The best sources are whole grains, including wheat bran, cereal, whole wheat flour and brown rice, oats, whole grains, beans, vegetables and fruits, etc., all of them have made up different types of dietary fiber. The role of dietary fiber on human body is to promote gastrointestinal peristalsis, speed up food through the gastrointestinal tract, reduce absorption. In addition, dietary fiber absorbs water in the large intestine to soften the stool, which can play a role in prevention and treatment of constipation.