Dietary fibre, phenolic compounds and colon health

Dietary fibre, phenolic compounds and colon health. Dietary fibre is composed of cell-wall polysaccharides, to which a number of phenolic compounds, so called co-passangers, are attached. Liberation of these components occur partly along the gastrointestinal tract depending on the structure of the cell-walls and food matrix. Liberated components are absorbed and metabolised. The unabsorbed components are delivered to the colon, where bacterial conversion occurs. In the colon cell-wall polysaccharides are converted to short-chain fatty acids: acetic acid, propionic acid and butyric acid. Bacterial conversion of several flavonoids have been under inverstigation e.g. quercetin derivatives were converted to hydroxyphenylacetic acid and catechins to hydroxyphenylpropionic acids. Little is still known about bacterial conversion of anthocyanins. Colourful anthocyanins have an instabile flavylium cation, which causes difficulties in identification of its breakdown products. Plant lignans, abundant in flax and rye, are converted to mammalian ones, enterodiol and enterolactone. The conversion involves several intermediary metabolites and the conversion efficiency is dependent on the bacterial flora. A large individual variation of has been shown in clinical trials and animal experiments. As the bacterial metabolites are converted in the colon and found in blood and urine, they are present on the colonic epithelium. However, the effects of these compounds are still largely unknown. Butyric acid is essential to nutrition of colonocytes and it has been shown to stimulate apoptosis in human colorectal tumour cell line in vitro. The dual role of butyrate as both a source of energy and inducer of pathways of cellular differentiation shows the complexicity of mechanisms, which affect concommintantly even for a single metabolite. The bacterial metabolites are still to be verified for many dietary components, after which their health effects can be investigated.