Glucomannan is a natural, water-soluble dietary fiber extracted from the roots of the elephant yam, also known as konjac. It may be an effective weight-loss supplement. However, only when taken properly.

Weight loss is not always easy, and long-term success requires dedication and perseverance.

Countless supplements and diet plans are marketed as effective weight loss strategies, claiming to make things easier.

This article takes a detailed look at the science behind glucomannan and whether it’s something you should be taking.

Glucomannan is available as a supplement, in drink mixes and is also added to food products, such as pasta and flour. It’s also the main ingredient in shirataki noodles.

Glucomannan comprises 40% of the dry weight of the elephant yam, which is originally from Southeast Asia. It has a long history of use in herbal mixtures and traditional foods like tofu, noodles and konjac jelly.

In addition to being sold as a dietary supplement, it’s used as a food additive — an emulsifier and thickener denoted with the E-number E425-ii.

Glucomannan has an exceptional ability to absorb water and is one of the most viscous dietary fibers known.

It absorbs so much liquid that a small amount of glucomannan added to a glass of water turns the entire content into a gel. These unique properties are believed to mediate its effects on weight loss.


Glucomannan is a water-soluble dietary fiber, extracted from the roots of the elephant yam. It has gained considerable attention as a weight loss supplement.

Glucomannan is a water-soluble dietary fiber.

Like other soluble fibers, it’s believed to promote weight loss in several ways (1):

  • It’s very low in calories.
  • It takes up space in your stomach and promotes a feeling of fullness (satiety), reducing food intake at a subsequent meal.
  • It delays stomach emptying, contributing to increased satiety (2).
  • Like other soluble fibers, it reduces the absorption of protein and fat (3).

It also feeds the friendly bacteria in your intestine, which turn it into short-chain fatty acids like butyrate, shown to protect against fat gain in some animal studies (4, 5).

Feeding your gut bacteria may also have other benefits. Some studies have shown a correlation between altered gut bacteria and body weight (6, 7).

Glucomannan differs from most other soluble fibers, as it’s exceptionally viscous, making it particularly effective for weight loss.


Like other soluble fibers, glucomannan absorbs water in your stomach and contributes to feelings of fullness. In addition, it may promote reduced calorie intake and weight loss in other ways.

Several randomized controlled trials have studied the effects of glucomannan on weight loss. These types of studies are the gold standard of scientific research in humans.

In the largest study, 176 healthy but overweight people on a calorie-restricted diet were randomly assigned either a glucomannan supplement or a placebo (8).

Three different glucomannan supplements with varying dosages were tested. Some also contained other fibers.

These were the results after 5 weeks:

As you can see, weight loss was significantly greater among those who supplemented with glucomannan.

Several other studies agree with these results. Glucomannan caused modest weight loss in overweight and obese individuals when regularly ingested before a meal (9, 10, 11).

It’s particularly effective when combined with a weight-reducing diet.

The same applies to all weight-loss methods — they work best in combination.


When taken before meals, glucomannan may lead to modest weight loss in overweight individuals, mainly by creating a feeling of fullness and reducing calorie intake.

In addition to promoting weight loss, glucomannan may improve some heart disease risk factors.

According to a systematic review of 14 studies, glucomannan can lower (10):

  • Total cholesterol by 19 mg/dL (0.5 mmol/L).
  • “Bad” LDL cholesterol by 16 mg/dL (0.4 mmol/L).
  • Triglycerides by 11 mg/dL (0.12 mmol/L).
  • Fasting blood sugar by 7.4 mg/dL (0.4 mmol/L).

It primarily reduces blood cholesterol by decreasing the absorption of cholesterol in your gut.

According to this research, adding glucomannan to your diet could potentially lower your risk of developing heart disease and type 2 diabetes.

As a water-soluble fiber, glucomannan has also been successfully used to treat constipation (12, 13).


Glucomannan can improve several important heart disease risk factors, including total cholesterol, “bad” LDL cholesterol, triglycerides and fasting blood sugar.

For weight loss, a dosage of 1 gram, 3 times per day is considered sufficient (14).

Mixed with water, glucomannan expands and can absorb up to 50 times its weight. Therefore, the recommended dosage of glucomannan is lower compared to other fiber supplements.

Glucomannan does not have any effect on weight loss unless it is taken before a meal. Timing recommendations range from 15 minutes to 1 hour before a meal (14, 8).

Glucomannan is well tolerated and generally considered safe.

However, if glucomannan expands before reaching the stomach, it may cause choking or blockage of the throat and esophagus, the tube that moves food from your mouth to your stomach.

To prevent this, it should be washed down with 1–2 glasses of water or another liquid.

Some people may experience mild side effects, such as bloating, flatulence, soft stools or diarrhea, but these negative effects are uncommon.

Glucomannan can also reduce the absorption of oral medications like sulfonylurea, a diabetes drug. This can be avoided by taking the medication at least four hours after or one hour before ingesting glucomannan.


Glucomannan is generally considered safe. The suggested dosage is 1 gram, taken 3 times per day with water. Make sure to take it before a meal, as it has no effect on weight loss otherwise.

Judging by the evidence, glucomannan is an effective weight-loss supplement. But as with any weight-loss strategy, it doesn’t work in isolation.

The only known way to lose weight in the long term is to make a permanent change to your lifestyle.

Glucomannan may help make that easier, but it won’t work miracles on its own.