Glucomannan
Description
Glucomannan is a dietary fiber derived from tubers of the Amorphophallus konjac plant, commonly referred to as the konjac root. It is grown primarily in the tropical, sub-tropical and temperate zones of Asia. The tubers of the konjac plant contain large amounts of mannan, known as "konjac mannan" or glucomannan. Glucomannan is an unabsorbable poylsaccharide, composed of glucose and mannose, in 1:1.6 ratio, bound through beta-1, 4-glycosidic linkages.
Glucomannan has been consumed in the Orient, particularly Japan, for at least 1,000 years. Recent interest in glucomannan fiber comes from the public's increased awareness of the effects of high fiber diets on health problems, such as diabetes, postprandial hypoglycemia, hyperlipidemias (e.g. high cholesterol levels), various gastrointestinal disorders, some cancers and obesity. In response to consumer interest, a number of suppliers now provide konjac glucomannan products in powdered, tablet or capsule forms. Some products are up to 60% pure glucomannan.
Glucomannan is a dietary fiber with much stronger gel-forming properties than either pectin or guar gum. Glucomannan holds the advantage of being tasteless and not sticking to the palate when eaten, which are common drawbacks of using other gel-forming dietary fibers such as pectin. Glucomannan species vary in their gel-forming activity. For example, the Shina-shu (Chinese species) harvested in the Gumma Prefecture in Japan, has more activity than the Zairai-shu (Japanese species), harvested in either the Saitama Prefecture or Fukushima Prefecture of Japan.
Method of Action
Glucomannan and other gel-forming fibers interfere with the motility and absorption of nutrients from the gut. One gram of glucomannan expands in volume and will absorb about 100 milliters of water in vitro. However, the inhibitory effect of dietary fiber on gastric emptying has been shown in some, but not all studies. Nevertheless, studies in rats have shown glucomannan gels around the food particles, interfering with the action of digestive enzymes and thus slowing the rate at which sugars and fats enter the blood stream.
It has been suggested fiber modifies gut hormone response, as already demonstrated for GIP and suggested for glucagon. Fiber can also directly affect fat and protein metabolism.
Glucomannan seems not only to affect energy intake but also energy expenditure, since it reduces postpandrandial thermogenesis significantly. Studies suggest simultaneous changes in postprandial hormone response and in substrate metabolism may contribute to the effect of fiber on postprandial thermogenesis.
Therapeutic Approaches
Postprandial hypoglycemia
Glucomannan has been shown to lower postprandial ("after eating") blood glucose concentrations in healthy subjects and those with type II diabetes. In studies on normal subjects given glucose loads, it has been shown that 5 grams of glucomannan can be beneficial, depressing plasma glucose levels and the insulin response. Its usefulness in the management of type II diabetes is thought to be due to glucomannan's ability to slow stomach emptying, modify responses of gastrointestinal hormones and delay glucose diffusion in the intestinal lumen.
About 10 to 20% of patients with previous gastric surgery suffer from dumping syndrome. Such patients suffer from reactive hypoglycemia several hours after a meal. The symptoms include a feeling of warmth, sweating, shakiness, dizziness and difficulty breathing. These symptoms can be prevented by ingestion of frequent small meals and supplementation of the diet with alpha-glycoside hydrolase inhibitors or gel-forming dietary fibers. This benefit was demonstrated in small animals and humans. Double-blind, placebo-controlled hospital studies of gastrectomized patients have shown that small amounts (5.2 grams per meal) of glucomannan may be beneficial to ptients with reactive postprandial hypoglycemia, without the disadvantage of being unpalatable or causing carbohydrate malabsorption. By comparison, pectin, which is generally poorly tolerated because of poor taste and adherence to the palate, requires 14.5 grams per meal to be equally effective.
Diabetes
Dietary fibers are effective in the treatment and management of diabetes. Until research was done on glucomannan, guar gum (galactomannan) was the most effective fiber for reducing blood sugar levels in diabetics. However, glucomannan was found to not only reduce the need for insulin or hypoglycemic agents, but to also lower serum cholesterol levels over 10% by the end of three weeks. Similarly, reductions in blood glucose levels (7.3% at 30 minutes) and serum insulin (13% at 30 minutes) were observed in healthy men given 2.6 grams of glucomannan daily.
Hypocholesterolemic agent
The Shina-shu species of glucomannan has been shown to cause significant decreases in plasma cholesterol levels as compared to controls in laboratory animals. This effect was strain specific, since some konjac root strains were found to have little or no cholesterol lowering effect.
Similar effects were reported in humans in three other studies. In one study elderly subjects reported a 23 mg/dl drop in serum cholesterol over a two week period while on glucomannan. A second study found a 21.7 mg/dl drop in serum cholesterol (and a 15.0 mg/dl drop in low-density lipoprotein cholesterol) levels in obese subjects within a four week period. The third study showed a 11.2% drop in serum cholesterol in diabetic patients within three weeks of daily supplementation with 3.6 grams of glucomannan. Glucomannan is thought to owe hypocholesterolemic ability to its binding and subsequent removal of bile through the feces.
Obesity and Weight Management
Glucolannan supplements are becoming important agents for weight control.
Twenty obese subjects were given one gram of glucomannan with 8 ounces of water, one hour prior to each of three daily meals. Results from this double-blind placebo-controlled study showed a significant mean weight loss of 5.5 pounds over an 8-week period. Serum cholesterol and low-density lipoprotein cholesterol (LDL) levels were also significantly reduced in the glucomannan treated group, but not in the control group. No adverse reactions were reported by these subjects.
However, in Australia, seven cases of esophageal obstruction have been reported by doctors seeing patients who took glucomannan for weight management. These cases were probably due to the inappropriate use of the glucomannan powder as a dietary supplement.
In Japan, where millions of people eat konjac root as a food, no such adverse reactions have been reported because their traditional method of ingestion requires the gel to be allowed to expand before ingestion. In the case of glucomannan powders, this expansion may only begin once the product is inside the gastrointestinal tract, with the potential for obstructing the passageway.
Drug interactions
Glucomannan may slow gastric emptying, thus resulting in the decreased absorption of some drugs.
Glucomannan Is Useful in the Treatment of Diabetes
Glibenclamide (a sulfonlyurea) is one of the most extensively-used hypoglycemic drugs.
In a controlled-study, patients were given 3.9 grams of glucomannan powder to see what effect, if any, glucomannan might have on the intestinal absorption of glibenclamide. Oral administration of glucomannan decreased plasma glibenclamide levels by more than 50% in comparison to levels in controls. This may suggest glucomannan adversely affects absorption of glibenclamide and possibly other drugs.
Toxicity Factors
Konjac glucomannan may reduce fat-soluble vitamin absorption while removing bile acids in humans. The absorption of vitamin E (alpha acetate) was reduced following administration of glucomannan. However, glucomannan did not interfere with the absorption of water soluble, fat-insoluble vitamin B-12 (mecobalamin).
There is some concern the effect of glucomannan on transit time of carbohydrates may influence the bioavailability of minerals. For example, in one study measuring the effect of unavailable carbohydrates on the intestinal absorption of calcium in rats during a 7 to 8 week period, calcium absorption was compromised by nearly 20%. This compromise was partially due to the loss of calcium-binding protein caused by the gastrointestinal transit of large amounts of undigested food.
Seven cases of esophageal obstruction caused by adding glucomannan to the diet have been reported. This occurs because glucomannan can absorb great amounts of water and expand at a rapid rate after consumption. Obstruction of the esophagus has also been reported by those using either guar gum or bulk laxatives. In the case of guar gum or bulk laxatives, most reports were from elderly patients. However, in the case of glucomannan, most of the complaints were much younger patients.
Although konjac root has been used for centuries by the Japanese, they have not reported the occurrence of similar problems, probably owing to the fact that glucomannan is allowed to expand before ingestion.
It is also noteworthy that glucomannan has been given to children with dumping syndrome in a hospital study. This study determined that glucomannan was not suitable for dumping syndrome in all children, because of side effects. Contrarily, three out of four children had considerably more complaints during one of the glucomannan trial phases.
There has been no report of subacute or acute toxicity (LD50) toxicity induced by glucomannan.
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