Also known as: Konjac glucomannan, Amorphophallus konjac, Konjac root fibre, KGM
Fat MetabolismStrong evidence
A soluble fibre from konjac root, and one of the only supplement ingredients anywhere with an authorised weight-loss claim. It works through sheer physical bulk, and the safety instructions matter more here than almost anywhere else in this index.
Typical dose
3.0–4.0 g per day
30-60 minutes before meals, with plenty of water
What it is
Glucomannan is a soluble fibre extracted from the root of Amorphophallus konjac, a plant grown across East Asia and used to make shirataki noodles and konjac jelly.
It is exceptionally absorbent, capable of taking up many times its own weight in water and forming a thick viscous gel.
That absorbency is both the mechanism and the hazard. Taken with insufficient water it can swell before reaching the stomach, which is why the dosing instructions are not a formality and should be followed exactly.
How it works
Glucomannan absorbs water in the stomach and expands, increasing the volume of the stomach contents and triggering stretch receptors that signal fullness.
The resulting gel also slows gastric emptying, so that fullness persists, and slows carbohydrate absorption, flattening the blood glucose rise after a meal.
Like other viscous fibres it traps bile acids and carries them out of the body, prompting the liver to make replacements from cholesterol. It is also fermented by gut bacteria, acting as a prebiotic.
What the evidence shows
The evidence was considered sufficient for a health claim on weight loss to be formally authorised, at 3 g daily in the context of an energy-restricted diet. Very few supplement ingredients clear that bar.
Controlled trials have reported modest but consistent reductions in body weight alongside calorie restriction, along with reduced hunger between meals.
There is also good evidence for effects on cholesterol and on post-meal glucose. The weight effect depends on the energy-restricted diet being there in the first place; the fibre makes the deficit easier to hold rather than creating one.
Dosing
Three grams daily is the figure the claim and the research rest on, taken as 1 g three times a day, 30 to 60 minutes before meals.
Take each dose with at least one to two large glasses of water. This is the single most important instruction on this page. Never take it dry, never take capsules without substantial fluid, and never take it lying down or immediately before bed.
Separate it from any medication by at least two hours, since viscous fibre reduces absorption. Build up gradually to reduce bloating and gas, which are common in the first week and settle.
Acts during transit; weight effects accrue over weeks
Timing
30-60 minutes before meals, with plenty of water
Competition status
Permitted — not on the WADA Prohibited List
Category
Fat Metabolism
Safety and side effects
The important warning is physical. Glucomannan expands rapidly on contact with water, and taken with too little fluid it can swell in the gullet and cause an obstruction. Always take it with plenty of water, never dry, and never lying down.
Anyone who has difficulty swallowing, or any narrowing anywhere in the digestive tract, should avoid it entirely.
Bloating and gas are common initially and settle. Because it slows carbohydrate absorption and reduces the uptake of medication taken alongside it, speak to your doctor before use if your blood sugar is being managed or you take regular medication.
Who should avoid it
Always take with plenty of water, never dry, and never lying down or immediately before bed — it can swell and cause an obstruction. Avoid entirely if you have difficulty swallowing or have been told there is any narrowing in your digestive tract. Separate from medication by at least two hours. Speak to your doctor before use if your blood sugar is being managed or you take regular medication.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
