Glucomannan

What is glucomannan?

Glucomannan is a soluble fibre from the tuber of the konjac plant, grown and eaten across east Asia. Reviews call it a hydrocolloid: it takes up a great deal of water and turns thick, which is why konjac flour has long been used in food to thicken and to gel. As a supplement it is sold purified, as capsules and as loose powder, usually taken with water before a meal. On British shelves it is best known as a slimming fibre, though the research spreads across blood fats, blood sugar and the bowels too.

Why do people take glucomannan?

What does glucomannan do in the body?

Glucomannan is a fibre the body does not digest. What it does instead is take up water: reviews call it a hydrocolloid, the family of substances that turn liquid thick, and industry uses it for exactly that, as a thickener and a gelling agent. Swallowed with water, it makes a bulky, viscous gel, and one review describes it as sticky enough to form a covering over the lining of the gut.

Almost everything studied about it is traced back to that thickness. A slower, thicker mixture is what researchers hold responsible for the smaller rise in blood sugar after a meal, for what happens to blood fats, and for the laxative effect. It also feeds the bacteria in the gut, which reviews call a prebiotic effect and which the trial in athletes saw as a shift in the mix of species. How thick it gets can be altered on purpose: reviews describe deacetylated and broken-down forms made for different textures.

What do people say about taking glucomannan?

These are anecdotes people have shared, not evidence.

What forms of glucomannan are there?

What else is worth knowing about glucomannan?

• It swells the moment it meets liquid, and that is the whole point of it: glucomannan capsules sold in the UK carry a choking-hazard warning on the pack and say to take them with plenty of water. The trials worked the same way — one gave the capsules with 237 ml of water an hour before each meal, and a children’s trial dissolved each sachet in about 125 ml of fluid.
• The side effects the trials reported were gut ones: a review of the weight trials lists abdominal discomfort, diarrhoea and constipation, and a crossover in children recorded brief intestinal discomfort in a few. An eight-week trial that measured gut symptoms, liver enzymes and kidney markers found none of them different from placebo, and a review of the children’s constipation trials said the risks of glucomannan products need weighing and longer safety testing is still wanted.
• Trials used from half a gram a day, as two gummy sweets, up to about 4 g, usually split and taken before meals; the cholesterol pooling settled on around 3 g a day, and the children’s trials used 3 g. Labels give the amount per serving, and a serving may be several capsules.
• The people studied were mixed: adults carrying extra weight, adults and children with raised cholesterol, adults with type 2 diabetes, hospital inpatients, male athletes and children aged 6 to 17. In the trial in children carrying extra weight, blood pressure readings came out higher on glucomannan at twelve and twenty-four weeks, while their cholesterol came out lower.

If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.