Those plant-derived carbohydrates that are unavailable nutrients.
Including: unavailable polysaccharides, unavailable oligosaccharides, and lignin.
A distinction can be made by:
- solubility, between soluble fibers and insoluble fibers, replaced by the following
- viscosity, between viscous fibers and non-viscous fibers
- fermentability, between fermentable fibers (including prebiotic fibers) and non-fermentable ones
Their roles depend ont their type: viscous fibers delay digestion and increase satiety while fermentable fibers are energy-yielding nutrients and some of them have even prebiotic activity.
I should be noted that fibers are sometimes excluded from ‘carbohydrates’, which actually restricts the definition of the latter to available carbohydrates.