Final answer:
Fiber, specifically cellulose, cannot be digested by humans as the enzyme cellulase is absent. However, fiber is included in the diet to increase food bulk and aid in gastrointestinal mobility.
Step-by-step explanation:
Fiber, specifically cellulose, cannot be digested by human beings as the enzyme cellulase (ß-glycosidase) is absent in our digestive system. However, fiber is included in the diet because it increases the bulk of the food and helps with the mobility of the food through the gastrointestinal tract.