glucomannan

Glucomannan is a soluble dietary fiber that comes from the root of the konjac plant (Amorphophallus konjac). It can absorb a large amount of water relative to its weight, and when taken with fluid it forms a thick, viscous gel in the gastrointestinal tract. This water-absorbing, gel-forming property is the basis for how it is thought to act.

According to the NIH Office of Dietary Supplements, glucomannan's effects on weight loss are inconsistent. The trials it summarizes are mixed: some found no significant difference from placebo, while others found modest reductions. One 2015 systematic review of six randomized controlled trials (293 participants) concluded that 1.24 to 3.99 g/day for up to 12 weeks did not significantly affect body weight versus placebo, and a 2014 meta-analysis reached a similar conclusion; an older meta-analysis focused on lipids and glucose reported only a small (about 0.79 kg) statistically significant loss over roughly 5 weeks. The same source notes glucomannan appears to have beneficial effects on blood lipids and glucose levels. Given the inconsistent and at best modest weight-loss data, this site does not consider glucomannan a reliable weight-loss treatment.

Doses studied in trials have varied widely, roughly 1.2 to 15.1 g/day, commonly taken before meals with plenty of water. There is no established effective dose for weight loss, and over-the-counter products differ in form and strength. Talk to a clinician or pharmacist before starting glucomannan, particularly if you take medications or have a swallowing, esophageal, or digestive condition.

The NIH Office of Dietary Supplements notes that little is known about glucomannan's long-term safety, but that it appears to be well tolerated for short-term use. Importantly, tablet forms were linked to seven reported cases of esophageal (food-pipe) obstruction in Australia in 1984-1985, so tablets should be used with caution; powdered and capsule forms have not been associated with this problem. Always take it with adequate fluid to lower the risk of choking or blockage, and avoid it if you have difficulty swallowing.

Reported side effects are generally mild and gastrointestinal. They can include gassiness and belching, bloating, abdominal discomfort, loose or watery stools and diarrhea, and in some people constipation.

The NIH fact sheet cited here does not list specific drug interactions for glucomannan. As a general precaution, viscous gel-forming fibers can slow or reduce how much of an oral medication or nutrient is absorbed when taken at the same time, so spacing glucomannan apart from other medicines is commonly advised. Ask a clinician or pharmacist about timing, especially if you take oral diabetes medicines or other prescription drugs, since glucomannan may also affect blood sugar.