Dietary Fibre

The term 'dietary fibre' or 'dietary fiber' (Am.) is 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.)

Recall from study of digestion that indigestible materials eventually amass in the lower sections of the large intestine forming faeces which are eliminated from the body by defecation.

Dietary Fibre Foods

Benefits of High Fibre Foods:

  • Usually filling due to bulk of fibrous structure
  • Dietary fibre content serves the dietary fibre functions listed below.
  • Associated with lower incidence of colon cancer and (together with sufficient fluid intake) reduced risk of other gastrointestinal conditions such as diverticular disease

Other effects of High Fibre Foods:

The following are sometimes cited as potential disadvantages of excessive dietary fibre as a proportion of overall diet:

  • Possible (significant) intestinal gas production, resulting in sensation of bloating.
  • Risk of constipation if insufficient fluid is consumed with a high-fibre diet.

In general:

  • Generally less filling than high fibre foods
  • (Perhaps as a result of being 'less filling') low-fibre foods may be consumed in larger quantities than high-fibre foods, leading to increased risk of obesity and associated health issues.
  • Smaller amount of undigested food moving through bowels, hence smaller amount of undigested material passed as faeces.

Low-fibre diets are occasionally suggested due to certain medical conditions - if affected, follow expert advice.

Sources of Dietary Fibre

'Dietary fibre sources' and 'dietary fibre foods' refer to foods containing significant amounts of dietary fibre.

Examples of High Fibre Foods:

  • Wholegrain cereals
  • Wholegrain breads and other wholegrain products
  • Fruits (fresh or dried), incl. apples, plums, berries.
  • Vegetables, esp. potato skins and leafy vegetables such as cabbage, broccoli, cauliflower, Brussels sprouts, green beans.
  • Sugar, honey, syrups
  • White rice and white flour
  • Plain pasta or noodles
  • Tender cuts of meat, or ground meat
  • Tender well-cooked fruits & vegetables without seeds, stems, skins or pulp (e.g. pulp in juices)

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

More 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.

See also types of sugar, carbohydrates, fatty acids, fats and proteins.

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