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.
Most of the studies described here used 3 to 5 g per day in divided doses before meals. However, there are concerns regarding the form of glucomannan used (see Safety Issues below).
What Is the Scientific Evidence for Glucomannan?
#High Cholesterol and High Blood Pressure
In a double-blind study , 63 people were given either 3.9 g per day of glucomannan or placebo for 4 weeks and then switched to the other treatment. ^ While taking glucomannan, participants showed significant reductions in total cholesterol, LDL cholesterol, and triglycerides, as compared to placebo. In addition, their systolic blood pressure (the upper number in the blood pressure reading) was also reduced. However, there was no significant increase in HDL cholesterol and no improvement in the ratio of LDL to HDL cholesterol.
Participants in another study were given either 3 g per day of glucomannan or placebo over an 8-week period. ^ The glucomannan group showed improvements in total and HDL cholesterol as well as a reduction in systolic blood pressure. Those taking glucomannan also lost weight, whereas the placebo group gained weight over the length of the trial.
Several other controlled studies have found similar results. ^ And, in a mathematical review combining the results of 14 studies, glucomannan significantly reduced total and LDL cholesterol levels. ^
A few small double-blind studies suggest that glucomannan may be helpful for people trying to lose weight; however, in other studies, no such benefit was seen. ^ One double-blind, placebo-controlled trial of 20 women who were more than 20% over their ideal weight found glucomannan to be more effective than placebo at promoting weight loss. ^ All participants were instructed not to change their eating or exercise habits while on the treatment. Those in the treatment group took 1 g of glucomannan 3 times a day for 8 weeks and lost an average of 5.5 pounds during that period; in comparison, those in the placebo group gained an average of 1.5 pounds, a significant difference. The glucomannan group also had a reduction of total and LDL cholesterol as well as triglyceride levels.
Benefits were also seen in a double-blind, placebo-controlled trial of 28 overweight people who had just experienced a heart attack. ^ However, another double-blind trial of 60 obese children did not find a significant difference in weight loss between the glucomannan and the placebo groups. ^ In this study, the children received either 1 g of glucomannan or placebo twice a day for 8 weeks.
A study of individuals with diabetes tested the effectiveness of glucomannan fiber-enriched biscuits against wheat bran biscuits for blood sugar control. ^ While using the glucomannan biscuits, people experienced a significant improvement in glucose control as compared to the wheat bran biscuits.
Other studies have also found evidence that glucomannan can improve blood sugar control. ^