It’s well known that soluble fiber, as found in oats, barley, and psyllium (used in bulk laxatives such as Metamucil), helps lower blood cholesterol levels and promote “regularity” and has other potential health benefits. Now another soluble fiber is gaining the attention of researchers and marketers. Fiber-rich extracts from the bulb-like corm (underground storage organ) of the Asian konjac plant (Amorphophallus konjac), called glucomannan, are being added to foods and made into dietary supplements that are touted not just for lowering cholesterol, but also to control blood sugar and help with weight loss. Should you give glucomannan a try?
Konjac (pronounced like "cognac") has been consumed for centuries in many parts of Asia as both a food and a traditional medicine to treat everything from asthma to tumors. In Japan, thick, semi-translucent, and slightly rubbery shirataki (also called konnyaku) noodles are made from konjac flour; they contain just fiber and water (no carbs, fat, or protein, so near-zero calories). In China, konjac curd (tofu), often braised with duck or chicken, is a popular dish. But konjac’s gelatinous texture and fishy smell make it unappealing to some Westerners.
In the West, some food companies use konjac glucomannan as a gelling or thickening additive. It is “generally recognized as safe” (GRAS) by the FDA and Health Canada. It’s also used to increase shelf life of baked goods, prevent ice crystals in ice cream, serve as a vegan substitute for gelatin, and provide texture in imitation seafood products.
The most potent soluble fiber?
In terms of potential health benefits of konjac supplements (tablets, capsules, or powders), the Natural Medicines database rates glucomannan as “possibly effective” for lowering cholesterol, relieving constipation, and controlling blood sugar levels in people with diabetes—similar to other fiber supplements.
Like other soluble fibers, glucomannan may affect blood cholesterol by reducing the absorption of cholesterol in the intestines and increasing the activity of intestinal bacteria that can affect cholesterol metabolism. Also like other soluble fibers, glucomannan absorbs water in the colon, which can help form a bulkier stool, and delays gastric emptying, which can help control blood sugar levels and improve satiety.
Health Canada (similar to the U.S. FDA) now allows foods that contain PGX—a viscous fiber extract made from glucomannan, sodium alginate, and xanthan gum—to claim that they can help lower blood cholesterol.
A new analysis in the American Journal of Clinical Nutrition looked at 12 small studies (totaling 370 people) and concluded that konjac glucomannan can lower LDL (“bad”) cholesterol by about 10 percent. What’s more, it found that this can be accomplished by a much lower dose of glucomannan than other soluble fibers because of its greater viscosity (which is key to the cholesterol-lowering effect). It should be noted that one of the authors of this analysis and some other research on glucomannan holds glucomannan-related patents, including one for the proprietary fiber blend used in the development of PGX.
What about weight-loss claims?
Research on glucomannan for weight control has been less positive.
In 2013, for instance, a study in the Journal of Obesity compared a glucomannan supplement to a placebo capsule, taken before each meal, in 53 overweight or obese people. After eight weeks, there were no significant changes in body weight, body fat, hunger, blood cholesterol, or blood sugar levels.
In 2014, an analysis in the Journal of the American College of Nutrition, combined data from eight small studies on glucomannan’s effect in overweight or obese people and concluded that it does not produce significant weight loss.
In 2015, a study in the Journal of the American College of Nutrition tested a glucomannan supplement versus a placebo in 83 overweight people for two months. Though it found no benefits overall, when the researchers included only participants who were compliant with the protocol, they found significant reductions in body weight, body fat, and LDL cholesterol.
A paper in Nutrition in Clinical Practice in 2015 reviewed various dietary supplements promoted for weight loss andconcludedthat while there is “some evidence” of benefit for glucomannan, it is weak and more research is needed.
As with any fiber supplements…
In studies, some people taking glucomannan supplements reported diarrhea and abdominal discomfort. It’s important to drink plenty of water with glucomannan, since its soluble fiber can rapidly absorb water, and there have been reports of glucomannan supplements causing choking or a blockage in the throat, esophagus, or intestines. For this reason, Health Canada has warned that glucomannan products should not be taken right before going to bed. Because glucomannan, like psyllium supplements, may affect blood sugar, it should be used with caution by people who have diabetes. To be on the safe side, it’s best not to take glucomannan at the same time as medications, since it might impair their absorption or effectiveness.
Bottom line: Studies on the potential health benefits of glucomannan, though relatively small and short so far, have been promising for lowering blood cholesterol but not for weight control. If you are taking a fiber supplement like psyllium to help lower your cholesterol or stay “regular” and want to try another kind of fiber, glucomannan is another option (as is beta-glucan). Still, it’s always best to get as much fiber as you can from food. So if glucomannan sounds interesting to you, you might try Japanese-style shirataki noodles, sold in many markets and online. Often dubbed “miracle noodles” (that’s also the name of one brand), they are hardly a miracle food, but they can be a good way to add filling, heart-healthy fiber to your diet with few calories—albeit with a rubbery texture and possibly fishy smell. You can use them in soups, stews, and stir-fries, where they’ll pick up the flavors of the other ingredients, or as a pasta substitute.
Also see 6 Asian Noodles to Try.