Glucomannan is an unabsorbable polysaccharide composed of the simple sugars glucose and mannose. It occurs as a natural dietary fiber in tubers from certain species of the plant genus Amorphophallus, which grow in tropical and subtropical regions of Asia. Although at least five Amorphophallus species are known to produce glucomannan, most of what is used commercially is derived from the konjac plant, Amorphophallus konjac, which is why the fiber is commonly called konjac glucommanan or konjac mannan. Recently gaining popularity as a dietary fiber supplement, glucomannan has stronger gel-forming properties than pectin or guar gum, and has the advantage of being tasteless and easy to swallow.
Dosage recommendations for glucomannan vary with intended use. Some studies on glucose tolerance and cholesterol control have used as much as 13 grams per day. Most studies on constipation have used three grams per day; however, one study found four grams per day to be more effective. Although glucomannan has no known toxicity and has not been associated with any negative side effects, research indicates that it may interfere with the absorption of some fat-soluble vitamins; thus, multivitamin supplements may be advisable for long-term glucomannan users.