Metabolic epidemiology of colorectal cancer.

Colorectal cancer, one of the "diseases of western civilisation", is a multistage process in which the first stage is the formation of a benign adenoma, followed by adenoma growth, increasing severity of epithelial dysplasia, and finally carcinoma. Adenoma formation is caused by environmental factors, probably acting primarily on a genetically predisposed fraction of the population. Adenoma growth is probably caused by a bacterial metabolite of the bile acids, the concentration of which is increased by dietary fat or meat and decreased by fibre. This probably acts in concert with hormonal factors. Similar factors are probably responsible for the increasing severity of epithelial dysplasia.