Glucomannan

When glucomannan reaches the stomach it absorbs water and forms a thick viscous gel that adds bulk, slows gastric emptying, and prolongs the sense of fullness, which may reduce food intake. In the small intestine the gel traps bile acids and cholesterol, reducing their reabsorption and forcing the liver to pull LDL cholesterol from the blood to make new bile acids — the same mechanism by which oat beta-glucan and psyllium lower cholesterol. The gel also slows the absorption of glucose, flattening post-meal blood sugar and insulin spikes. In the colon, glucomannan is fermented by gut bacteria into short-chain fatty acids, which nourish the gut lining and may contribute to improved bowel regularity and modest effects on appetite hormones. Its laxative effect comes from increased stool bulk and water content.

Glucomannan has a sizeable evidence base. A 2008 meta-analysis of 14 RCTs (531 participants) found that glucomannan significantly reduced total cholesterol by about 19 mg/dL, LDL by 16 mg/dL, triglycerides, body weight (by about 0.8 kg), and fasting glucose, but not HDL or blood pressure. A 2017 meta-analysis of 12 trials in adults and children confirmed an LDL reduction of about 10%. Weight-loss results are less convincing: a 2014 systematic review of 9 RCTs concluded that glucomannan did not produce statistically significant weight loss compared with placebo, and a 2013 US trial of 53 overweight adults found no difference over 8 weeks. The EU health claim rests on older trials showing 1–2 kg greater loss with 3 g/day alongside a calorie-restricted diet. For constipation, several trials in adults and children show increased stool frequency, with a 2008 review supporting its use as a bulking laxative, though a pediatric trial found no benefit. Studies on diabetes show improved fasting glucose and HbA1c in some but not all trials. Most studies are short (4–16 weeks) and small, and product quality varies. The evidence is moderate for cholesterol and constipation, weak for meaningful weight loss.

Glucomannan, a water-soluble polysaccharide of glucose and mannose (beta-1,4 linked) with acetyl groups; absorbs up to 50 times its weight in water

Glucomannan is not absorbed; it exerts its effects by forming a gel in the stomach and intestine and is then partly fermented in the colon. Timing matters: taking it 15–60 minutes before a meal with at least 250 ml of water allows the gel to form before food arrives, which is when the trials showing satiety and cholesterol benefits dosed it. Powder forms thicken within minutes, so they must be drunk immediately after mixing. Because the gel physically binds other substances, it can trap medications and fat-soluble nutrients, so separate it from drugs by at least an hour before or four hours after.

• Shirataki noodles and konjac rice (nearly calorie-free, mostly water and glucomannan)
• Konnyaku jelly blocks (Japanese cuisine)
• Konjac jelly snacks (banned in some countries due to choking deaths)
• Used as a thickener and vegan gelatin substitute