Glucomannan is a polysaccharide (long chains of simple sugars, primarily mannose and glucose) that is classified as a soluble fiber. Because glucomannan can absorb up to 200 times its weight in water, it has been used as a dietary supplement to promote weight loss (via increasing feelings of fullness).
Glucomannan is derived from several plants, but the primary source is an Asian plant called Konjac. Glucomannan is a water-soluble dietary fiber that is derived from konjac root (Amorphophallus konjac). Like other forms of dietary fiber, glucomannan is considered a “bulk-forming laxative.” Glucomannan promotes a larger, bulkier stool that passes through the colon more easily and requires less pressure, and subsequently less straining - to expel.
Glucomannan delays stomach emptying, leading to a more gradual absorption of dietary sugar; this effect can reduce the elevation of blood sugar levels that is typical after a meal. Like other soluble fibers, glucomannan can bind to bile acids in the gut and carry them out of the body in the feces, which requires the body to convert more cholesterol into bile acids. This can result in the lowering of blood cholesterol and other blood fats. Controlled and double-blind studies have shown that supplementation with several grams per day of glucomannan significantly reduced total blood cholesterol, LDL ("bad") cholesterol, and triglycerides, and in some cases raised HDL ("good") cholesterol. One double-blind study reported that glucomannan (8-13 grams per day) lowered total and LDL cholesterol in people with the insulin resistance syndrome.22
Glucomannan, which is also classified as a soluble dietary fiber, is derived from konjac flour. Konjac flour itself is derived from the Amorphophallus species, plants which are related to the common philodendron house plant and which grow in only certain parts of the world, including some regions in China and Japan. One member of the Amorphophallus genus called Amorphophallus konjac, is also known as voodoo lilly, devil's tongue and konjac. Konjac flour, however is derived from the tubers of various species of Amorphophallus, and the term konjac is used generically for the various species, as well as for the flour from their tubers. In addition to being known as konjac, the plant is called ju ruo (pronounced in Chinese) by the Chinese people, and called konjaku or konnyaku by the Japanese.
Konjac flour has a long history of use in both China and Japan as a food substance and as a folk remedy. Glucomannan products are widely used in Japan and China as general health aids, topically, for skin care and as a thickening agent for foods, among other things. Glucomannan, sometimes called konjac mannan, is marketed in the United States as a dietary supplement. Polysaccharides containing D-mannose and D-glucose in similar proportions to that found in konjac flour are found in other organisms, such as certain yeasts. Yeast glucomannan is not marketed as a dietary supplement.
As a weight loss aid, glucomannan is sometimes combined with other fibers, such as chitosan, to increase the fat-binding effects and feelings of fullness – both of which may be effective in promoting weight loss and weight maintenance as well as for controlling mildly elevated cholesterol and blood sugar levels.