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Glucomannan (konjac)

The most viscous fibre of all — an EFSA nod for weight and cholesterol, but respect the choking warning.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden)Last reviewed July 2026Supplement guide

SupplementsFiberSoluble fibreWeightCholesterol

Glucomannan comes from the root of the konjac plant. It absorbs a huge amount of water — the source of both its benefits and its one real hazard.

Key takeaways

  • EFSA-approved: glucomannan aids weight loss within an energy-restricted diet at 3 g/day (1 g before three meals with water).1
  • Also an approved claim for maintaining normal blood cholesterol (4 g/day).1
  • Works by swelling into a gel that promotes fullness and slows digestion.
  • ⚠️ Choking/obstruction risk — always take with plenty of water, never dry, and avoid if you have swallowing difficulties.

What it is

A very high-molecular-weight soluble fibre (glucomannan) from konjac (Amorphophallus konjac). It forms an exceptionally viscous gel — up to 50× its weight in water — increasing satiety and slowing gastric emptying.

The evidence

EFSA authorised a weight-loss claim (3 g/day alongside calorie restriction) and a cholesterol claim (4 g/day).1 Trials show a modest effect on body weight and LDL; it is not a stand-alone slimming cure.2

Dose

1 g with a large glass of water before each of three meals (3 g/day) for weight; up to ~4 g/day for cholesterol. Take 30–60 min before eating.

Safety — read this

Because it swells so much, dry tablets or too little fluid can cause throat or gut obstruction — always take with ample water and don’t use it if you have swallowing problems or a narrowed gut. It can also slow and reduce how well medicines are absorbed, because the gel physically traps them in the gut. Give it real spacing: take any medication at least 1 hour before, or 2–3 hours after glucomannan — a few minutes’ gap is not enough. For drugs where the exact dose matters — thyroid hormone (levothyroxine), the contraceptive pill, or diabetes medication — keep to the wider end and check with a pharmacist.

Bottom line: a genuinely useful satiety/cholesterol fibre with EFSA backing — effective in modest amounts, but only when taken with plenty of water.

References

Adult research summary for education, not a prescription.

  1. EFSA NDA Panel: scientific opinions on glucomannan and reduction in body weight / maintenance of normal blood cholesterol. EFSA Journal 2010. www.efsa.europa.eu
  2. Sood N, et al. Effect of glucomannan on plasma lipid and glucose concentrations, body weight, and blood pressure: systematic review and meta-analysis. Am J Clin Nutr 2008. pubmed.ncbi.nlm.nih.gov

This article summarises nutrition science for education. It is not a substitute for individual medical advice; consult a professional before starting a supplement, in pregnancy, or alongside medication.