Limited evidenceWeight management

Glucomannan

Glucomannan is an extremely viscous soluble fibre with decent evidence for lowering LDL cholesterol, but the weight-loss case has weakened badly: the best meta-analysis found no significant weight difference versus placebo. It also carries a real, documented choking and obstruction hazard.

What the science says

Weight loss Limited evidence
A meta-analysis of 8 RCTs found no statistically significant weight difference versus placebo (-0.22 kg; 95% CI -0.62 to 0.19, I² 65%). A broader 2021 review did find a significant -1.27 kg (95% CI -2.45 to -0.09), but explicitly noted this fell below its 2.5 kg threshold for clinical significance. Earlier, more favourable reviews were revised after data errors were identified.
Lowers LDL cholesterol Moderate evidence
Pooling 14 studies (531 participants), glucomannan lowered total cholesterol by 19.3 mg/dL (95% CI -24.3 to -14.3), LDL cholesterol by 16.0 mg/dL (95% CI -21.3 to -10.7) and triglycerides by 11.1 mg/dL. This is the most consistent benefit and is in line with what viscous fibres generally do to lipids.
Modestly lowers fasting glucose Limited evidence
The same pooled analysis found fasting blood glucose fell by 7.4 mg/dL (95% CI -14.2 to -0.7). Notably, people with impaired glucose metabolism, children, and those already making dietary changes did not benefit to the same degree — the opposite of what you would want.
Increases fullness Limited evidence
Glucomannan absorbs many times its weight in water and slows gastric emptying, which plausibly increases satiety. Since this has not translated into reliable weight loss in controlled trials, the satiety effect appears too small or too easily compensated for to matter.

Dosage & safety

Studied dose1-4 g/day is the usual range. The EFSA-authorised weight-management claim is based on 3 g/day taken as three 1 g doses with 1-2 glasses of water, 15-60 minutes before meals, in the context of an energy-restricted diet. Lipid trials generally used 3-4 g/day. Powder or capsules are preferred over tablets.

SafetyThe serious risk is mechanical: glucomannan swells rapidly and has caused oesophageal and intestinal obstruction. Tablet formulations have been restricted or withdrawn in several countries for this reason, and konjac jelly confectionery has caused choking deaths in children and older adults. Always take it with a large glass of water, never dry, never immediately before lying down, and avoid it entirely if you have swallowing difficulty, oesophageal stricture or a history of bowel obstruction. Expect bloating, flatulence, abdominal discomfort and either loose stools or constipation. Because it is highly viscous it can impair absorption of oral medications — take other drugs at least 1 hour before or 4 hours after, with particular care for sulfonylureas, levothyroxine and fat-soluble vitamins.

Scientific references

The efficacy of glucomannan supplementation in overweight and obesity: a systematic review and meta-analysis of randomized clinical trials
Onakpoya I, Posadzki P, Ernst E — J Am Coll Nutr, 2014
Effect of glucomannan on plasma lipid and glucose concentrations, body weight, and blood pressure: systematic review and meta-analysis
Sood N, Baker WL, Coleman CI — Am J Clin Nutr, 2008
Efficacy of dietary supplements containing isolated organic compounds for weight loss: a systematic review and meta-analysis of randomised placebo-controlled trials
Bessell E, Maunder A, Lauche R, et al. — Int J Obes (Lond), 2021

Where to buy

NOW Foods, Glucomannan, 575 mg, 180 Veg Capsules

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