This ingredient benefits from allowed claim on reduction of body weight by EFSA but is supported by little scientific evidence.
In Japan, konjac (Amorphophallus konjac), from where is extracted glucomannan, is already considered like a functional food since many years thanks to published studies proving its efficacy on reduction of body weight and other applications.
EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA); Scientific Opinion on the substantiation of health claims related to konjac mannan (glucomannan) and reduction of body weight (ID 854, 1556, 3725), reduction of post-prandial glycaemic responses (ID 1559), maintenance of normal blood glucose concentrations (ID 835, 3724), maintenance of normal (fasting) blood concentrations of triglycerides (ID 3217), maintenance of normal blood cholesterol concentrations (ID 3100, 3217), maintenance of normal bowel function (ID 834, 1557, 3901) and decreasing potentially pathogenic intestinal microorganisms (ID 1558) pursuant to Article 13(1) of Regulation (EC) No 1924/2006. EFSA Journal 2010;8(10):1798. [27 pp.].
Keithley J and Swanson B. Glucomannan and obesity: a critical review. Alternative Therapies in Health and Medicine. 2005;11:30-34.