Glucomannan is a water-soluble polysaccharide that is considered a dietary fiber. It is a hemicellulose component in the cell walls of some plant species. Glucomannan is a food additive used as an emulsifier and thickener. Products containing glucomannan, marketed under a variety of brand names, are also sold as nutritional supplements for constipation, obesity, high cholesterol, acne vulgaris and type 2 diabetes. Although there is some clinical support for potential health benefits, the U.S. Food and Drug Administration (FDA) has not approved any product containing glucomannan for the treatment of these medical conditions. Health Canada has authorized some products containing glucomannan for the purposes of appetite reduction, weight management, treatment of constipation and management of high cholesterol levels.
Glucomannan is mainly a straight-chain polymer, with a small amount of branching. The component sugars are β-(1→4)-linked D-mannose and D-glucose in a ratio of 1.6:1. The degree of branching is about 8% through β-(1→6)-glucosyl linkages.
Glucomannan comprises 40% by dry weight of the roots, or corm, of the konjac plant. Another culinary source is salep, ground from the roots of certain orchids and used in Turkish cuisine. Glucomannan is also a hemicellulose that is present in large amounts in the wood of conifers and in smaller amounts in the wood of dicotyledons. Glucomannan is also a constituent of bacterial, plant and yeast cell wall with differences in the branches or glycosidic linkages in the linear structure.
Potential health benefits
Treatment of constipation
Glucomannan is a soluble fiber, and as such, has been investigated for the treatment of constipation. Glucomannan may relieve constipation by decreasing fecal transit time. In the treatment of chronic constipation, glucomannan significantly improved symptoms of constipation while being well-tolerated and free of relevant side effects.
Cholesterol and other lipids
Glucomannan has demonstrated statistically significant improvements in the total cholesterol of obese patients. In healthy men, four weeks of taking 3.9 grams per day of glucomannan decreased total cholesterol, low-density lipoprotein, triglycerides, and systolic blood pressure; notably, triglycerides dropped by 23%. Glucomannan has also been tested in children with high cholesterol in conjunction with a diet. A gender difference has been shown, greater decreases in total cholesterol and low-density lipoprotein were observed in girls when compared to boys. When used in conjunction with chitosan, glucomannan decreases serum cholesterol, possibly by increasing steroid excretion via the feces.
Type 2 diabetes
Glucomannan may be useful as a therapeutic adjunct for type 2 diabetes. It has been shown to improve the lipid profile and alleviate the fasting blood glucose levels of type 2 diabetics. Glucomannan also helps to increase insulin sensitivity and improves glycemia and risk factor for coronary heart disease 
Clinical trials examining the use of glucomannan for weight loss have produced mixed results. A 2014 systematic review and meta-analysis of clinical trials failed to show that glucomannan supplementation generated statistically significant weight loss.
Potential health risks
A health advisory was released by Health Canada stating the following: "Natural health products containing the ingredient glucomannan in tablet, capsule or powder form, which are currently on the Canadian market, have a potential for harm if taken without at least 250 ml or 8 ounces of water or other fluid. The risk to Canadians includes choking and/or blockage of the throat, esophagus or intestine, according to international adverse reaction case reports. It is also important to note that these products should not be taken immediately before going to bed." The health advisory was issued after authorization of some products containing glucomannan for the purposes of appetite reduction, weight management, treatment of constipation and management of high cholesterol levels.
Several companies selling products containing glucomannan have been disciplined by the U.S. Federal Trade Commission (FTC) for misleading or exaggerated claims pertaining to the health benefits of glucomannan supplements.
Glucomannan is an ingredient in a variety of dietary supplement products marketed via television advertisements claiming to aid in weight loss. According to the FTC, there is no clinical data supporting many of the manufacturer claims and several companies have been determined by the Federal Trade Commission (FTC) or the Food and Drug Administration (FDA) to have, at some time, violated the Federal Food, Drug, and Cosmetic Act. The companies include Vitacost, PediaLean, Herbal Worldwide Holdings, BioTrim, and others. The company Obesity Research Institute, the marketer of FiberThin, Zylotrim, Propolene and Lipozene, settled FTC charges that their misleading weight-loss claims violated federal laws by agreeing to pay $1.5 million in consumer redress.
- Kaname Katsuraya; Kohsaku Okuyamab; Kenichi Hatanakab; Ryuichi Oshimab; Takaya Satoc; Kei Matsuzakic (2003). "Constitution of konjac glucomannan: chemical analysis and 13C NMR spectroscopy". Carbohydrate Polymers. 53 (2): 183–189. doi:10.1016/S0144-8617(03)00039-0.
- "Glucomannan: Uses, Side Effects, Interactions and Warnings".
- Elbein, A. D. (1969). Biosynthesis of a cell wall glucomannan in mung bean seedlings. Journal of Biological Chemistry, 244(6), 1608-1616.
- Tokoh, C., Takabe, K., Sugiyama, J., & Fujita, M. (2002). CP/MAS 13C NMR and electron diffraction study of bacterial cellulose structure affected by cell wall polysaccharides. Cellulose, 9(3-4), 351-360.
- Chorvatovičová, D., Machová, E., Šandula, J., & Kogan, G. (1999). Protective effect of the yeast glucomannan against cyclophosphamide-induced mutagenicity. Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 444(1), 117-122.
- Marzio L, Del Bianco R, Donne MD, Pieramico O, Cuccurullo F (August 1989). "Mouth-to-cecum transit time in patients affected by chronic constipation: effect of glucomannan". Am. J. Gastroenterol. 84 (8): 888–91. PMID 2547312.
- Passaretti S, Franzoni M, Comin U, et al. (1991). "Action of glucomannans on complaints in patients affected with chronic constipation: a multicentric clinical evaluation". Ital J Gastroenterol. 23 (7): 421–5. PMID 1742540.
- Walsh DE, Yaghoubian V, Behforooz A (1984). "Effect of glucomannan on obese patients: a clinical study". Int J Obes. 8 (4): 289–93. PMID 6096282.
- Arvill A, Bodin L (March 1995). "Effect of short-term ingestion of konjac glucomannan on serum cholesterol in healthy men". Am. J. Clin. Nutr. 61 (3): 585–9. PMID 7872224.
- Martino F, Martino E, Morrone F, Carnevali E, Forcone R, Niglio T (June 2005). "Effect of dietary supplementation with glucomannan on plasma total cholesterol and low density lipoprotein cholesterol in hypercholesterolemic children". Nutr Metab Cardiovasc Dis. 15 (3): 174–80. doi:10.1016/j.numecd.2004.04.004. PMID 15955465.
- Gallaher DD, Gallaher CM, Mahrt GJ, et al. (October 2002). "A glucomannan and chitosan fiber supplement decreases plasma cholesterol and increases cholesterol excretion in overweight normocholesterolemic humans". J Am Coll Nutr. 21 (5): 428–33. doi:10.1080/07315724.2002.10719246. PMID 12356785.
- Chen HL, Sheu WH, Tai TS, Liaw YP, Chen YC (February 2003). "Konjac supplement alleviated hypercholesterolemia and hyperglycemia in type 2 diabetic subjects--a randomized double-blind trial". J Am Coll Nutr. 22 (1): 36–42. doi:10.1080/07315724.2003.10719273. PMID 12569112.
- Vuksan, V. L. A. D. I. M. I. R., et al. "Konjac-mannan (glucomannan) improves glycemia and other associated risk factors for coronary heart disease in type 2 diabetes. A randomized controlled metabolic trial." Diabetes Care 22.6 (1999): 913-919.
- Salas-Salvadó J, Farrés X, Luque X, Narejos S, Borrell M, Basora J, Anguera A, Torres F, Bulló M, Balanza R (2008). "Effect of two doses of a mixture of soluble fibres on body weight and metabolic variables in overweight or obese patients: a randomised trial.". Br J Nutr. 99 (6): 1380–7. doi:10.1017/S0007114507868528. PMID 18031592.
- JK. Keithley,J., Swanson, B., Mikolaitis, M., DeMeo, J., Zeller, L., Fogg, Adamji, J. 2013. Safety and Efficacy of Glucomannan for Weight Loss in Overweight and Moderately Obese Adults. J Obes. 2013; 2013: 610908
- Onakpoya I, Posadzki P, Ernst E. 2014. The efficacy of glucomannan supplementation in overweight and obesity: a systematic review and meta-analysis of randomized clinical trials. J Am Coll Nutr. 2014;33(1):70-8.
- Health Canada Advises Canadians that Natural Health Products containing Glucomannan May Cause Serious Choking if Used with Insufficient Fluid, Health Canada Advisory
- "Ads for Various Diet Supplements and Topical Gels Don’t Cut the Fat, Says the FTC". Federal Trade Commission
- Food and Drug Administration letter
- C. Landis Plummer (June 15, 2004). DOCKET NO. 9318. Federal Trade Commission.
- Federal Trade Commission complaint
- Kovacic, William E.; Chun, Barbara Y.K. (November 3, 2004). "COMPLAINT FOR PERMTLNENT INJUNCTION AND OTHER EQUITABLE RELIEF". United States District Court for the Central District of California.
- "FTC Settles Claims with Marketers of FiberThin and Propolene". Federal Trade Commission. June 20, 2005
- "Import Alert 33-15, Detention Without Physical Examination of Gel Candies Containing Konjac" US FDA, Retrieved April 1, 2016
- Current EU approved additives and their E Numbers, Food Standards Agency, 26 November 2010
- "E425(ii) Konjac Glucomannan". CyberColloids. Retrieved January 25, 2013.