Glucomannan is a fibre, and the evidence that it lowers weight is thin. It is the soluble fibre of the konjac root, sold in capsules and powders and as the base of konjac noodles. Where trials find a weight effect, it is about a kilogram or less. Regulators have split on it: the EU authorises one narrow weight claim, while FDA's only decision about its health effects is about cholesterol. This page reports the trial record and the regulators' records, dated, and says what is not established.
Read on 10 November 2026 from PubMed, Regulation (EU) No 432/2012, FDA's constituent update of 10 January 2020 and the FTC's 2005 settlement.
What it is
Glucomannan comes from the root of Amorphophallus konjac, also called elephant yam. It absorbs water and forms a thick gel, which is the reason it is sold for fullness. In the US it is a dietary supplement: it is not a drug, FDA does not approve it before sale, and no weight-loss drug contains it. That puts it in a different category from the medicines on GLP-1 alternatives and on our register of weight-loss pills.
What the trials measured
A PubMed search on 10 November 2026 for randomised controlled trials naming glucomannan and weight or obesity in the title or abstract returned 31 records, from 1984 to 2026, many of them small and short. Four reviews have pooled them:
| Review | Trials included | Weight difference over placebo | What the authors concluded |
|---|---|---|---|
| Sood et al., Am J Clin Nutr 2008 | 14 studies, 531 people | −0.79 kg (95% CI −1.53 to −0.05) | A benefit on weight, larger on cholesterol; less benefit for people already on a diet |
| Onakpoya et al., J Am Coll Nutr 2014 | 8 trials pooled (9 included), overweight or obese people only | −0.22 kg (95% CI −0.62 to 0.19), not significant | The trials "do not show" statistically significant weight loss |
| Zalewski et al., Nutrition 2015 | 6 trials, one in children | Not pooled; three adult trials found short-term differences, none on BMI | "Some evidence" of short-term weight effects in adults, none for children |
| Bessell et al., Int J Obes 2021 | Glucomannan trials within 67 supplement trials | −1.27 kg (95% CI −2.45 to −0.09) | Below its 2.5 kg threshold for clinical significance; insufficient evidence to recommend |
Why the reviews differ. Sood's review was not limited to people with overweight or obesity, and the two later reviews that limited themselves to overweight or obese adults found the smallest effects. The largest single US trial in that group, Keithley et al., 2013, gave 53 adults 1.33 g capsules before three meals for eight weeks and found no difference from placebo in weight, body composition or hunger. The newest systematic review, published in October 2026 (Kusuma et al., Food & Function), found that much of the mechanism evidence comes from animal studies and that several human studies reported null results.
The scale, with populations named. These figures are kilograms over a few weeks to a few months. In their own trials of adults with obesity and no diabetes, semaglutide 2.4 mg lowered weight by 14.9% against 2.4% on placebo over 68 weeks (STEP 1), and tirzepatide 15 mg by 20.9% against 3.1% over 72 weeks (SURMOUNT-1). No trial has compared glucomannan with either, and the populations, lengths and measures differ.
What the regulators have said
The regulators read the same evidence differently, and what each one decided is narrower than marketing suggests.
The EU authorises one weight claim. Regulation (EU) No 432/2012, based on EFSA's 2010 opinion, allows food to state: "Glucomannan in the context of an energy restricted diet contributes to weight loss." The claim may be used only on food containing 1 g of glucomannan per portion, and the label must tell the consumer that the effect was seen with 3 g a day, in three 1 g doses, with one to two glasses of water before meals, on an energy-restricted diet. It must also carry a choking warning for people with swallowing difficulties or who take it with too little fluid. The same regulation authorises a separate cholesterol claim at 4 g a day.
FDA's decision is about cholesterol, not weight. On 10 January 2020 FDA granted a citizen petition and said it intends to add glucomannan to the regulatory definition of dietary fibre, because "the scientific evidence suggests that glucomannan can help reduce blood cholesterol." Until that rulemaking is finished, it lets manufacturers count glucomannan as fibre on the label. The update says nothing about weight, and no FDA weight claim for glucomannan exists.
The FTC has acted on glucomannan weight advertising. In June 2005 the FTC settled with FiberThin, LLC, Obesity Research Institute, LLC, their two owners and two endorsers over FiberThin and Propolene, fibre supplements said to contain glucomannan. The complaint challenged claims of rapid, substantial weight loss "without any need to diet or exercise" and of more than 2 pounds a week. The defendants agreed to $1.5 million in consumer redress, with a $41 million suspended judgment. The FTC's release called these "Red Flag" claims, alongside weight loss regardless of what a person eats and weight loss in all users.
What is not established
- No trial has shown that glucomannan produces a weight loss its own reviewers call clinically meaningful in adults with obesity.
- No long trial exists: the trials run for weeks to a few months, so nothing shows whether any effect lasts.
- The EU claim depends on a diet that cuts energy; no regulator has found that glucomannan lowers weight on its own.
- Whether glucomannan changes GLP-1 release in people, and whether that would matter for weight, is not shown by these trials. The wider claim that foods and fibres act as a "natural" GLP-1 is examined on natural GLP-1, and fibre products sold as boosters on GLP-1 boosters.
Where a reader might act on this, a pharmacist or doctor is the right person to ask, especially for anyone with a swallowing problem, a bowel condition or on other medicines, since a swelling fibre can affect when other tablets are absorbed. This page reports the record; it does not recommend glucomannan or any product containing it.
Sources
- Regulation (EU) No 432/2012, Annex (authorised health claims: glucomannan) — EUR-Lex, read 10 November 2026; EFSA Journal 2010;8(10):1798.
- FDA, "FDA Grants Citizen Petition on Glucomannan as a Dietary Fiber", constituent update, 10 January 2020 — fda.gov.
- FTC, "FTC Settles Claims with Marketers of FiberThin and Propolene", 20 June 2005 — ftc.gov.
- Sood N et al., Am J Clin Nutr 2008, PMID 18842808. Onakpoya I et al., J Am Coll Nutr 2014, PMID 24533610. Zalewski BM et al., Nutrition 2015, PMID 25701331. Bessell E et al., Int J Obes 2021, PMID 33976376. Keithley JK et al., J Obes 2013, PMID 24490058. Kusuma RA et al., Food & Function 2026, PMID 42765172.
- Wilding JPH et al., NEJM 2021, PMID 33567185 (STEP 1). Jastreboff AM et al., NEJM 2022, PMID 35658024 (SURMOUNT-1).
- PubMed query, 10 November 2026:
glucomannan[tiab] AND (weight[tiab] OR obesity[tiab]) AND randomized controlled trial[pt]→ 31 records.