Originally published August, 2003. Updated June, 2011
Konjac fiber is a starch from the root of the konjac plant (Amorphophallus konjac) that grows in both China and Japan. The popular name for the root in Japan is devil's tongue or konnyaku, and it is prepared there into foods that look like squares or strips of stiff gelatin, with a rubbery texture. The Japanese regard konjac as a health food, and consider it to be especially good for intestinal function.
Its main component is glucomannan, a water-soluble dietary fiber consisting of mannose and glucose sugars. Glucomannan hasn’t been widely studied in the West, but in 2008, researchers at the University of Connecticut published an analysis of 14 studies involving consumption of capsules, biscuits and energy bars containing this type of fiber. The analysis concluded that glucomannan significantly lowered total cholesterol, LDL ("bad") cholesterol, and triglycerides; kept blood sugar (glucose) stable, and had a small effect on weight. The researchers found no evidence that it helped lower blood pressure or had a beneficial effect on HDL ("good") cholesterol. Its effect on blood sugar appeared to be no different from that of other types of dietary fiber.
An article published in the Wall Street Journal in January, 2010 quoted a researcher at Rush University College of Nursing as saying that evidence so far suggests that regular consumption of glucomannan might lead to modest, short term weight loss. More study is needed to see if these findings hold up over time.
Konjac fiber and glucomannan are widely promoted online and elsewhere for weight loss and cholesterol control. I wouldn’t rely on any of these products. The best supplement for cholesterol control is red yeast rice extract, a source of naturally occurring statins. Because it delivers a mix of those compounds rather than a single molecule, red yeast rice extract is much less likely to cause the side effects that sometimes occur with pharmaceutical statins. For weight loss, I know of no supplements or shortcuts that are effective. The best way to lose weight is by healthy eating and increasing physical activity.
If you're interested in konjac fiber, I suggest trying some of the traditional Japanese foods such as shirataki noodles or konnyaku. I've eaten them frequently in Japan and like their novel textures - although less adventurous eaters may find them a bit challenging. You can find them in Asian markets or order them online.
Andrew Weil, M.D.