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A Review of Glucomannan (Konjac Fiber)
By Judith L. Balk, MD, Dr. Balk is Assistant Research Professor, University of Pittsburgh; she reports no consultant, stockholder, speaker's bureau, research, or other financial relationships with companies having ties to this field of study.
Glucomannan is a soluble fiber obtained from the tubers of the Japanese konjac plant, Amorphophallus konjac. One report notes that it has been safely consumed as food for more than 1,000 years in the Orient.1 The Massachusetts General Hospital web site lists high cholesterol as the principal indication for glucomannan, with other proposed uses being constipation, diabetes, high blood pressure, and weight loss.2 Given the prevalence of these conditions in the United States, glucomannan may prove to have public health significance.
Glucomannan is a pectin-like gel fiber composed of a polysaccharide chain of repeating units of β-1,4-linked glucose and mannose.1 Glucomannan particles, derived from the konjac root, are tasteless, odorless, and white, consisting of long thread-like macromolecules tangled together.3
The factor that makes glucomannan different from other types of soluble fiber is its ability to retain water. On contact with water glucomannan particles swell to roughly 200 times their original volume, turning glucomannan into a viscous liquid.3 Put another way, 1 g of glucomannan will absorb about 100 mL of water in vitro.1
Because of its high absorptive properties, lower doses are needed for glucomannan than for other fiber supplements. Lower doses may lead to improved adherence to a fiber regimen.
High-fiber diets are often recommended for patients with hyper-cholesterolemia due to the cholesterol-lowering properties of water-soluble fiber such as oat. However, the mechanism of action is controversial; promotion of bile-acid excretion in the stool and/or blockage of cholesterol absorption are possible mechanisms.3
Serum cholesterol concentrations decreased in healthy men with hypercholesterolemia when taking glucomannan compared to placebo.3 After a two-week baseline, subjects were given either 3.9 g glucomannan or placebo daily for four weeks, then were crossed over to the opposite treatment after a two-week washout period. Total cholesterol lowered by 10%, LDL by 7.2%, triglycerides by 23%, and systolic blood pressure by 2.5%. Each of these changes was statistically significant. HDL did not change, and no adverse effects were observed.
In another study assessing weight loss, cholesterol levels were also measured.1 At baseline, serum cholesterol in all subjects averaged 198 mg/dL, which is high normal. At eight weeks, both total and LDL cholesterol had decreased significantly in the glucomannan group compared to the placebo group, by 26.2 mg/dL and 20.9 mg/dL, respectively. In a study of subjects with insulin resistance, serum cholesterol improved with glucomannan compared to wheat bran, changing the classification of eight of 11 subjects into normal cholesterolemia.4 Apo-B also fell significantly, but neither Apo-A-1 nor triglycerides decreased. HDL decreased significantly in both treatments, but the decreases in total cholesterol were sufficient to improve lipid ratios.
Similarly, in overweight, normocholesterolemic subjects fed a supplement containing both chitosan and glucomannan, total, HDL, and LDL cholesterol concentrations were significantly lower at the end of the four-week study period than at baseline.5
Insulin Resistance and Diabetes
Fiber may have benefit in prediabetic metabolic conditions such as insulin resistance. Vuksan et al investigated the effect of glucomannan on metabolic control in subjects with insulin resistance in a double-blind, placebo-controlled crossover design.4 All subjects had an eight-week baseline following the NCEP Step 2 program to eliminate possible effects of dietary change on metabolic parameters. After the baseline, there were two successive three-week experimental phases, separated by a two-week washout interval of the NCEP Step 2 diet. Five subjects were on glucomannan for the first phase and six were on the control supplement. Both treatments consisted of a three-day rotating Step 2 diet with three meals per day provided, for consumption at work or at home. The two treatments differed only in the type of fiber: glucomannan vs. wheat bran. Weight decreased in both treatment periods, but not statistically significantly. The glucomannan group had an improvement in glycemic control compared to the wheat bran group, as determined by serum fructosamine levels. No changes were seen in blood pressure.
Eleven hyperlipidemic and hypertensive Type 2 diabetics were enrolled in a trial with the same methodology as the previous study.6 Compared with placebo, glucomannan significantly reduced the primary outcomes: serum fructosamine, total/HDL cholesterol, and systolic blood pressure. The authors concluded that glucomannan may help to improve the effectiveness of conventional treatment in Type 2 diabetes. Similarly, compared with placebo, glucomannan reduced total and LDL cholesterol, total/HDL ratio, ApoB, and fasting glucose in Type 2 diabetic subjects.7 Triglycerides, HDL, LDL/HDL ratio, postprandial glucose, and body weight did not differ. Fecal neutral sterol and bile acid concentrations were increased with glucomannan compared to placebo.
A letter in the Lancet discussed the use of glucomannan in both diabetics and healthy men.8 When 13 diabetics supplemented their diets with glucomannan for 90 days, fasting glucose fell by 29%. Five healthy men underwent a 50 g glucose tolerance test with and without glucomannan. The glucomannan reduced blood glucose and serum insulin levels by 7.3% and 13%, respectively, at 30 minutes.
Glucomannan could potentially affect obesity via several mechanisms. One mechanism would be to increase satiety, if taken with water prior to meals. Another mechanism would be to lower the glycemic index of food, causing slower release of sugars and fats, leading to less hyperinsulinemia. Other potential mechanisms would be to lower the absorption of fats in the gut, and to decrease transit time. On the other hand, differences in postprandial availability of food may in turn lead to changes in energy expenditure.9
Glucomannan has been shown to delay gastric emptying, increase bile acid excretion in the stool, and lower postprandial glucose concentrations.10 Keithley and Swanson reviewed the role of glucomannan in obesity, concluding that, at 2-4 g/d, glucomannan was well tolerated and resulted in significant weight loss in overweight and obese individual, and that further investigation of safety, efficacy, and mechanisms of action is needed to determine whether glucomannan can help to decrease the high prevalence of overweight and obesity in the United States.10
In a double-blind, randomized, controlled trial, 20 obese women were randomized into two groups, one receiving two capsules of 500 mg of glucomannan three times daily before each meal, and the other group receiving the same dosage and schedule of a placebo starch capsule.1 Both groups were identical at baseline, averaging about 184 pounds and 64 inches tall. Acceptance of the supplement was high, with many subjects subjectively noting that they had a "full" feeling after taking glucomannan, and some reported that the supplement had relieved mild constipation. No adverse effects were noted in either group. At the end of the eight-week period, the subjects in the glucomannan group had lost 5.5 pounds, whereas those in the placebo group had gained 1.5 pounds (P < 0.005).
A series of placebo-controlled trials investigated different fiber agents that each included glucomannan vs. placebo.11 While each supplement included glucomannan, one was glucomannan only, and the other two had additional types of fiber, such as guar and alginate. The dosage in the fiber supplements ranged from 420 mg to 4,320 mg glucomannan. All subjects followed a 1,200 kcal/d diet. Subjects in both the placebo and fiber groups lost weight, but all three fiber supplements resulted in increased weight loss compared to placebo (P < 0.01). Over the five-week observation period, the fiber groups lost roughly 4 kg, whereas the placebo group lost roughly 2.4 kg. There were no differences between fiber groups, demonstrating that glucomannan alone is equally effective as glucomannan plus other types of fiber. The large range of glucomannan dosages limits the utility of this study.
In contrast, 90 children, mean overweight 46%, were placed on either glucomannan or placebo, 1 g twice per day for two months.12 During this time period, the children followed a normocaloric diet and were evaluated every two weeks. At the end of the study period, the mean overweight of the glucomannan group decreased from 49.5% to 46.1%, and that of the placebo group from 43.9% to 41.7%. Both decreases were significant, but the difference between groups was not statistically significant. The only significant difference concerned lipid metabolism; the children under glucomannan treatment had a significant decrease of α-lipoprotein and an increase in pre-β-lipoprotein and triglycerides, whereas the children in the placebo group had a decrease in triglycerides and apo-β-lipoprotein. The authors suggest that the metabolic alteration may derive from a primary decrease in α-lipoprotein, causing an accumulation of pre-β-lipoprotein and triglycerides. The cause for the decrease in α-lipoprotein was postulated to be most likely a result of inadequate water intake. The authors question the use of glucomannan in childhood obesity, as it could have a potentially atherogeneous effect.
Because glucomannan is a fiber supplement, it can relieve constipation. Fiber can promote defecation by three mechanisms: increasing colonic contents, stimulating colonic motility, and promoting the growth of certain bifidobacteria and lactobacilli.13 One study investigated the mechanisms of action of glucomannan.13 Subjects in this placebo-controlled trial ingested 1.5 g/meal of glucomannan, and their stools were collected on days 15-21 of the study period to determine fecal mass, components, microflora, and short-chain fatty acid contents. The glucomannan supplement significantly increased the mean defecation frequency, wet and dry stool weight, and daily fecal output of lactobacilli, bifidobacteria, and total bacteria. The supplement also increased fecal short-chain fatty acid concentrations. No adverse effects were noted. The authors summarize the incorporation of 4.5 g/d of glucomannan to a low-fiber diet could improve colon health by enhancing bowel movement and stool bulk, and improving the colonic ecology without causing gastrointestinal side effects.
A case report of a 31-year-old man with cholestatic hepatitis questioned whether glucomannan was involved.14 The man had used glucomannan, along with two other botanical agents, and had stopped the glucomannan four weeks prior to developing jaundice. The reason that glucomannan was implicated was that the other botanicals had been found to be safe, and that one "potential, but speculative mechanism may be related to the presence of serotonin and its derivatives." The reference cited is a Japanese article noting that a byproduct in the manufacturing of konjac jelly (80% glucomannan) is called tobiko, which "has an irritant taste and unpleasant smell. Therefore it cannot be disposed of as such as industrial waste."15 This byproduct was found to have serotonin in it, but it is not clear that glucomannan itself has serotonin in it. In addition, it is not clear if the subject was consuming botanicals, including glucomannan, that were contaminated with liver-damaging chemicals.
Another case report describes esophageal obstruction from a pharmacobezoar containing glucomannan.16 A bezoar is a concretion of partly or wholly undigested material formed in the alimentary canal, and a pharmacobezoar is made of medication. In this case, a 37-year-old female took a diet pill containing glucomannan with a glass of water, and had the initial feeling of a lump in her throat. As the day progressed, this sensation had worsened. She presented to the emergency department, where she forcefully vomited, and dislodged the pill. The tablet had a firm gelatinous texture with an intact gelatin coating, but it had formed a large cast of the patient's esophagus. On taking the patient's history, she had experienced transient partial obstruction of the esophagus several times in the past, and endoscopy did note an esophageal web. This case illustrates that glucomannan and other hygroscopic medications are contraindicated in patients with a history of upper gastrointestinal pathology, difficulty swallowing, or inability to drink adequate fluid. Glucomannan bottles have the warning, "Taking this product without adequate fluid may cause it to swell and block your throat or esophagus, and may cause choking. Do not take this product if you experience difficulty in swallowing." This warning is based on information reported to the FDA MedWatch program.
If glucomannan is effective in increasing bile acid excretion, it may also be able to remove fat-soluble vitamins. This was studied by giving a control test meal with added vitamin B12 and vitamin E, with and without glucomannan.17 The absorption rate of vitamin E was reduced when glucomannan was added, but absorption of B12 was not affected. It is thus suggested that glucomannan can reduce fat-soluble vitamin absorption, but not water-soluble vitamin absorption. The dosage of glucomannan was high, however, at 3.9 g per meal; usual dosage is 1.5 g per meal.
Patients may also note excess gas, abdominal distention, or diarrhea. These symptoms usually resolve within several days, or they can be managed by a reduction in dosage.
Although the published research reports have some inconsistencies, the majority of studies do find that total cholesterol, LDL, fasting glucose concentrations, weight, and constipation are improved with glucomannan. Dosage is not standardized, but many research studies use 2-4 g per day in divided dosages, taken about one hour before meals. This appears to be safe in patients without gastrointestinal tract pathology, but more research is indicated. To be safe, fat-soluble vitamins should be taken at a separate time to assure vitamin absorption.
1. Walsh DE, et al. Effect of glucomannan on obese patients: A clinical study. Int J Obesity 1984;8:289-293.
2. Learning about health and illness. [web site] October 2006. Available at: http://healthgate.partners.org/browsing/LearningCenter.asp?fileName=21743.xml&title=Glucomannan.
3. Arvill A, Bodin L. Effect of short-term ingestion of konjac glucomannan on serum cholesterol in healthy men. Am J Clin Nutr 1995;61:585-589.
4. Vuksan V, et al. Beneficial effects of viscous dietary fiber from Konjac-mannan in subjects with the insulin resistance syndrome: Results of a controlled metabolic trial. Diabetes Care 2000;23:9-14.
5. Gallaher DD, et al. A glucomannan and chitosan fiber supplement decreases plasma cholesterol and increases cholesterol excretion in overweight normo-cholesterolemic humans. J Am Coll Nutr 2002;21:428-433.
6. Vuksan V, 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 1999;22:913-919.
7. Chen HL, et al. Konjac supplement alleviated hyper-cholesterolemia and hyperglycemia in type 2 diabetic subjects—a randomized double-blind trial. J Am Coll Nutr 2003;22:36-42.
8. Doi K, et al. Treatment of diabetes with glucomannan (konjac mannan). Lancet 1979;1:987-988.
9. Doi K. Effect of konjac fibre (glucomannan) on glucose and lipids. Eur J Clin Nutr 1995;49(Suppl 3):S190-S197.
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11. Birketvedt GS, et al. Experiences with three different fiber supplements in weight reduction. Med Sci Monit 2005;11:PI5-8.
12. Vido L, et al. Childhood obesity treatment: Double blinded trial on dietary fibres (glucomannan) versus placebo. Padiatr Padol 1993;28:133-136.
13. Chen HL, et al. Konjac acts as a natural laxative by increasing stool bulk and improving colonic ecology in healthy adults. Nutrition 2006;22:1112-1119. Epub 2006 Oct 4.
14. Villaverde AF, et al. Acute hepatitis of cholestatic type possibly associated with the use of glucomannan (amorphophalus konjac). J Hepatol 2004;41:1061-1062.
15. Niwa T, et al. Cis-N-(p-Coumaroyl)serotonin from Konnyaku, Amorphophallus konjac K. Koch. Biosci Biotechnol Biochem 2000;64:2269-2271.
16. Vanderbeek PB, et al. Esophageal obstruction from a hygroscopic pharmacobezoar containing glucomannan. Clin Toxicol (Phila) 2007;45:80-82.
17. Doi K, et al. Influence of dietary fiber (konjac mannan) on absorption of vitamin B12 and vitamin E. Tohoku J Exp Med 1983;141(suppl):677-681.