Insoluble fiber is important to a healthy body. For the most part, insoluble fiber passes through the large intestine mostly intact. It helps control and balance the pH in the intestines. Insoluble fiber promotes a healthy bowel system (and helps prevent constipation) as well as remove toxic waste through the colon. Insoluble fiber can be found in vegetables, fruit skins, whole-wheat products, corn bran, seeds, and nuts.