Dietary fiber, dietary fibre, or sometimes roughage and ruffage is the indigestible portion of food derived from plants. There are two main components: Soluble fiber dissolves in water. It is readily fermented in the colon into gases and physiologically active byproducts, and can be prebiotic and/or viscous. Soluble fibers tend to slow the movement of food through the system. Insoluble fiber does not dissolve in water.