Numerous studies have shown that a diet high in fiber can help prevent colon cancer by maintaining bowel regularity. Fiber helps lower cholesterol, and thus can reduce your risk of heart attack and stroke. Fiber-rich foods are a stabilizing influence on blood sugar levels. They are "slow burn" foods, raising blood sugar slowly and steadily to keep your energy level high all day.
(Source: From Tired to Terrific in 10 Days: Natural Energy, by Erika Schwartz, M.D., and Carol Colman.)