Why soluble is not specific enough
Soluble fibre is a broad label. Some soluble fibres dissolve but do not build much viscosity. Others hydrate into thick, gel-forming systems. For bile acid effects, that physical behaviour matters.
A gel can slow movement, change contact between digestive contents, and hold bile acids more effectively than a fibre that simply appears on a nutrition panel. Psyllium and oat beta-glucan are often discussed because they can form viscous systems when used correctly. The lesson for fibre communication is discipline: do not claim a mechanism from the fibre gram alone. Ask whether the ingredient creates the structure that the mechanism requires.