Fiber is a type of carbohydrate that the body can’t digest, but it certainly can break down into useful parts. Fiber is attacked by bacteria that live in the colon, breaking it down into acids and gases through the process of fermentation. Dietary fiber is only partially fermented, because certain plant cells are able to resist bacterial attacks. The acids produced by this process of fermentation are absorbed by the body for the most part, and nourish the lining of the colon. This also provides some fuel for organ function, particularly the liver, which has important consequences for metabolism because half of the calories in fiber are made available to the body.