Glucomannan is a natural dietary fiber extracted from the roots of the elephant yam, also known as konjac. It has been used for centuries in traditional Asian medicine and cuisine. With its exceptional water-absorbing ability, glucomannan is one of the most viscous dietary fibers known. In fact it swells 50s time larger when added to liquid.
This means that a 2000mg capsule dose swells to about the same size as a bagel, but would be substantially more dense. As in the visual below you can see a coin sat on top of a 2000mg glucomannan and water solution.
The main function of glucomannan is that it helps people stick to diets by making them feel full. It’s a relatively straightforward concept, however a lot of studies and directions for use don’t seem to use the supplement correctly.
Glucomannan also has a lot of positive functions for digestion, although most other claimed benefits of glucomannan are spurious at best.
What is Glucomannan?
Glucomannan, derived from the konjac plant, is a soluble fiber that has gained considerable attention as a weight loss supplement. With its high fiber content, glucomannan helps regulate digestion speed, blood sugar levels, and nutrient absorption. It is low in calories and contains minimal proteins and vitamins. 
Glucomannan is available in various forms, including supplements, drink mixes, and as an additive in food products like pasta and flour. It can also be found in popular food items like shirataki noodles often known as 0 calorie noodles. 
Glucomannan Weight Loss Studies
In generally the studies are overwhelmingly positive, at least when we filter for people who have used it correctly.
Some of the negative studies used glucomannan after food. Which obviously doesn’t do anything.
There were other reports, particularly in the case of very obese participants where the trial subjects simply ate through the pain, despite reporting that it did make overeating more difficult.
A few examples of positive glucomannan studies include:
A trial of 78 men, with a BMI over 28 (obesity is classified as 30). The trial was placebo controlled and the men were given either 4g of flour or 4g of glucomannan to take twice daily with water 30 minutes before breakfast and dinner. Both groups were told to continue to diet as they pleased. The none glucomannan group presented no difference at the end of the trial and the glucomannan group dropped 1.4 BMI points over the course of 5 weeks. Their gut flora were also healthier. 
The largest study of it's type was conducted on 176 men and women who were dieting. Three 3 different dosages of glucomannan were used. Spefically, pure glucomannan (group a), glucomannan and guar gum (group b) and glucomannan, guar gum and alginat (group c). All 3 groups reported increased weight loss agains the placebo, with group a, or standard glucomannan performing the best.
The results were
Group A - 10lb Glucomannan / 6lb Placebo
Group B - 8lb Glucomannan mix / 5.8lb Placebo
Group C - 8.5lb Glucomannan mix / 4.5lb Placebo
As part of this study the group was also asked to take the supplement before meals and with water. 
How to use glucomannan for weight loss
As you’d expect the most best way to use glucomannan for weight loss is to take it about 20-40 minutes before food preparation. This generally results in people producing smaller meals, consuming less calories and still feeling full.
There are two other effective options. The first is using shirataki noodles or konjac root rice. Replacing a high carb, high calorie and thus high glycemic index part of their meal. Again this is generally effective, although the anecdotal consumer reports do outweigh the direct studies in terms of evidence there is no shortage of it.
Note from the author, I have myself used glucomannan this way and recommended it to friends who have used it successfully. The main issue with konjac noodles is that they are expensive and of course they don’t actually taste as good as pasta.
The third use that is actually effective when it comes to glucomannan is moderate supplementation through the day. 1000-2000mg three times daily does seem to prevent snacking, however, it is not as effective as taking it before a meal.
Glucomannan should also be taken with water. This is often mentioned as a way to prevent a choking hazard, although generally this is not the case as it comes in a capsule form already for safety reasons. But, the glucomannan itself needs water to expand, so to function correctly you should take 300-500ml of water with the supplement to maximise it’s effects.
Other benefits of Glucomannan
There are a few other proven benefits for Glucomannan in relation to bowel health. Although ironically most of the reported glucomannan side effects are related to gastric issues.
Glucomannan Bowel Health
Glucomannan acts as a natural laxative, increasing stool bulk and improving colonic ecology. It promotes regular bowel movement and aids in relieving constipation. 
Glucomannan Prebiotic Properties
As a dietary fiber, glucomannan acts as a prebiotic, providing nourishment for beneficial gut bacteria. This can improve gut health and support a healthy digestive system. 
Stops Absorption Of Some Other Calories
Like all dietary fibers, adding them to your diet can block some fat and protein calorie absorption. 
Less well studied benefits of Glucomannan
Managing Blood Sugar
As a soluble fiber, glucomannan slows down the absorption of sugar, preventing rapid spikes in blood glucose levels. This can be particularly beneficial for individuals with type 2 diabetes or those at risk of developing the condition.
By regulating blood sugar levels, glucomannan helps maintain stable energy levels and reduces cravings for sugary foods, contributing to weight management. 
Increasing Fat Metabolism
Glucomannan has been found to stimulate the metabolism of fat cells, promoting fat burning and weight loss. It activates brown adipose tissue (BAT), which converts other fat cells into "beige fat." This beige fat is then burned for energy, aiding in weight loss.
Furthermore, glucomannan stimulates the uncoupling protein, leading to the burning of white fat tissue, the most abundant fat in the body. This process enhances fat metabolism and contributes to overall weight reduction. 
Research suggests that glucomannan may have anti-diabetic effects. It helps regulate blood sugar levels, reducing the risk of developing type 2 diabetes and aiding in the management of the condition. 
Glucomannan has been studied for its potential anti-inflammatory properties. It may help reduce inflammation in the body, which is associated with various chronic diseases.
Side Effects of Glucomannan
Glucomannan is generally considered safe when taken as directed. However, it is important to follow the recommended dosage and drink plenty of water to prevent potential choking or blockage in the throat or esophagus, although this as mentioned earlier is a low risk in capsule or noodle form.
Glucomannan Benefits Conclusion
Glucomannan, derived from the konjac plant, is a natural dietary fiber that has gained attention as an effective weight loss supplement. Its ability to reduce appetite and improve digestive health are well documented.
In addition to appetite suppression, glucomannan has shown potential benefits in managing blood sugar and increasing fat metabolism contributes to its success in aiding weight loss efforts, although these are less well backed.
While glucomannan is generally safe, hense it's common appearance in fat burner and weight loss aids it is important to follow recommended dosages and consult with a healthcare professional before starting any new supplementation regimen.
2 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6925537/
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6392846/
8 - https://pubmed.ncbi.nlm.nih.gov/10372241/
9 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9099655/
10 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9919128/