Glucomannan
Fermentable fibers
The most viscous fiber in the plant world: it absorbs up to fifty times its weight in water.
A single food in the catalog provides it: konjac.
Effects and benefits
The thirstiest fiber in the plant world: one gram of konjac holds up to fifty grams of water. This gel fills the stomach and slows everything else; on a calorie-restricted diet, it helps weight loss, and it contributes to maintaining normal cholesterol.
Glucomannan comes from the konjac tuber, the basis of shirataki noodles and Japanese konnyaku. It swells so much that a small amount makes a considerable mass of gel: the stomach feels full, and it empties more slowly.
As part of a calorie-restricted diet, 3 g a day, in three doses of 1 g with one or two glasses of water before meals, contribute to weight loss; and 4 g a day contribute to the maintenance of normal blood cholesterol levels (Regulation 432/2012).
The caution comes from the same strength: dry, it swells very fast. As a supplement, it is always taken with plenty of water, never just before lying down, and people who have trouble swallowing should avoid it in this form. In noodles, it is already swollen and poses no such problem. In the colon, it is fermented slowly.
Sources: EU Regulation 432/2012 · EFSA, 2010