The role of dietary fiber in health and disease.

Dietary fiber has been, for several years, the glamour ingredient in popular nutrition. Based on epidemiological evidence, lack of fiber in the diet has been impugned as a major risk factor for development of colon cancer, heart disease, diabetes and a variety of lesser ills. Animal experiments suggest that some components of the complex mixture of substances called fiber will reduce cholesterol levels to a modest extent and will inhibit atherosclerosis induced by diet. In man the data center on the effects of fiber on plasma cholesterol levels and some fibers such as pectin or guar exert significant hypocholesterolemic effects whereas others, such as bran, do not. The situation is similar with regard to colon cancer. Some types of fiber, bran and cellulose for instance, inhibit experimentally induced colon cancer. There are a number of ways of establishing experimental colon cancer; feeding the carcinogenic agent, injecting it, or instilling it intrarectally. There also exists a variety of carcinogenic agents. The effect of fiber is the sum of the type of fiber and carcinogen used and the mode of establishing the cancer. Different combinations give different results in animal studies. In man the data bearing on this subject are wholly epidemiological. A few case-control studies have provided suggestions that low fiber diets may predispose to colon cancer but these studies point to a dietary life-style in which many components other than fiber vary. The most notable success in wedding practice to hypothesis has been in the area of diabetes. Here it has been shown clearly that increasing dietary fiber results in reductions in lipemia, glycemia and insulin requirement. What remains? More work in the cancer and heart disease fields but mainly a greater effort to identify the specific structure of those fibers which exert a beneficial effect. This will have the two-fold benefit of identification of specifically useful structural types of fiber and of possibly providing clues to mechanism of action or of carcinogenicity. Most experts agree that a modest increase in intake of fiber will have a generally beneficial effect but they can only support these statements with epidemiological proof. Future research must include studies designed to confirm the epidemiological findings and to identify the specific components responsible for them.