The other type of fiber, which is not a prebiotic, is called insoluble. It isn't digested, dissolved, nor is it fermented in the large intestine. But it does provide a valuable service by acting as "roughage" which offers some significant health benefits.
While soluble prebiotic fibers have only received serious attention since the mid 1990s, insoluble fibers have long been recognized. In the 1960s, their importance to our digestive systems elevated from being just bulk, to a recognition as providing important stimulation to our digestive tract.
Insoluble fibers can also absorb and carry significant volumes of water as they pass through your GI tract, making them valuable for softening stool and easing the demands on your digestive system's transport mechanism.
Insoluble fibers collect carcinogens and other toxic waste, and help keep them from being absorbed into your cells and bloodstream, where they can cause trouble. By helping in the transport of food waste out of the digestive system, a number of health problems may be avoided.