Glucomannan (Konjac)

Claimed to suppress appetite and produce weight loss by expanding in the stomach, lower cholesterol, improve blood glucose and relieve constipation. The weight claim fails on the evidence while being authorised in law. A random-effects meta-analysis of eight randomised trials in overweight or obese participants found body weight −0.22 kg (95% CI −0.62 to 0.19), I² = 65% — not statistically significant, a confidence interval that comfortably includes zero and slight weight gain, and a point estimate inside the range of normal day-to-day body-weight fluctuation. The European Union nevertheless authorises a weight-loss claim for glucomannan, one of very few such claims it grants.

The cholesterol effect holds up better but on a thin base: 12 studies with only 370 participants in total gave LDL cholesterol −0.35 mmol/L (95% CI −0.46 to −0.25), about a 10% relative reduction, and non-HDL cholesterol −0.32 mmol/L. Apolipoprotein B did not move in the six trials that measured it, and since apoB is the particle-count measure, its being unmoved while LDL-C falls weakens the atherogenic interpretation.

Safety knowledge is moderate. The gel-forming property that supplies the mechanism is also the hazard: choking and oesophageal and intestinal obstruction, which is the explicit basis for the warning the European Union mandates on every authorised claim. Recorded adverse events were abdominal discomfort, diarrhoea and constipation. No hepatotoxicity signal; long-term safety data are limited and the included trials were short.