Insoluble fiber does not dissolve in water. 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.
Dietary fibers can act by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed.[2] .[3] Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers.[1]