Although dietary fibre or 'dietary fiber' is now widely used in health and nutritional contexts, the word 'roughage', also used in colloquial speech and in older textbooks, has the same meaning.
What is Dietary Fibre ?
Dietary fibre is the indigestible parts of plant materials.
There are two types (or 'components' i.e. 'parts') of dietary fibre:
- Soluble Dietary Fibre - sometimes called 'prebiotic' or 'viscous', absorbs water to form a viscous 'gel' and is fermented by bacteria in the digestive tract resulting in production of gases, hence possible sensation of 'bloating'.
- Insoluble Dietary Fibre - does not ferment but absorbs water while passing through the digestive system, resulting in bulky yet soft faeces, easing defecation.
In terms of chemical structure, dietary fibre consists of non-starch polysaccharides (NSPs). Recall that polysaccharides consist of molecules that take the form of long chains of simple sugars (monosaccharides) attached together by chemical bonds. Examples of non-starch polysaccharides (NSPs) include arabinoxylans, dextrin, cellulose, and other plant-derived compounds such as inulin, lignin, waxes, chitins, pectins, beta-glucans and other carbohydrates with β-glycosidic linkages. (More about monosaccharides, oligosaccharides and polysaccharides.)
Dietary Fibre Foods
Sources of Dietary Fibre
'Dietary fibre sources' and 'dietary fibre foods' are expressions used to refer to human foods containing significant amounts of dietary fibre.
Functions of Dietary Fibre
Dietary fibre functions include:
- Increases rate of passage of foodstuffs through the digestive system.
- Absorbs water, forming a gel.
- A source of nutrients and energy for bacteria within the gut and can act as a substrate for bacterial metabolism.
- Binds to certain residues within the digestive tract, helping to remove unwanted materials from the body via accumulation in faeces followed by excretion of faeces.
Dietary fibre can bind to:
- cations of some elements, e.g. calcium (Ca), magnesium (Mg), Iron (Fe) and Zinc (Zn)
- bile acids
- nitrogenous waste
- Decreases rate of emptying of stomach, hence increases efficiency of digestion of food.
- Decreases carbohydrate absorption.
Additional benefits of dietary fibre include:
- Sufficient ingestion of dietary fibre together with sufficient fluids is believed to reduce risk of certain diseases and disorders of the gastrointestinal tract, e.g. diverticular disease
Interesting Facts about Dietary Fibre
High fibre foods such as wholegrain breads, potatoes, fruit and vegetables increase the bulk of stools (faeces) by approx. 5g per every 1g of fibre. This is due to water absorbed by the fibre and matter that binds to it in the gut.