Dietary fibers are the indigestible portion of plant foods that move food through the digestive system, absorbing water and easing elimination.
There are two general categories of fiber – soluble and insoluble. Soluble fibers, which are easily digested, can be divided into three major types: pectins (found in root vegetables, cabbage, apples, whole-wheat bran, and beans); gums (which can be obtained from oatmeal, dried beans, and other legumes); and mucilages (which are synthesized by plant cells and are found in food additives).
There are also several types of insoluble fibers. One is cellulose, which can be found in cabbage, peas, apples, root vegetables, whole-wheat flour, beans, bran, and wheat. Another is hemicellulose, which is found in bran, cereals, and whole grains. Lignan, most abundantly found in flaxseed, is a phytochemical that works very much like an insoluble fiber.
Fiber is actually classified as a carbohydrate; in the U.S. the total carbohydrates listed on a food label will include dietary fiber, although the fiber is listed separately.