woman looking in her fridge with arms full of fruit and vegetables

Glucomannan: benefits, dosage & side effects

Find out all about glucomannan, including what it does, the benefits to taking it and how much you might need

Written by Beth Gibbons on January 18, 2019 Reviewed by Dr Sarah Schenker on January 30, 2019


What is glucomannan and what does it do?

Glucomannan, or konjac as it is sometimes known, is a natural, water-soluble fibre derived from the roots of a south-east Asian plant called the elephant yam.1 It has been traditionally used by the Chinese to detoxify and ease symptoms of asthma, and is still used to make noodles and tofu.2

As with any fibre, glucomannan can help you feel more satisfied after eating, but what makes it interesting to scientists is that glucomannan has extraordinary water absorption capacities – it’s able to absorb up to 50 times its own weight in water!3

Glucomannan is often used as a bulking agent in foods, appearing as E425 on food labels.4 It’s also available as capsules and powder, and is added to some diet foods such as pastas and noodles.

Benefits of glucomannan

What does glucomannan do in the body?

It may help you maintain your weight

Like all soluble fibres, glucomannan is highly absorbent, binding to water to form a gel.5 This helps:6
  • create a sensation of fullness that can reduce appetite
  • delay stomach emptying, keeping you feeling fuller for longer
  • break down carbohydrates more slowly, stabilising post-meal blood sugar levels
  • decreases absorption of fat

A 2005 study by Norway’s University of Tromso reported that dieters who took glucomannan lost more weight than those taking other high-fibre supplements.7 However, a 2013 study in Journal of Obesity found it had no effect on weight.8

It may support gut health

As a viscous dietary fibre, glucomannan is the favourite food for certain friendly bacteria in our guts. A 2010 laboratory study by the University of Reading found that it helped encourage healthy levels of gut bacteria populations, for example bifidobacterium, but more research on humans is still needed.9

And like all fibres, glucomannan can reduce constipation by increasing bulk, and speeding up food transit time through the colon.10

It can lower cholesterol levels

According to a 2008 review of evidence in The American Journal of Clinical Nutrition, glucomannan can reduce levels of total cholesterol and cause a drop in ‘bad’ LDL cholesterol, too. Researchers suggested that glucomannan has this effect by altering both the way cholesterol is produced in the liver and how fats in the body are stored or used for energy.11


How much glucomannan is safe to take?

A dose of around 2g to 4g per day has been used safely in studies, but always read the packaging or food label first.12 Take glucomannan 15 minutes to an hour before a meal, and make sure you swallow it with 1-2 glasses of fluid to avoid it expanding before it reaches your stomach, as this could be dangerous.13

Glucomannan has not been proved safe for the following people:

  • pregnant or breastfeeding women
  • children
If you are diabetic, speak to your doctor before taking glucomannan as it can reduce the absorption of some diabetes medications.14


What are the side-effects of taking glucomannan?

Generally, glucomannan is well-tolerated. Mild side-effects can include:15
  • bloating
  • flatulence
  • soft stools
  • diarrhoea
Shop Supplements Advice is for information only and should not replace medical care. Please check with your GP before trying any remedies.


1. Keithley J, Swanson B. Glucomannan and obesity: a critical review 2. Chua M, et al. Traditional uses and potential health benefits of Amorphophallus konjac K. Koch ex N.E.Br 3. Keithley JK, et al. Safety and Efficacy of Glucomannan for Weight Loss in Overweight and Moderately Obese Adults 4. Mortensen A. Re‐evaluation of konjac gum (E 425 i) and konjac glucomannan (E 425 ii) as food additives

5. As Source 3

6. J. Slavin, H. Green. British Nutrition Foundation. Dietary fibre and satiety 7. Birketvedt GS, et al. Experiences with three different fiber supplements in weight reduction

8. As Source 3

9. Connolly ML, Lovegrove JA, Tuohy KM. Konjac glucomannan hydrolysate beneficially modulates bacterial composition and activity within the faecal microbiota 10. Chen HL, et al. Konjac acts as a natural laxative by increasing stool bulk and improving colonic ecology in healthy adults 11. Sood N, Baker WL, Coleman CI. Effect of glucomannan on plasma lipid and glucose concentrations, body weight, and blood pressure: systematic review and meta-analysis

12. As Source 1

13. Atli Arnarson. Healthline. Glucomannan – Is It an Effective Weight Loss Supplement?

14. As above

15. As Source 13
Weight LossWeight Management