Thickness can change digestion conditions
Viscosity sounds like a texture word, but in digestion it can become a mechanism. A viscous fibre system changes the thickness of the gut contents. That can influence how food particles mix, how enzymes reach starches, and how quickly digestion products move toward the absorptive surface.
This is one reason gel-forming fibres such as psyllium and beta-glucan receive attention in glucose-response discussions. The key is physical behaviour in the real matrix. A fibre that performs in water may behave differently in milk, protein, minerals, acid, sweeteners, or heat. For product teams, viscosity should be measured, not assumed from the ingredient name.