Studies on the role of specific dietary fibres in protection against colorectal cancer

Although dietary fibre is generally thought to protect against the development of colorectal cancer, some of the results of animal and epidemiological studies are equivocal. We believe that this may be because the term dietary fibre covers a range of complex materials and some may protect but others may not. Dietary fibre is mainly composed of plant cell walls which vary in composition and properties according cell type and plant species. In addition to polysaccharides, the walls of some plant cell types contain the hydrophobic polymers lignin or suberin. Two groups of mechanisms have been proposed for the way dietary fibres may protect against colorectal cancer: those in which the dietary fibre may act directly and those in which the dietary fibre may have an indirect effect as a consequence of it being degraded by colonic bacterial enzymes and the products fermented. Direct mechanisms include the adsorption of carcinogens onto undegraded dietary fibres which pass out of the intestinal tract in the faeces. We have shown that different types of plant cell walls adsorbed a range of carcinogens, including heterocyclic aromatic amines, to different extents. Cell walls that contained lignin or suberin adsorbed hydrophobic carcinogens particularly well. Furthermore, the presence of lignin, and probably suberin, in the walls makes them resistant to degradation in the colon. Wheat bran, which is a good source of dietary fibre, contains some cell types with lignified walls. We used Fischer-344 rats to test the ability of wheat bran to protect against the formation of aberrant crypts (which are considered to be precursors to colon cancer) caused by the heterocyclic aromatic amine, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Our results indicate that wheat bran protects and probably does so by a direct mechanism.