Insoluble fiber, which does not dissolve in water. It can be metabolically inert and provide bulking, or it can be prebiotic and metabolically ferment in the large intestine. Bulking fibers absorb water as they move through the digestive system, easing defecation.Fermentable insoluble fibers mildly promote stool regularity, although not to the extent that bulking fibers do, but they can be readily fermented in the colon into gases and physiologically active byproducts. Insoluble fibers tend to accelerate the movement of food through the system