What Is Glucomannan? The Wording, the Trials and the Choking Rule

Glucomannan is the gel-forming fibre of the konjac tuber. It is the one fibre with an authorised GB weight wording, "Glucomannan in the context of an energy restricted diet contributes to weight loss", at 3 g a day in three 1 g doses with 1 to 2 glasses of water before meals; across eight trials the pooled difference against placebo was 0.22 kg and not statistically significant. It swells to many times its size, so every dose needs water.^{1, 2, 3} Part of the Wellgard fibre guide.

Evidence at a glance

• Trials: 8 weight trials pooled in 2014; 12 cholesterol trials pooled in 2017, 370 people.
• Largest single trial cited: 80 children with constipation, 4 weeks; the US weight trial, 53 adults, 8 weeks.
• Strongest measured outcome: LDL cholesterol 0.35 mmol/L lower, about 10%, at about 3 g a day.
• Not tested in any trial we found: glucomannan against psyllium, head to head.

Sources checked on 13 October 2026.

Never take glucomannan dry. The register requires this warning: "Warning of choking to be given for people with swallowing difficulties or when ingesting with inadequate fluid intake - advice on taking with plenty of water to ensure substance reaches stomach."¹ Drink each dose with a full glass of water before it thickens, not just before sleep, and not at all if you have difficulty swallowing. Chest pain, vomiting, or difficulty swallowing or breathing after a dose needs urgent help: call 999.

What is glucomannan, and where does it come from?

Glucomannan is the main polysaccharide in the tubers of the Amorphophallus konjac plant, used as a dietary fibre in eastern cuisines for more than 1,000 years.³ It is a soluble fibre that holds a great deal of water, forms a very thick solution and expands rapidly to many times its original size, hence its choking warning.³

What is glucomannan allowed to claim in Great Britain?

The register rejected every other glucomannan wording it assessed, including bowel, blood glucose, insulin and prebiotic wordings.¹

Does glucomannan work for weight loss?

Glucomannan carries "in the context of an energy restricted diet contributes to weight loss" at 3 g a day in three 1 g doses with water before meals; across eight randomised trials in overweight and obese adults, the pooled difference against placebo was 0.22 kg and not statistically significant, and results varied between trials.^{1, 2}

A 2015 review of six trials found three adult trials with significant reductions at single time points, from 0.21 kg at week 2 to 3.17 kg at week 8; only one showed a benefit at more than one time point, and none changed body mass index.⁴ A 2008 pooling of 14 trials (531 people) found body weight 0.79 kg lower than control, though people already making dietary changes benefited less.⁵

One US trial tested glucomannan without a diet: 53 overweight adults took 1.33 g with a glass of water an hour before each meal, about 4 g a day, for 8 weeks. Weight changed by 0.40 kg against 0.43 kg on placebo, with no difference in body composition, hunger, cholesterol or glucose.⁶ The authorised wording assumes an energy-restricted diet, which this trial did not use. Before-and-after photos are not trial data.

Does glucomannan lower cholesterol or blood sugar?

Across 12 randomised trials of 370 people (8 in adults, 4 in children), about 3 g a day of konjac glucomannan lowered LDL cholesterol by 0.35 mmol/L, about 10%, and non-HDL cholesterol by 0.32 mmol/L, about 7%; six trials showed no change in apolipoprotein B.⁷ The authorised cholesterol wording applies at 4 g a day.¹

Blood sugar has no authorised wording: the register rejected every glucomannan glucose and insulin wording.¹ In the 2008 pooling, fasting blood glucose was 7.44 mg/dL lower (the US unit), and people with impaired glucose metabolism benefited less.⁵ Glucomannan is not a replacement for a prescribed cholesterol or diabetes medicine.

How do you take glucomannan safely?

The authorised pattern is 1 g at a time, three times a day, before meals, each dose with 1 to 2 glasses of water.¹

• Stir powder into a full glass of water and drink it straight away, before it thickens.
• Swallow capsules one at a time with a full glass of water, sitting or standing upright.
• Take the last dose "at least an hour before going to bed", as the NHS says for ispaghula.⁸
• Take other medicines and supplements "more than an hour before or after", the NHS spacing for ispaghula.⁹

How to take psyllium husk has the same fluid rules.

Who should not take glucomannan, and what should you not take with it?

Side effects in the weight trials were abdominal discomfort, diarrhoea and constipation.² The serious risk is blockage: a glucomannan diet aid obstructed a 37-year-old woman's oesophagus until forceful vomiting cleared it just before an endoscopy, which later found a narrowing called an oesophageal web.³ Ask a GP or pharmacist first if you:

• have difficulty swallowing or any throat or gullet problem;
• have tummy pain with sickness or vomiting, or a change in your poo lasting more than 2 weeks, including blood (the NHS list for ispaghula, a similar gel);¹⁰
• take regular medicines, especially lithium, carbamazepine, digoxin, insulin or other diabetes medicines;⁹
• are pregnant or breastfeeding, or are thinking of giving it to a child.

The case report's authors note glucomannan may change how other medicines are absorbed.³ On its ispaghula pages the NHS points people to the Yellow Card scheme to report suspected side effects.¹¹

How do you compare glucomannan supplements?

Compare labels against the authorised wording:¹

• Grams of glucomannan per dose: the weight wording needs 1 g per portion.
• Doses a day: 3 g in three doses for weight, 4 g for cholesterol.
• The water instruction and the choking warning on the pack.

A glucomannan complex combines glucomannan with other ingredients, and each added nutrient carries only its own wording. Chromium, for example, may carry "Chromium contributes to the maintenance of normal blood glucose levels" where the product is at least a source of chromium (15% of its nutrient reference value).¹ Which fibre supplement is best compares fibres by job.

Glucomannan or psyllium: which does what?

Both are gel-forming fibres with different evidence. In a pooling of 16 fibre trials in 1,251 adults with constipation, psyllium and pectin showed significant effects, at doses above 10 g a day.¹² Glucomannan has the two authorised wordings, and its constipation trials we found are in children; psyllium carries no authorised GB claim at any dose.¹ What psyllium husk does has its trials and is psyllium husk safe its cautions.

What we don't know

• We found no trial comparing glucomannan with psyllium head to head for weight, cholesterol or bowel habit.
• No trial cited here measured heart attacks or strokes; the cholesterol results are blood measures.
• The weight trials varied in reporting quality, and data in children are too limited for any conclusion on weight.

Frequently asked questions

Does glucomannan help you poo?

The trials we found are in children. In 80 constipated children aged 3 to 16, about 2.5 g a day for 4 weeks was no better than placebo for treatment success, with more tummy pain in the first and fourth weeks.¹³ Across three trials of 122 children it raised stool frequency by 1.4 a week without changing stool consistency or treatment success.¹⁴ How much fibre children need has the UK amounts.

How long does glucomannan take to work?

The weight trials cited here measured change at 2 to 8 weeks, and the cholesterol trials ran for at least 3 weeks.^{4, 6, 7} None reported a result within days.

Can you take glucomannan every day?

The authorised pattern is daily.¹ In the 8-week US trial it was well tolerated, with no difference from placebo in liver or kidney blood tests.⁶ It is not for anyone with difficulty swallowing.

Is konjac jelly or konjac noodles the same thing?

They are foods made from the same tuber, whose main polysaccharide is glucomannan, so they contain fibre; the label gives the amount.³ The weight trials tested supplements, and a food may carry the weight wording only with 1 g of glucomannan per portion and every condition in the table.¹

Is glucomannan banned in the UK?

We found no UK ban on glucomannan supplements. The GB register authorises two wordings for it, each with the choking warning, and konjac (E425) is on the government's list of approved food additives for England, Wales and Northern Ireland.^{1, 15}

Sources

1. Department of Health and Social Care. Great Britain nutrition and health claims register, dated 19 May 2026. gov.uk
2. Onakpoya I et al. Journal of the American College of Nutrition 2014. Pooled analysis of 8 randomised trials. PubMed 24533610
3. Vanderbeek PB et al. Clinical Toxicology 2007. Case report, 1 woman. PubMed 17357389
4. Zalewski BM et al. Nutrition 2015. Systematic review of 6 randomised trials. PubMed 25701331
5. Sood N et al. American Journal of Clinical Nutrition 2008. Pooled analysis of 14 trials, 531 people. PubMed 18842808
6. Keithley JK et al. Journal of Obesity 2013. Randomised trial, 53 adults, 8 weeks; funded by a charitable trust. PubMed 24490058
7. Ho HVT et al. American Journal of Clinical Nutrition 2017. Pooled analysis of 12 randomised trials, 370 people. PubMed 28356275
8. NHS. Ispaghula husk: how and when to take it. Page last reviewed 13 January 2026. nhs.uk
9. NHS. Ispaghula husk: other medicines and supplements. Page last reviewed 13 January 2026. nhs.uk
10. NHS. Ispaghula husk: who can and cannot take it. Page last reviewed 13 January 2026. nhs.uk
11. NHS. Ispaghula husk: side effects. Page last reviewed 13 January 2026. nhs.uk
12. van der Schoot A et al. American Journal of Clinical Nutrition 2022. Pooled analysis of 16 randomised trials, 1,251 adults. PubMed 35816465
13. Chmielewska A et al. Clinical Nutrition 2011. Randomised trial, 80 children, 4 weeks; funding not checked. PubMed 21320737
14. Han Y et al. Asia Pacific Journal of Clinical Nutrition 2017. Pooled analysis of 3 randomised trials, 122 children. PubMed 28429913
15. Food Standards Agency. Approved additives and E numbers. Published 25 June 2026. gov.uk

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