There are two types of fiber, both of which can promote regularity and are found in naturally healthy foods. Soluble fiber, the key ingredient in fiber therapy products, is found in oats, apples, barley and many other foods. It dissolves in water. Psyllium and methylcellulose are both soluble fibers, but methylcellulose does not ferment in the intestines and cause gas. Insoluble fiber is found in whole g rains, nuts and many vegetables. It does not dissolve in water.