Glucomannan is a fiber with many beneficial effects. It lowers cholesterol, helps control blood sugar in diabetes, and may even help with weight loss. Surprisingly, it also decreases inflammation, potentially helps with allergies, protects the skin from UV radiation, and may delay aging.
Long used in traditional Chinese medicine, glucomannan is now sold as a dietary supplement and weight loss aid. The herbal remedy is made from the yam-like tubers of the konjac plant. The tubers, commonly referred to as elephant yams, are used in Japanese kitchens to make yam cakes (konnyaku) and noodles (shirataki).
In addition to being sold as a dietary supplement, it’s used as a food additive — an emulsifier and thickener denoted with the E-number E425-ii.
Glucomannan has an exceptional ability to absorb water and is one of the most viscous dietary fibers known.
It absorbs so much liquid that a small amount of glucomannan added to a glass of water turns the entire content into a gel. These unique properties are believed to mediate its effects on weight loss.
Like other soluble fibers, it’s believed to promote weight loss in several ways (1):
- It’s very low in calories.
- It takes up space in your stomach and promotes a feeling of fullness (satiety), reducing food intake at a subsequent meal.
- It delays stomach emptying, contributing to increased satiety (2Trusted Source).
- Like other soluble fibers, it reduces the absorption of protein and fat (3Trusted Source).
It also feeds the friendly bacteria in your intestine, which turn it into short-chain fatty acids like butyrate, shown to protect against fat gain in some animal studies (4Trusted Source, 5Trusted Source).
Glucomannan may help with weight loss in certain groups of people but doesn’t seem to have an effect on others. Hopefully, future studies will explain why this supplement works for some and not others, and show which groups benefit most from glucomannan supplementation.
Glucomannan appears to have beneficial effects on blood lipids and glucose levels , but its effects on weight loss are inconsistent. In one study in Italy, 2 g/day glucomannan in two divided doses in 60 obese children (mean age 11.2 years) for 2 months did not significantly affect weight loss compared to placebo . In a small study conducted in the United States, 20 obese women (weighing ≥20% more than ideal body weight) consumed 3 g/day glucomannan (1 g before each meal) or placebo for 8 weeks . At the end of the study, glucomannan produced significantly greater weight loss (mean loss of 2.5 kg) than placebo (mean gain of 0.7 kg). In another study conducted in the United States, supplementation with glucomannan (3.9 g/day) for 4 weeks in 63 healthy men did not significantly reduce body weight compared with placebo . Eight weeks of glucomannan supplementation (1.33 g before each meal for a total of 3.99 g/day) also failed to significantly reduce body weight compared to placebo in 53 overweight or obese adults who continued their usual dietary and physical activity habits .
Glucomannan may help keep cholesterol levels in check, suggests a 2008 review of studies published in the American Journal of Clinical Nutrition.2
Based on a review of 14 clinical trials, glucomannan appeared to lower total cholesterol and “bad” low-density lipoprotein (LDL) cholesterol. On the other hand, it did nothing to improve “good” high-density lipoprotein (HDL) cholesterol or blood pressure.
The quality of reviewed studies was moderate to poor. Further research is needed to establish whether glucomannan is a safe and viable treatment option for hyperlipidemia (high cholesterol).
Contrary to popular belief, glucomannan has less impact on blood sugar levels than expected.6
This is surprising given that glucomannan passes through the intestines largely intact and can absorb up to 50 times its weight in water.
While it would fair to assume that this would slow the absorption of sugar (glucose) in the digestive tract, most research has shown that this doesn’t occur.
Konjac root powder is a prebiotic that feeds the friendly bacteria in the intestine. A study published in 2008 demonstrated that glucomannan supplementation increases the fecal concentration of probiotics in general as well as specific probiotics like bifidobacteria and lactobacilli. (4)
Higher intakes of prebiotics are linked to benefits, including: (5)
- lower risk for cardiovascular disease
- healthier cholesterol levels
- better gut health
- improved digestion
- lower stress response
- better hormonal balance
- higher immune function
- lower risk for obesity and weight gain
- lower inflammation and autoimmune reactions
Glucomannan powder or flour is LIKELY SAFE when consumed as food. Glucomannan powder and capsules are POSSIBLY SAFE for most adults and children when used in medicinal amounts for up to 4 months. However, solid tablets containing glucomannan are POSSIBLY UNSAFE for adults and LIKELY UNSAFE for children. These can sometimes cause blockages of the throat or intestines.
Severe esophageal and GI obstruction have been reported with glucomannan tablets. The hypoglycemic effects are potentially dangerous to patients with diabetes. Glucomannan has been linked in case reports to cholestatic hepatitis and occupational asthma. Minor adverse effects are normally GI related and include diarrhea, flatulence, abdominal discomfort, and bloating.
Clinical studies of glucomannan in diabetes, cholesterol control, and obesity have used dosages of 1 to 13 g daily.