Glucomannan (Konjac) for Weight Loss: What the Evidence and Regulators Say

Glucomannan is a soluble fiber from the root of the konjac plant that turns into a thick gel in water^[1]. In the US it is sold as a dietary supplement, which means the FDA does not approve it, or its label claims, before it goes on sale^[3]. In trials, the effect on weight ranges from no significant difference to roughly 1.3 kg (about 3 lb) more than placebo, in short studies^[9,11]. The real safety concern is choking: dry glucomannan can swell before it reaches the stomach^[17].

Below I go through what it is, what US and other regulators allow companies to claim, what the trials show on weight and fullness, why Australia banned the tablets, and what to know if you take a GLP-1 medication.

What glucomannan is

Glucomannan is the main carbohydrate in the tuber of Amorphophallus konjac, a plant from East Asia, where it has been eaten for more than a thousand years^[1]. Chemically it is a chain of mannose and glucose, in an 8 to 5 ratio, joined by beta linkages^[1].

What sets it apart from other fibers is how much water it holds. A pharmacology review describes it as the dietary fiber with the highest known molecular weight and viscosity, with an extraordinarily high water-holding capacity^[1]. The NIH Office of Dietary Supplements puts it at up to 50 times its own weight^[2]. How soluble fibers like this one differ from insoluble fiber is in soluble vs. insoluble fiber.

That is where the fullness idea comes from: by soaking up water in the gut, it has been proposed to increase feelings of fullness and slow stomach emptying^[2]. Whether that turns into eating less and losing weight is a question for the trials below. How stomach volume fits with the other hunger signals is in my guide to the science of satiety.

It is sold in two very different forms:

• As a supplement: capsules, tablets or powder.
• As food: shirataki noodles, konjac “rice” and other products made from the already-hydrated gel.

What US regulators say

FDA: a dietary supplement, not an approved treatment

Under the 1994 Dietary Supplement Health and Education Act, the FDA does not have the authority to approve dietary supplements before they are marketed, and it generally does not approve their claims or labeling either^[3]. The company is responsible for making sure its product is not adulterated or misbranded^[3].

A supplement label can carry a structure/function claim, which describes an effect on the body. The FDA’s own example is “fiber maintains bowel regularity”^[4]. The company must have evidence that the claim is truthful, notify the FDA within 30 days of marketing it, and print a disclaimer that the FDA has not evaluated the claim and that the product is not intended to “diagnose, treat, cure or prevent any disease”^[4]. When you see that disclaimer next to a weight claim on a glucomannan bottle, it means exactly what it says.

Separately, in January 2020 the FDA said it intends to propose adding glucomannan to the regulatory definition of dietary fiber and, until that rulemaking is done, lets companies count it in the fiber line on Nutrition and Supplement Facts labels. The reason given was that the evidence suggests it can help lower blood cholesterol^[6]. Weight was not part of that decision.

FTC: the ad needs real evidence

The Federal Trade Commission polices the advertising. Its December 2022 guidance says health claims for supplements need “competent and reliable scientific evidence”, and that for health benefits this generally means randomized, controlled trials in people^[5]. It also says a “Results not typical” line does not fix a testimonial that shows dramatic weight loss; the ad has to say what a typical buyer can expect^[5].

Other regulators, for context

• European Union. The EU allows the claim “Glucomannan in the context of an energy restricted diet contributes to weight loss”, only for foods with 1 g of glucomannan per serving. The label must say the effect comes with 3 g a day in three 1 g doses, taken with one or two glasses of water before meals, alongside a reduced-calorie diet, and must warn about choking^[7]. The claim rests on a 2010 EFSA scientific opinion on glucomannan and body weight^[7,8]. These are legal conditions for the label wording, not a recommendation to take it.
• Canada. Health Canada’s 2024 glucomannan monograph lists uses for cholesterol and constipation, not weight loss, and accepts only capsules and powder^[19].

What the trials show on weight

Here are the most useful evidence summaries, with average extra weight lost compared with placebo:

The NIH also cites a 2015 systematic review of 6 trials with 293 participants: 1.24 to 3.99 g a day for up to 12 weeks had no significant effect on body weight compared with placebo^[2]. Its overall verdict is that glucomannan’s effects on weight loss are inconsistent^[2].

One well-run US example: in an 8-week double-blind trial in Chicago, 53 adults with overweight or obesity took 1.33 g of glucomannan or placebo with 8 ounces of water an hour before each meal, eating as they normally did. There was no difference in weight, body composition, hunger or fullness^[13].

My read: if there is an effect, it is around 1 kg (2 lb), in short trials with a lot of variation between them. For a 200-lb person, that is about 1 % of body weight.

Does it make you feel full?

That glucomannan fills the stomach makes sense on paper, but in people the evidence on appetite is thinner than the ads suggest. A 2026 systematic review pooled 20 studies of glucomannan and resistant starch: 6 randomized or crossover trials, 3 non-randomized studies and 11 animal studies. Several reported more fullness or lower intake, but the results were heterogeneous, the mechanistic evidence came mostly from animals, and several human studies found nothing^[14].

The Chicago trial, which measured hunger and fullness directly, saw no change either^[13].

Shirataki and other konjac foods

Shirataki noodles are glucomannan as food: an already-hydrated konjac gel with very few calories^[15]. Our food database does not have a konjac entry yet, so I am not quoting per-serving figures here; they vary by product, so check the Nutrition Facts panel.

The most useful study is a crossover trial in Toronto with 16 healthy adults^[15]. Before an all-you-want dessert, they ate the same volume, 325 ml (about 11 fl oz), of pasta, half pasta and half konjac noodles, or konjac noodles only:

Source: Au-Yeung et al., 2018^[15].

On average, people did not eat more dessert afterwards, so total energy for the test was lower^[15]. But swapping all the pasta for konjac left people hungrier, not fuller^[15]. It is a small, single-meal study in people without obesity.

That fits what I cover in the most filling foods: volume helps, but a plate with almost no protein or energy does not keep you full. And if a GLP-1 already has you eating little, a 77-kcal bowl can crowd out what your body needs; see what to eat on GLP-1 medications.

To add fiber from food, the fiber calculator compares your day with a 25 g reference.

Risks: choking and blockage

The risk regulators worry about comes from the very property that is advertised: glucomannan swells with water. If it does so before reaching the stomach, it can block the throat or esophagus.

• FDA rules for OTC drugs. Over-the-counter drugs that contain glucomannan or other water-soluble gums in a dry form (capsules, granules, powders, tablets, wafers) must carry a bold “Choking” warning and say to take them with at least 8 ounces of fluid. The regulation cites reports of esophageal obstruction and asphyxiation, especially with too little fluid or in people with a narrowed esophagus or trouble swallowing^[16]. That rule covers drugs; supplement labels are not bound by it.
• Australia. Seven cases of esophageal obstruction linked to glucomannan tablets were reported in 1984 and 1985^[2]. Australia now has a permanent ban on glucomannan tablets, because a tablet can swell with saliva before it reaches the stomach and block the throat. Capsules and powder are not banned^[17].
• Canada. In January 2010, Health Canada warned that glucomannan tablets, capsules and powders can cause choking or blockage of the throat, esophagus or intestine if taken without at least 8 ounces of water or other fluid. It advised against taking them right before bed or using them with difficulty swallowing^[18]. Its 2024 monograph says to get medical help right away for chest pain, vomiting, or trouble swallowing or breathing after a dose^[19].
• Konjac jelly candies. In 2001 the FDA warned about mini-cup jelly candies containing konjac after several choking deaths and near-deaths among children and older people; these candies do not melt in the mouth, and an import alert targets them^[21]. The EU banned konjac as an additive (E 425) in jelly confectionery, including mini-cups, in 2003, concluding that a label warning was not enough^[22].

A published case shows how it happens: a 37-year-old woman developed a blocked esophagus after taking an over-the-counter diet aid containing glucomannan; a later endoscopy showed a pre-existing narrowing of her esophagus^[20].

Other side effects in trials were mild: abdominal discomfort, diarrhea and constipation^[9], plus gas and bloating^[2]. The NIH notes that little is known about its long-term safety^[2].

Interactions with medicines

Because it forms a gel in the gut, glucomannan may change how other medicines are absorbed^[1,20]. Health Canada’s monograph says to take it a few hours before or after other medications or health products, and to ask a health professional before use with diabetes or during pregnancy or breastfeeding^[19].

If you take prescription medicine every day, talk to your doctor or pharmacist before adding any fiber supplement.

On Wegovy, Zepbound or another GLP-1: talk to your prescriber first

I found no study in people that combined glucomannan with semaglutide or tirzepatide. What the labels and regulators do say comes down to four points:

1. GLP-1 medicines already slow the stomach. The Wegovy label says it causes a delay of gastric emptying and thereby has the potential to impact the absorption of concomitantly administered oral medications^[23]. Zepbound delays gastric emptying and thereby has the potential to impact the absorption of concomitantly administered oral medications^[24]. Glucomannan can also affect drug absorption^[1,19].Dotted underline: a drug-label fact; hover over it to see the date it was checked.
2. Both can affect the bowel. Both labels list ileus, intestinal obstruction, severe constipation including fecal impaction among postmarketing reports^[23,24], and glucomannan taken with too little fluid has been linked to blockage of the throat, esophagus or intestine^[18]. More on this in constipation on GLP-1 medications.
3. Gastroparesis. Both labels say the drug is not recommended in patients with severe gastroparesis^[23,24].
4. Fluid matters. Glucomannan’s safety depends on enough water^[16,18]. On Wegovy, the label notes gastrointestinal adverse reactions leading to dehydration such as nausea, vomiting, or diarrhea^[23]; see hydration on GLP-1 medications.

None of this proves the combination is dangerous, but there is no evidence that it is safe or useful either. If you take a GLP-1 and are thinking about a glucomannan supplement, bring it up with your prescriber or pharmacist first.

And if the goal is to feel full so you can eat as little as possible, that is worth raising with a professional too: fullness is a tool for eating well, not for eating less than you need.

How it fits with other “natural GLP-1” supplements

Glucomannan often shows up in blends sold as “nature’s Ozempic” or “GLP-1 boosters”. It is not a GLP-1 and does not work like the medicines. I compare its evidence with berberine, psyllium, probiotics and other ingredients in “Natural Ozempic” and GLP-1 supplements.

FAQ

Does glucomannan work for weight loss?

In trials, the effect ranges from no significant difference to a little over 1 kg (about 3 lb) more than placebo, in short studies^[9,11]. Even the most favorable figure fell short of what its authors counted as clinically meaningful^[11].

Is glucomannan FDA approved?

No. The FDA does not approve dietary supplements before they are sold, or their structure/function claims^[3,4]. It has said glucomannan can count as dietary fiber on labels, based on cholesterol evidence^[6].

Is glucomannan safe?

In short trials, side effects were digestive and mild^[9]. The serious risk is a blocked throat or esophagus when it is taken with too little fluid or by someone who has trouble swallowing, which is why Australia banned the tablets and Canada requires warnings^[17,18].

Are shirataki noodles good for weight loss?

They are very low in calories. In one trial, swapping pasta for konjac noodles cut the energy of that meal without people making up for it afterwards, but it left them hungrier^[15]. They add volume; they do not replace protein or other nutrients.

Can I take glucomannan with Ozempic, Wegovy or Zepbound?

That is a question for your prescriber. No study has tested the combination. The Wegovy and Zepbound labels say these medicines slow stomach emptying and may affect the absorption of oral medicines^[23,24]; Ozempic contains the same active ingredient as Wegovy, semaglutide. Glucomannan can affect absorption too^[1,19].