Glucomannan

What is glucomannan?

Glucomannan is a soluble fiber extracted from the tuber of the konjac plant, Amorphophallus konjac. It has been part of East Asian diets for centuries, for example in konjac noodles. Its defining feature is how much water it absorbs: a 2007 case report describes it as expanding rapidly to many times its original size and forming very thick, viscous solutions.

What the research says

The bowel-habit evidence comes from small studies. In a 2006 placebo-controlled study of eight healthy adults on a low-fiber diet, 4.5 g a day of konjac glucomannan for three weeks increased bowel movement frequency by about 27% and stool weight by about 30%, along with higher counts of lactobacilli and bifidobacteria in stool. A 2008 follow-up in seven constipated adults found that the same dose raised weekly bowel movements from about 4.1 to 5.3. These are encouraging results, but in very few people.

In children, a 2017 meta-analysis of three randomized trials (122 participants) found glucomannan increased bowel movement frequency but did not significantly improve stool consistency or overall treatment success. The authors urged caution because of the small sample size.

Glucomannan is often marketed for appetite and weight. A 2014 meta-analysis of eight randomized trials found no statistically significant weight loss compared with placebo, and reported adverse events such as abdominal discomfort, diarrhea and constipation.

How it’s used in supplements

Studies used several grams a day, usually split across meals with water. Multi-ingredient gut capsules contain much less. Because glucomannan swells so quickly, it carries a specific safety concern: case reports describe esophageal obstruction after taking glucomannan products, particularly in people with existing swallowing or esophageal problems. Take any glucomannan product with a full glass of water, never dry, and avoid it if you have trouble swallowing.

Glucomannan is also fermented by colon bacteria. In the 2006 and 2008 studies, stool samples showed more lactobacilli and bifidobacteria and higher levels of short-chain fatty acids during supplementation, which suggests a prebiotic-like effect, similar to other fermentable fibers. Whether this translates into meaningful health benefits beyond bowel frequency is not yet clear.

Because glucomannan can slow the absorption of other substances, it may affect how oral medicines work, including diabetes medicines; the 2007 case report notes reported blood-sugar-lowering effects and possible changes in how well other medications are absorbed. Take other medicines at least an hour before or several hours after glucomannan, and check with your pharmacist if you take prescriptions.