[Dietary fibre: more than a matter of dietetics. I. Compounds, properties, physiological effects]

Dietary fibre is a heterogeneous group of substances which have only one common characteristic: the non-digestability in the small bowel. With one exception all fibres are carbohydrates (poly- or disaccharides). Some fibres are water-soluble, others are unsoluble. This property is associated with physiological effects. Soluble (viscous) fibres can bind water and thus form hydrocolloids or gels, unsoluble ones cannot. Dietary fibres play an essential role in the physiology of the gastrointestinal tract. They modify the absorption of nutrients (particularly carbohydrates and lipids) in the small bowel. They accelerate the gut transit time and determine stool composition and quantity. They are the main nutritional source for the colonic microflora. During the bacterial fermentation short-chain fatty acids are formed which are essential for nutrition and integrity of the colonocytes and for colonic function. Moreover gases, detoxicating enzymes, antioxidants and carcinogen-inactivating compounds arise. The most important fibres are cellulose, hemicellulose, pectin, guar, psyllium, beta-glucan, Klason lignin and digestion-resistant starch; they are present in varying amounts in plant foods and in fibre preparations. The usual daily intake of dietary fibre in Europe and the USA amounts to only 15-20 g, while health authorities and nutrition societies recommend a reference value of at least 30 g. Dietary fibres are applied as food-integrated, as supplement and as purified substances.