insoluble fiber that not soluble in water, composed mainly of lignin, cellulose, and hemicelluloses and primarily found in the bran layers of cereal grains.Its actions include increasing fecal bulk and decreasing free radicals in the gastrointestinal tract.rude fiber the fiber that remains after food is digested with alkali and acid, which destroys all soluble and some insoluble fiber. It is mainly lignin and cellulose. depressor f's afferent nerve fibers that when stimulated reflexly cause diminished vasomotor tone and thus decreased arterial pressure.