It occurs on the outside of many seeds, fruits, legumes and other foods. Wheat bran is an example of insoluble fiber. It is generally the chewiest part of the food in its natural form. With wheat products it's absence is the reason that white bread is generally softer than whole wheat or whole grain breads. In western cultures we often remove the natural fiber from our foods by peeling, milling and other processes which occur in commercial food processing.
Insoluble fiber is key in the treatment of constipation and other intestinal disorders.
It has also been linked to the prevention of colon cancer. Crude and Dietary fiber are terms that are used in the measurement process when determining the fiber content of certain foods. Older methods of measurement used harsh chemical processes to determine the fiber content of foods. This process destroyed much of the soluble and insoluble fiber in foods, and the resulting residue was called crude fiber. The Department of Agriculture often refers to crude fiber content in its literature.