Dietary Fibre

Dietary fibre is a member of a family of dietary complex carbohydrates which have individual and diverse actions. The chemistry of individual polymers cannot, however, identify or predict the biological action in the gastrointestinal tract. Each fibre is peculiar in its biological action, and is affected by extraction, physical format and processing. One way of classifying dietary polysaccharide is as dietary complex carbohydrates. Another name for dietary fibre is non-starch polysaccharides (NSP).

Definition of dietary fibre

The definition of dietary fibre has proved to be difficult, largely as the renewed interest was generated by epidemiological observations rather than experiments. Different fibre has varying effects along the gastrointestinal tract. Fibre is said to protective against a number of conditions, from hiatus hernia to colonic cancer, coronary heart disease, obesity and haemorrhoids and different fibres may have varying protective effects. An early definition of dietary fibre by Hugh Trowell was the skeletal remains of plant cells that are resistant to digestion ( hydrolysis ) by human enzymes. Later skeletal remains was changed to plant polysaccharides and lignins. A recent North American definition separates fibre into two groups, which together add up to total dietary fibre.

Dietary fibre consists of non-digestible, intact naturally occurring plant polysaccharides and lignin. Added fibre consists of isolated non-digestible carbohydrates which have beneficial physiological effects in humans.The persisting dilemma in defining dietary fibre is that dietary fibre acts as a sponge along the gastrointestinal tract and as such its properties are affected by the manner of processing prior to ingestion.

Recommended Intake

This is not precisely defined. An intake of 30g / day has ben suggested but then one could question how much of what sources of fibre, cereal, fruit or vegetable. Whatever is decided, a wide range of sources should be the rule. The five plan for fruit and vegetables would be a good start.

Key Points

1. Dietary fibre is a term for the family of dietary complex carbohydrates and lignins in plant cell walls. Alternative descriptions include roughage and non-starch polysaccharides (NSP). These plant cell wall polymeric carbohydrates and lignins are not digested in the upper gastrointestinal tract.

2. These complex carbohydrates have individual and diverse actions along the gastrointestinal tract. The chemistry cannot, however, identify or predict the biological action of individual fibres in the gastrointestinal tract. Each fibre is peculiar in its biological action, and is affected by extraction, physical format and processing.

3. In the upper gastrointestinal tract the physical properties of the dietary fibre are important in slowing the rate of absorption of nutrients.

4. Some dietary fibres may alter sterol turnover, usually by increasing faecal bile acid excretion.

5. Some dietary fibres increase caecal bacterial growth and metabolism.

6. Some dietary fibres increase faecal weight through a combination of the water-holding capacity of the fibre not fermented by bacteria, by bacterial growth, and the osmotic effect of bacterial fermentation products in the colonic lumen.

Further reading

Eastwood MA (1992) Physiological effect of dietary fiber. Annual Review of Nutrition, 12 , 19-35.
Hellendoorn, E.W (1978) Fermentation as the principal cause of the physiological activity of indigestible food residue, in Topics in Dietary Fiber Research (ed. G.A. Spiller), Plenum Press, New York, pp. 127–216.
Kritchevsky, D, and Bonfield, C. (1994) Dietary Fibre in Health and Disease, Eagan Press, St Paul Minnesota, USA.
Kritchevsky, D., Bonfield, C. and Anderson, J.W (eds) (1990) Dietary Fiber, Plenum Press, New York.
Report of the British Nutrition Foundations Task Force (1990) Complex Carbohydrates in Foods, Chapman & Hall, London
Symposium on “The nutritional consequences of complex carbohydrates” (1996) Proceedings of the Nutrition Society .55, 863-943
McCleary B , Prosky L eds.( 2001). Advanced dietary fibre technology. Iowa State Press. Iowa.
Trowell, H., Burkitt, D. and Heaton, K. (1985) Dietary Fibre, Fibre-Depleted Foods and Disease, Academic Press, London.



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