Prevention of colon cancer: role of short chain fatty acids produced by intestinal flora.

Any polysaccharide, whether starch or fibre (ie non-starch polysaccharides) may be fermented in the large bowel by resident microflora (anaerobic bacteria). Amongst other substances, the short chain fatty acid butyrate is produced during fermentation. Butyrate is important in the maintenance of normal epithelial biology; it is probably the means by which dietary fibre prevents colonic epithelial atrophy. Starch which escapes digestion in the small intestine (resistant starch) also prevents colonic epithelial atrophy. Dietary fibres differ greatly in their physicochemistry and also in their biological effects. As a general rule, resistant starch (especially of type 2) tend to behave more like soluble then insoluble nonstarch polysaccharides. In humans, resistant starch results in substantial production of butyrate in the colon. Butyrate can be shown to have "antitumour" effects at various levels (cell and molecular), and this could explain the important inverse association between starch intake and colon cancer incidence (on a country by country basis). The nature of the variables affecting butyrate production from dietary polysaccharides by resident microflora need to be explored with a view to better understanding the practical application of this to cancer prevention.