A diet high in fiber may reduce the risk for colon cancer by nearly half, according to the American Institute for Cancer Research. Researchers tracked the diets of more than 500,000 people for 4-1/2 years. Good sources of fiber include fruits and vegetables, whole grains and beans. Fiber may not be the only nutrient causing the protective effect. Vitamins and minerals in the plants, as well as other dietary habits of people who eat a high-fiber diet may play a role.