Dietary fiber is a complex mixture of plant materials that are resistant to breakdown (digestion) by the human digestive system. You may hear some people refer to fiber as "roughage" or the material that gives food its 'crunch'. Technically, fiber is the material that gives plants their strength and structure. Since fiber is a plant material, this means that any food that comes from animals, (beef, chicken, milk, cheese, etc.) contains no fiber.

There are two major kinds of dietary fiber -- insoluble (cellulose, hemicellulose, lignin) and soluble (gums, mucilage, pectin, psyllium). Insoluble fiber is mostly found in whole-grain products such as whole-wheat bread. Soluble fiber is found in fresh fruits and vegetables, dried beans, peas, lentils, and psyllium. Insoluble fibers do not dissolve in water, whereas, soluble fibers do. Because of differences in solubility, the effect on the body is different.