A high fiber diet is a diet containing large or significant amounts of materials which are actually indigestible, like cellulose, from grains or fruits. The fiber is used to improve digestion, remove toxins, and improve stool quality. Lack of fiber in diet is considered a cause of constipation, auto-toxicity, (self-poisoning) and other potentially serious metabolic disturbances caused by retaining wastes. High fiber supplements are often recommended for people with weak digestion or other digestive problems to promote health.