Glucomannan is a dietary fiber derived from the tubers of Amorphophallus konjac. Konjac flour (made from these tubers) is used to make a jelly called konyaku, a common food product in Japan.

Fiber-containing foods, such as oats, are known to help reduce cholesterol and improve constipation and may also help regulate blood sugar and assist in weight reduction by creating a feeling of fullness. However, many people have a hard time consuming enough fiber from food, so turn to fiber supplements, such as guar gum and pectin, to help fulfill their daily requirements. Glucomannan offers one advantage over these forms of fiber: much smaller doses are necessary. When glucomannan is placed in water, it can swell up to 17 times its original volume. These qualities make it potentially quite convenient as a fiber supplement.

Although glucomannan can be derived from other sources such as yeast, most studies have used glucomannan purified from the konjac root.