Even when the causative factors are known, cancer may still occur in some circumstances. Many mutagenic substances occur in food. The carcinogenic potential of food compounds in humans is not yet clear. If they do play a role in the occurrence of cancer, it would be exceedingly difficult to remove them from the environment. Dietary fiber is generally believed to protect against colorectal cancer. Burkitt first proposed the fiber hypothesis based on his observation of low colon cancer rates in regions of Africa where fiber intake is high. Some case control studies of colorectal cancer have pointed the beneficial effect of total dietary fiber. Dietary fiber consists of plant cell wall polysaccharides and liginin, which are not hydrolyzable by human digestive enzymes, and includes pectin, cellulose, and hemicellulose. Several plausible physical and biochemical mechanisms for the beneficial effect of dietary fiber have been proposed. The risk of colorectal cancer decreases with high intake of total fiber and increased with diets high in animal fat.