Fibers are basically sugars we cannot digest.
The most known fibers are those composing the cell-walls of plants. For this reason they are only present in plant-derived foods. However, chitin and chitosan can also be considered fibers.
Fibers, therefore, have no food value either from the caloric point of view nor from the point of view of nutrient supply. However, they perform an important action because they absorb liquids and other substances contributing to the regularity and good functioning of the intestinal system.
Fibers are also essential for keeping the intestinal flora florid and vital. This is important not only for the digestive processes, but also for the proper functioning of the immune system.