Glucomannan is a water-soluble polysaccharide that is considered a dietary fiber. 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 also the main ingredient in shirataki noodles and konnyaku which are traditional Japanese foods. These products are made from the ground corm of the konjac plant. These natural products have been eaten in Japan for centuries. Because of the fiber content and health benefits, they are known as the "broom of the intestine" in Japan. They are now being marketed for weight loss and other health benefits in the United States and worldwide due to the low calorie count.
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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.[1] The degree of branching is about 8% through β-(1→6)-glucosyl linkages.
Glucomannan with α-(1→6)-linked galactose units in side branches is called galactoglucomannan.
Glucomannan comprises 40% by dry weight of the roots, or corm, of the konjac plant. It 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 a soluble fiber, and as such, has been investigated for the treatment of constipation. Glucomannan may relieve constipation by decreasing fecal transit time.[2] In the treatment of chronic constipation, glucomannan significantly improved symptoms of constipation while being well-tolerated and free of relevant side effects.[3]
Glucomannan has demonstrated statistically significant improvements in the total cholesterol of obese patients.[4] In healthy men, 4 weeks of taking 3.9 grams of glucomannan decreased total cholesterol, low-density lipoprotein, triglycerides, and systolic blood pressure; notably, triglycerides dropped by 23%.[5] Glucomannan has also been tested in children with high cholesterol in conjunction with a diet. Interestingly, greater decreases in total cholesterol and low-density lipoprotein were observed in female children when compared to male children.[6] When used in conjunction with chitosan, glucomannan decreases serum cholesterol possibly by increasing steroid excretion via the feces.[7]
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.[8]
In one study involving 200 obese subjects, Glucomannan or a placebo were given for 16 weeks. They found that Glucomannan & fiber group lost 4.52 kg on average compared to 0.79 of control group. The Glucomannan group also had increased satiety compared to control group. LDL cholesterol was significantly reduced as well during the 16 week period. The study also reported that the treatments were well tolerated, and did not report any side effects.[9]
An 8 week double blind trial involved 20 obese patients. A placebo or a glucomannan fiber supplement of 1 gram was given to the subjects one hour before each meal. No changes were made to the diet or exercise habits of the patients. The study found that over the 8 week period, cholesterol levels were significantly reduced, and the Glucomannan group had an average weight loss of 5.5 lbs.[4]
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 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."[10] The health advisory was issued after authorization of some products containing Glucomannan or the purposes of appetite reduction, weight management, treatment of constipation and management of high cholesterol levels.
Eating glucomannan in the form of shirataki noodles does not pose any choking risk due to the size of the noodles.
Several companies selling products containing glucomannan have been disciplined by the U.S. Federal Trade Commission (FTC) for misleading or exaggerated claims[11][12] 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 are 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,[13] PediaLean,[14] Herbal Worldwide Holdings,[15] BioTrim,[16] 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.[17]
In 2002, a number of products containing konjac-derived glucomannan were recalled as choking hazards.[18]
As a food additive, glucomannan is used as an emulsifier and thickener. It has E number E425(ii).