What is dietary fibre?
The term ‘fibre’ describes a group of carbohydrates that resist digestion in the gut (1). ‘Fibre’ describes a wide range of different substances, in a similar fashion to ‘FODMAPs’ describing a number of poorly absorbed sugars. In fact, anything that fits the description above is classified as fibre (from β-glucans through to wheat bran), and each can be further classified based on their chemical structure, chain length (molecular size), solubility (ability to dissolve), viscosity (ability to thicken liquids once dissolved) and fermentability (2, 3). These properties all influence how the fibre behaves in the gastrointestinal tract, although the fermentability of fibre may play a particularly important role in determining tolerability in IBS.
Fermentable fibres
Fermentability describes the rate and extent to which a fibre is broken down by our gut bacteria. The process of fermentation takes place in the large intestine, providing energy for our gut bacteria and releasing by-products into the body, such as gases and short-chain fatty acids (2, 4). More readily fermentable fibres include plant gums (e.g. guar gum) and resistant starch (see Table 1), whereas cellulose and sterculia are examples of minimally fermentable fibres (3, 5). Both the fermentation itself and its by-products have been shown to benefit health (6).
The key difference between fermentable fibres and FODMAPs is the rate at which they are fermented. The chemical structure of fibre differs from FODMAPs, in that fibre is composed of longer chains of sugars whereas FODMAPs are much shorter in chain length (7). As a result, even very fermentable fibres are fermented at a slower rate and produce gas at a more controlled and steady rate when compared to FODMAPs.
Table 1 - Examples of fermentable fibres and respective food sources
| Fermentable Fibres | Food Source |
|---|---|
| β-glucans | Oats; barley |
| Guar gum | Supplement; thickening agent/stabiliser |
| Resistant starch (specifically RS2) | Green bananas; legumes; raw potato; supplement |
| Pectin | Fruits (particularly citrus); root vegetables; jams; nuts; legumes |
| Xanthan gum | Supplement; thickening agent/stabiliser (often used in gluten free baking) |
Fermentable fibres are less likely to contribute to gastrointestinal discomfort in IBS than FODMAPs. However, IBS is a highly individualised condition, with differing degrees of sensitivity from person to person. Therefore, you may need to work with your dietitian to determine what fibres you tolerate, and in what quantity.
The other situations where these fibres may factor into symptoms and discomfort include:
Consuming a concentrated amount of such a fibre in the form of a supplement (where the overall quantity entering the large intestine may offset the slower rate of fermentation); or
“Stacking” large amounts of fermentable fibres with other high FODMAP foods (see: https://www.monashfodmap.com/blog/fodmap-stacking-can-i-overeat-green/).
What does this mean for me?
Fibre is an important component of a healthy diet and shown to be beneficial to health in multiple ways (8, 9). There’s no need to over-restrict fibre intake in IBS (particularly high fibre foods that contain a mixture of different fibre types) as most Australians don’t include enough fibre in their diet to start off with (10)! If gastrointestinal symptoms persist while following a low FODMAP diet, you may need to work with your dietitian to modify your fibres intake.
References
1. FAO/WHO. CODEX Alimentarius (CODEX) Guidelines on Nutrition Labeling CAC/GL 2–1985. 2013.
2. Holscher HD. Dietary fiber and prebiotics and the gastrointestinal microbiota. Gut Microbes 2017;8(2):172-84. doi: 10.1080/19490976.2017.1290756.
3. McRorie JW, Jr., McKeown NM. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. Journal of the Academy of Nutrition and Dietetics 2017;117(2):251-64. doi: 10.1016/j.jand.2016.09.021.
4. Dahl WJ, Agro NC, Eliasson AM, Mialki KL, Olivera JD, Rusch CT, Young CN. Health Benefits of Fiber Fermentation. Journal of the American College of Nutrition 2017;36(2):127-36. doi: 10.1080/07315724.2016.1188737.
5. Eswaran S, Muir J, Chey WD. Fiber and functional gastrointestinal disorders. The American journal of gastroenterology 2013;108(5):718-27. doi: 10.1038/ajg.2013.63.
6. Sekirov I, Russell SL, Antunes LC, Finlay BB. Gut microbiota in health and disease. Physiological reviews 2010;90(3):859-904. doi: 10.1152/physrev.00045.2009.
7. Cummings JH, Macfarlane GT. The control and consequences of bacterial fermentation in the human colon. The Journal of applied bacteriology 1991;70(6):443-59.
8. SACN. Internet: https://www.gov.uk/government/publications/sacn-carbohydrates-and-health-report (accessed October 31st 2016).
9. Stephen AM, Champ MM, Cloran SJ, Fleith M, van Lieshout L, Mejborn H, Burley VJ. Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutrition research reviews 2017:1-42. doi: 10.1017/s095442241700004x.
10. Global Nutrition and Policy Consortium. Internet: www.globaldietarydatabase.org/dietary-data-by-country.html (accessed Jan 15 2018).