That portion of our diet which is not broken down by our digestive enzymes and does not therefore serve as a (direct) source of nutrients. Also known as non-starch polysaccharides. Fibre is not, as is often thought, calorie-free; bacteria digest soluble fibre to form short-chain fatty acids, principally acetate, propionate and butyrate, which are used as energy sources for the intestinal lining (mucosa) and can also be absorbed into the bloodstream.
Dietary fibre holds water and thus softens the stools and adds bulk, assisting stool propulsion (peristalsis) and evacuation. Diets low in fibre promote constipation.
Types of dietary fibre
|Cellulose||Insoluble||Less digestible by bacteria. Adsorbs bile acids and cholesterol.||Found in plant cells walls (e.g. leafy vegetables, peas, beans, wheat bran)|
|Gums||Soluble||Can be digested by bacteria. Adsorb bile acids and cholesterol. Retard the rate of absorption of simple sugars from the small intestine.||Found in seeds and plant secretions. May be used as food additives.|
|Hemicellulose||Insoluble||Less digestible by bacteria. Adsorbs bile acids and cholesterol.||Found in plant cell walls|
|Inulin||Soluble||Can be digested by bacteria||Found in Jerusalem artichokes|
|Lignins||Insoluble||Less digestible by bacteria. Adsorb bile acids and cholesterol.||Found in bran|
|Mannosans, raffinose, stachyose, verbacose||Soluble||Can be digested by bacteria||Found in pulses (legumes)|
|Mucilages||Soluble||Can be digested by bacteria||Found in seeds (e.g. psyllium) and seaweeds|
|Pectins||Soluble||Can be digested by bacteria. Adsorb bile acids and cholesterol. Retard the rate of absorption of simple sugars from the small intestine.||Found in fruit and vegetables, especially apple peel and the white part of citrus fruit|