Starchy foods, type of fiber, and cancer risk.

Fibers have been classified broadly as predominantly "soluble" or "insoluble," The soluble fibers have little effect on fecal bulk because they are largely fermented to short-chain fatty acids, so lowering colonic pH, and their influence on cancer risk is in relation to these variables. They are associated with increased fecal bile acid losses. Insoluble fibers are much less fermented and enhance fecal bulk without greatly increasing bile acid losses. Their effect in diluting carcinogens or bile acid promoters relates to their protective role in colon cancer. However, high-fiber foods not only contain combinations of different fiber types but also usually contain starch. Under various conditions, and possibly influenced by fiber, substantial amounts of starch (3-20% of dietary intake) may also enter the colon depending on the nature of the foods eaten. These carbohydrates will contribute to the effective fiber load. In considering the action of fiber in the human gut, fiber type, starch, and other components as well as the nature of foods making up the diet must be assessed. These factors are of importance both in interpreting epidemiological leads and in devising logical trials of fiber in terms of intervention.