Psyllium stalks contain tiny seeds, also called psyllium. The seeds are covered by husks, which is the part of the plant used in foods. The seed husk contains glycosides and mucilage which are used in the food industry to form gels that give thickening and textural changes to foods. The psyllium husk is a source of water-soluble fiber, like fiber found in grains such as oats and barley. The amount of soluble fiber in psyllium is much higher than oat bran. Psyllium fiber is not broken down as it passes down the gastrointestinal tract and so psyllium has no nutritive value other than as a source of fiber. Adding water to dry psyllium causes it to swell to up to ten times its original volume. For many years products containing psyllium have been used to increase fecal bulk and loosen stools, as ways of treating constipation. You will often see psyllium fiber in hairball formulas.