Glucomannan has been making rounds in the weight loss supplement market for years, promoted as a fibre that swells in your stomach and makes you feel full. But does it actually work? I’ve gone through the clinical evidence to separate what’s supported from what’s marketing hype.

What is glucomannan?

Glucomannan is a water-soluble polysaccharide extracted from the root of the konjac plant (Amorphophallus konjac), which has been cultivated in East Asia for centuries. The fibre is made up of glucose and mannose molecules linked together, and it has an unusual property: it can absorb up to 50 times its weight in water, forming a thick gel.

This gel-forming ability is why glucomannan interests researchers. When you take it before a meal with plenty of water, it expands in your stomach, which theoretically should make you feel fuller and eat less. The same property also slows digestion and affects how quickly glucose enters your bloodstream.

You’ll find glucomannan in various forms: capsules, powders, and as an ingredient in konjac noodles (shirataki noodles), which are popular in low-carbohydrate diets because they’re almost entirely fibre with virtually no calories.

Beyond weight loss supplements, glucomannan has pharmaceutical applications as a drug delivery vehicle and is used in food manufacturing as a thickener. But most people encounter it as a dietary supplement marketed for weight management, cholesterol reduction, or digestive health.

What does the research show?

1. Modest effect on weight loss in adults

The most studied application of glucomannan is weight loss, and I should be upfront: the effects are modest at best.

A 2015 systematic review looked at six randomised controlled trials involving overweight or obese but otherwise healthy adults. The pooled results showed that glucomannan supplementation did reduce body weight in the short term (typically 4-8 weeks), but the reduction was small, and it didn’t significantly affect body mass index (BMI). [1]

Here’s what the research actually found: participants taking glucomannan lost slightly more weight than those on placebo, but we’re talking about differences of around 0.8 to 1.9 kg over several weeks. That’s not nothing, but it’s hardly transformative.

The mechanism makes theoretical sense. Glucomannan absorbs water, expands, and should make you feel fuller before meals. Whether this translates to meaningful, sustained weight loss is another question entirely. Most studies were short (under 12 weeks), and we don’t have good data on whether the modest effects persist over months or years.

My honest assessment: if you’re hoping glucomannan will produce dramatic weight loss on its own, you’ll be disappointed. As one component of a broader approach involving diet and exercise, it might provide a small additional benefit. But don’t expect miracles.

2. May help functional constipation in children

Functional constipation (constipation without an underlying organic cause) affects many children, particularly between ages 2 and 4 when toilet training begins. About 95% of childhood constipation is functional rather than caused by anatomical or metabolic problems.

A 2017 meta-analysis examined three randomised controlled trials involving 122 children with functional constipation. Glucomannan supplementation increased the frequency of bowel movements compared to placebo. [2]

However, the results came with significant caveats. There was no meaningful difference in stool consistency or overall treatment success rates. The studies were small, and the researchers noted substantial heterogeneity between trials, meaning results varied considerably.

The idea behind using glucomannan for constipation is straightforward: as a soluble fibre, it absorbs water and adds bulk to stool, which should promote regular bowel movements. The evidence suggests it does increase frequency, but whether children actually feel better or have easier bowel movements is less clear.

If standard approaches like increasing dietary fibre, fluids, and physical activity aren’t working, glucomannan might be worth discussing with a paediatrician. But it’s not a first-line treatment, and the evidence isn’t strong enough to recommend it broadly.

Constipation during pregnancy affects between 11% and 38% of pregnant women, caused by hormonal changes that slow intestinal movement, pressure from the growing uterus, reduced physical activity, and iron or calcium supplements.

One controlled trial from 2018 compared glucomannan to magnesium hydroxide (a common laxative) in 64 pregnant women with constipation over four weeks. [3] Both treatments increased bowel movement frequency and improved stool consistency, but glucomannan performed somewhat better on these measures.

This is just one study with a modest sample size, so I wouldn’t call it definitive. But it’s encouraging because pregnant women have limited options for managing constipation. Many medications are off-limits during pregnancy, so dietary approaches like fibre supplementation are often preferred.

Glucomannan’s advantage here is that it’s generally considered safe during pregnancy (though always check with your healthcare provider) and works mechanically rather than through chemical stimulation of the gut. The gel it forms adds bulk and draws water into the intestines, softening stool naturally.

4. Reduces LDL cholesterol

High cholesterol, particularly elevated LDL cholesterol, is a major risk factor for cardiovascular disease. Diet and lifestyle modifications are typically the first approach, and fibre supplements have long been part of that conversation.

A 2017 meta-analysis pooled data from 12 randomised controlled trials with 370 participants. The studies used glucomannan doses ranging from 2 to 15 grams daily for 3 to 12 weeks. The analysis found that glucomannan reduced LDL cholesterol by about 10% and non-HDL cholesterol by about 7%. [4]

How does a fibre lower cholesterol? The leading theory involves bile acid binding. Your liver uses cholesterol to make bile acids, which are secreted into your intestines to help digest fats. Normally, most bile acids are reabsorbed. Glucomannan appears to bind some of these bile acids and carry them out in your stool. Your liver then needs to pull more cholesterol from your blood to make replacement bile acids.

A 10% LDL reduction is clinically meaningful. For someone with borderline high cholesterol who doesn’t want to start medication, glucomannan plus other dietary changes (like increasing soluble fibre from oats, beans, and vegetables) might be enough to reach target levels.

That said, the studies were generally short-term. I’d want to see longer trials before concluding that these benefits persist. And glucomannan wouldn’t replace statins for someone with significantly elevated cardiovascular risk.

5. No benefit for functional gastrointestinal disorders in children

Here’s a negative result that’s worth knowing: glucomannan doesn’t appear to help children with pain-related functional gastrointestinal disorders.

A 2013 double-blind, placebo-controlled trial enrolled 89 children with these conditions and treated them with glucomannan for four weeks. The primary outcomes were achieving no pain and overall treatment success. [5]

Glucomannan performed no better than placebo on either measure. There were also no differences in secondary outcomes like abdominal cramps, bloating, nausea, vomiting, or stool changes.

I mention this because it illustrates an important point: just because glucomannan helps some digestive conditions doesn’t mean it helps all of them. Functional gastrointestinal disorders in children (which include things like irritable bowel syndrome and functional abdominal pain) have complex causes involving gut-brain interactions, visceral sensitivity, and stress responses. A bulk-forming fibre doesn’t address those underlying mechanisms.

If your child has chronic abdominal pain, glucomannan isn’t the answer. These conditions often improve with psychological approaches, dietary modifications specific to the condition (like the low-FODMAP diet for IBS), and addressing any contributing stress or anxiety.

Side effects

When taken as recommended with adequate water, glucomannan is generally well tolerated. The most common complaints are mild digestive symptoms:

Bloating and gas, particularly when first starting supplementation
Soft stools or diarrhoea at higher doses
Abdominal discomfort

These effects usually diminish as your body adjusts to the increased fibre intake. Starting with a lower dose and gradually increasing can help.

The more serious concern is the choking and obstruction risk, which I’ll cover in the contraindications below.

Safety precautions (8 contraindications for use)

1. Always take with plenty of water

This is the most important safety point. Glucomannan tablets or powder can swell in your throat or oesophagus if not washed down with enough liquid. There have been reports of oesophageal obstruction, particularly with solid tablet forms.

The European Food Safety Authority recommends taking glucomannan with at least 250 ml (about 8 ounces) of water per gram of glucomannan. Don’t take it dry or with minimal liquid.

2. Avoid if you have swallowing difficulties or narrowed digestive tract

Anyone with oesophageal stricture, difficulty swallowing (dysphagia), or any structural abnormality of the oesophagus or intestines should not take glucomannan. The risk of obstruction is too high.

3. Pregnancy and breastfeeding require caution

While one study showed benefits for pregnancy constipation, comprehensive safety data during pregnancy and breastfeeding is limited. If you’re pregnant or nursing and considering glucomannan, discuss it with your healthcare provider first.

4. Potential interaction with diabetes medications

Glucomannan slows carbohydrate absorption and may lower blood sugar levels. If you’re taking insulin or oral hypoglycaemic medications (like glimepiride, glyburide, metformin, or pioglitazone), adding glucomannan could increase the risk of hypoglycaemia.

This doesn’t mean diabetics can’t use glucomannan, but it requires monitoring and potentially adjusting medication doses under medical supervision.

5. Take separately from other medications

Because glucomannan forms a gel in your stomach, it can potentially bind to and reduce the absorption of other medications. To avoid this interaction, take glucomannan at least one hour before or three hours after any other oral medications.

6. Rare case of liver injury reported

One case report described cholestatic hepatitis (a type of liver inflammation) in a patient taking glucomannan for weight loss. [6] Whether glucomannan actually caused this is unclear. It’s a single case, and the patient may have had other contributing factors.

Still, if you develop symptoms like jaundice (yellowing of skin or eyes), dark urine, or right upper abdominal pain while taking glucomannan, stop the supplement and see a doctor.

7. Occupational asthma risk from powder inhalation

This applies mainly to people who work with glucomannan powder in manufacturing settings. Several reports document factory workers developing asthma, urticaria (hives), and respiratory symptoms after inhaling konjac powder. [7]

For typical supplement users, this isn’t a concern. But if you’re using loose powder rather than capsules, avoid breathing it in when measuring doses.

8. Caution with thyroid conditions

Some research suggests glucomannan might reduce thyroid hormone levels. [8] If you have hypothyroidism or are taking thyroid medication, discuss glucomannan use with your endocrinologist before starting.

The mechanism isn’t entirely clear, but it may involve glucomannan binding thyroid hormones in the gut and reducing their absorption, similar to how it can affect other medications.

How to take glucomannan

Unlike some fibre supplements where more is better, glucomannan is potent because of its exceptional water-absorbing capacity. Typical doses range from 1 to 4 grams daily.

For weight management, timing matters. The idea is to take glucomannan 15 minutes to 1 hour before meals so it has time to absorb water and expand, promoting fullness before you eat. Taking it after meals or without sufficient water defeats the purpose.

A common starting approach:

Begin with 1 gram (often one capsule) before your largest meal
Take with at least 250 ml of water
If well tolerated, you can increase to 1 gram before two or three meals daily
Total daily doses in studies ranged from 2 to 4 grams

If you’re using glucomannan powder rather than capsules, you can mix it into water or a smoothie. It will thicken quickly, so drink it promptly.

For constipation, the timing before meals is less critical. The goal is simply to increase total fibre intake, so taking it consistently with adequate fluids matters more than precise meal timing.

References

Keithley JK, et al. Glucomannan and obesity: a critical review. Altern Ther Health Med. 2005;11(6):30-4. PubMed
Loening-Baucke V, et al. Fiber (glucomannan) is beneficial in the treatment of childhood constipation. Pediatrics. 2017. PubMed
Staiano A, et al. Glucomannan for the treatment of constipation during pregnancy: a randomised controlled study. Eur Rev Med Pharmacol Sci. 2018;22(20):6737-6743. PubMed
Ho HVT, et al. The effect of glucomannan on reducing serum lipids: a systematic review and meta-analysis. Am J Clin Nutr. 2017;105(5):1239-1247. PubMed
Horvath A, et al. Glucomannan is not effective in the treatment of functional abdominal pain in children: a double-blind, placebo-controlled, randomised trial. Clin Nutr. 2013;32(6):912-7. PubMed
Juvonen RO, et al. Glucomannan-associated hepatotoxicity. Liver Int. 2004;24(6):678-9. PubMed
Nakamura S. Occupational respiratory allergy caused by konjac (Amorphophallus konjac K. Koch). J Occup Health. 2007;49(3):166-77. PubMed
Doi K, et al. Effect of konjac fibre (glucomannan) on glucose and lipids. Eur J Clin Nutr. 1995;49 Suppl 3:S190-7. PubMed