It is true that most fiber “passes through,” but it also becomes food for the flora in the large intestine — our microbiome. The resulting output from the flora can be gas, which we are all familiar with, and it can also be a fermentation product like butyric acid, a short-chain fatty acid associated with beneficial effects. We are in the early stage of learning how the regular presence, or lack, of fiber-rich foods can impact health.
There are two types of dietary fiber: insoluble and soluble, with the important distinction being whether the fiber dissolves in water. The health benefits of the two differ.
Insoluble fiber is found in wheat bran, vegetables, fruits, and whole grains such as corn, rye, barley and brown rice. Insoluble fiber increases the bulk and weight of the stool, as well as the rate at which food travels through the digestive system. This makes for potential protections from certain cancers: Population studies have reported that the incidence of colon cancer decreases as insoluble fiber goes up. The thinking is that fiber can effectively dilute or even bind potential cancer-causing substances and quickly usher them out of the body. Insoluble fiber might also help lower the risk of heart disease, perhaps because it can bind the bile acids and cause more cholesterol to exit before being absorbed.
Soluble fiber is found in oat bran, rice bran, legumes (beans, lentils and peas), fruits and vegetables. Although these fibers dissolve in water, the body cannot break them down or absorb them.