…Dietary fibre (DF) and all its benefits
A forum sponsored by the American Association of Cereal Chemists provides us with a consensus definition for dietary fibre1 as follows:
‘Dietary fiber is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substances. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation.’
We can see from this definition that dietary fibre:
- comes from plants
- consists mostly of the complex carbohydrates known as oligosaccharides and polysaccharides
- isn’t absorbed by the body, so isn’t a nutrient
- is fermented in the large intestine
- has a number of health benefits
By taking each of these points in turn, let’s examine DF further:
Dietary fibre originates from plants
DF occurs naturally in cereals, vegetables, legumes, fruits, nuts and seeds. There are two types of DF, soluble and insoluble. If soluble, the DF is easily fermented by bacteria in the large intestine, if insoluble fermentation is likely to be limited. The following table describes different types of DF, whether it is soluble or insoluble and the main food sources.
Table 1: Dietary fibre classification
| Fibre component | Solubility | Description | Food sources | Cellulose | Insoluble | The major component of cell walls in plants. There is very little digestion of cellulose by enzymes in the gut. | Vegetables and brans |
|---|---|---|---|
| Hemi-cellulose | Insoluble/Soluble | Polysaccharides in plant cell walls | Cereals |
| Lignin | Insoluble | Non-carbohydrate component of plant cell walls | Woody plants |
| Pectin | Soluble | Components of plant cell walls | Vegetables, legumes, fruits |
| Gum | Soluble | Secreted by plants at the site of an injury | Legumes, seaweed extract |
| Mucilages | Soluble | Synthesised by the plant to protect the seed. Used in the food industry as a stabiliser. | Gum extracts from plants, e.g. gum acacia |
2
Dietary fibre consists mostly of complex carbohydrates known as oligosaccharides and polysaccharides
Check back to the post on mono, di and polysaccharides, linked here, for a description of these.
Dietary fibre isn’t a nutrient
Fung describes DF as an anti-nutrient because its benefits lie in the capacity to reduce absorption of nutrients such as carbohydrates, particularly where someone is overweight or obese. As a consequence, blood glucose levels do not rise as much and less insulin is needed to shunt the glucose out of the bloodstream3.
Dietary fibre is fermented in the large intestine
It is known that DF plays a part in altering the form and function of the gut microbiome. Anaerobic fermentation of DF in the colon by these microbes produces short chain fatty acids such as acetate and butyrate, which are known to promote health and wellbeing4.
Dietary fibre has a number of health benefits
The evidence supporting the beneficial effects of DF on increasing faecal bulk, promoting colonic health and gut motility and the prevention and/or treatment of constipation is incontrovertible4.
The substitution of lower fibre foods for food higher in fibre reduces peak blood glucose levels after eating as well as serum cholesterol levels2 and the viscous properties of DF can inhibit the absorption of glucose and cholesterol2,5.
Overall, a high fibre diet is associated with a lower risk of cardiovascular disease (CVD) possibly due to a lower concentration of inflammatory markers which are linked to increased risk of mortality from CVD and cancer5.
There is also evidence to suggest a high fibre diet is associated with a lower risk for particular types of cancer, for example pancreatic, gastric and colon cancer as well as strokes and type 2 diabetes4.
How to eat more fibre
The results of this umbrella review serve to support current dietary recommendations for a high intake of fibre as part of a healthy diet5. Dependent on age and gender, in the US it is advised adults should consume up to 34g of DF per day6, whilst in the UK government guidelines state 30g per day7. On average, in the Western world, DF intake falls well short of the recommended amount4,6,7. One of the reasons for this is food retailers, particularly larger supermarket chains, dedicate much of their shelf space to processed, rather than fresh foods3,4. Ways to make good this shortfall include replacing ultra-processed food in our diet, such as chips, pizza, cake, biscuits, crisps and confectionary, with freshly prepared food abundant in vegetables, pulses, wholegrain breakfast cereals, wholemeal bread and pasta and fruits.
References
- Report of the Dietary Fiber Definition Committee to the Board of Directors of the American Association of Cereal Chemists. CEREAL FOODS WORLD, MARCH 2001, 46(3), pp 112-126. Available from: https://www.cerealsgrains.org/initiatives/definitions/Documents/DietaryFiber/DFDef.pdf
- DHINGRA, D., MICHAEL, M., RAJPUT, H., PATIL, R. T. (2012). Dietary fibre in foods: a review. Journal of food science and technology, 49(3), 255–266. Available from: https://doi.org/10.1007/s13197-011-0365-5
- Fung, Jason. The Obesity Code: Unlocking the Secrets of Weight Loss. London: Scribe, 2020, pp 180-185
- BARBER, T. M., KABISCH, S., PFEIFFER, A., WEICKERT, M. O. (2020). The Health Benefits of Dietary Fibre. Nutrients, 12(10), 3209. Available from: https://doi.org/10.3390/nu12103209
- VERONESE, N. et al. Dietary fiber and health outcomes: an umbrella review of systematic reviews and meta-analyses. The American Journal of Clinical Nutrition, March 2018, 107(3), pp 436–444. Available from: https://doi.org/10.1093/ajcn/nqx082
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. Available from: https://www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf
- NHS. How to get more fibre into your diet. Available from: https://www.nhs.uk/live-well/eat-well/how-to-get-more-fibre-into-your-diet/
