’ approach and that these recommendations are just a synthesis of what’s available.

We hope that highlighting these functional characteristics will show the number of different ways that fibres can impact our guts and shift the perspective of fibre research in IBS so that specific types of fibre can be investigated and then applied for to achieve specific therapeutic responses, eventually allowing for personalisation of fibres in IBS.

References:

1. So et al. Dietary fibres and IBS: translating functional characteristics to clinical value in the era of personalised medicine. Gut 2021. doi: 10.1136/gutjnl-2021-324891
2. Vasant et al. British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut 2021. doi: 10.1136/gutjnl-2021-324598
3. Lacy et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol 2021. doi: 10.14309/ajg.0000000000001036