Final answer:
Fiber is a type of carbohydrate that remains undigested due to the absence of necessary enzymes in the human body. It helps move food through the digestive system by stimulating peristalsis and absorbs unhealthy substances, reducing the risk of certain diseases.
Step-by-step explanation:
Fiber, which is a carbohydrate, moves through the digestive tract undigested because the enzymes required to digest cellulose, the main component of fiber, are not produced by the human body. Although fiber provides no energy and is not absorbed, it plays critical roles in maintaining our health. Insoluble fiber, like cellulose, does not dissolve in water and adds bulk to our diet, thereby facilitating bowel movements by stimulating peristalsis - the series of wave-like muscle contractions that move food through the digestive tract.
Fiber also acts like a sponge, absorbing unhealthy substances such as cholesterol and preventing their absorption into the bloodstream, lowering risks for conditions like colon cancer. Moreover, although it is not digested in the upper gastrointestinal tract, some forms of insoluble fiber can be fermented by intestinal flora in the colon, contributing to overall gut health.