Fiber is the non-digestible portion of plant foods that plays a critical role in digestion. There are two different categories that all fiber products fall into: 1) soluble or insoluble, and 2) fermentable or non-fermentable. Soluble fiber dissolves in water, whereas insoluble fiber does not. The fiber in Citrucel with SmartFiber is 100% soluble; the psyllium fiber in Metamucil is not 100% soluble. Citrucel with SmartFiber contains only 100% non-fermentable fiber, so none of it ferments to cause excess gas like the fermentable fiber in Metamucil. This is based on laboratory tests. Results may vary in humans.