Abstract
Non-starch polysaccharides (NSP; major components of dietary fibre) have been rather disappointing in the prevention and management of large bowel inflammatory diseases (IBD) or colorectal cancer (CRC). Resistant starch (RS) is that starch which escapes small intestinal digestion and enters the large bowel. RS contributes to total dietary fibre and could be as important as NSP in promoting large bowel health and preventing IBD and CRC. Indeed, it appears that some societies historically at low risk for these conditions eat relatively little NSP but have diets high in RS through their culinary practices. RS acts largely through its large bowel bacterial fermentation products which are, in adults, short chain fatty acids (SCFA). Collectively, SCFA have several non-specific positive actions on large bowel physiology including lowering of luminal pH. Of the major acids, butyrate has attracted the most attention. It is a major metabolic fuel for and promoter of a normal phenotype in colonocytes. Recent data from our laboratory support the latter suggestion. We have shown that, in rats, higher dietary protein (as casein, red meat or soy) increases colonocyte genetic damage and thinning of the colonic mucus barrier. However, feeding of RS as a high amylose maize starch opposed both of these changes in proportion to increased colonic butyrate. These data accord with prospective population data showing lower CRC risk with consumption of total dietary fibre. RS intakes appear to be low in most affluent industrialized countries, so increasing its consumption by modifying consumer foods is one strategy to improve public health. CSIRO and its partners are developing new high amylose cereal cultivars for this purpose. Colonic delivery of specific SCFA could also be useful clinically and we have shown that acetylated, propionylated and butyrylated starches resist small intestinal amylolysis. The bound SCFA are released by the large bowel microflora, raising their digesta levels, with the greatest increase being in the esterified acid. Feeding studies with butyrylated starch in rats have confirmed the opposition of diet-induced genetic damage, supporting a role for this SCFA in lowering risk of CRC and IBD. Further human and animal interventions are planned to determine the potential of these new types of RS in enhancing colonic health.