Fiber consists mainly of cellulose, commonly found in the skins of fruits and vegetables and in the coverings of cereal grains, such as wheat bran. Since it is not absorbed by the body, dietary fiber is not considered a nutrient and has little caloric value. Essentially, we have carbon coupled with water molecules. Insoluble fibers, such as cellulose, hemicellulose and lignin, are not soluble in water. Some examples of sources are wheat bran, whole grain products, and vegetables.
Insoluble fibers help with stool bulk and regulation of bowel movements.
Soluble fibers, such as gums, pectins, and mucilages, become gummy when water is added. By slowing the rate at which food passes through the digestive system, these fibers affect the absorption rate of nutrients. Some sources of soluble fiber include dried beans, oats, barley, and some fruits and vegetables. While dietary fiber may be helpful to digestion and to slowing down the rate of nutrient absorption, the issue of how to account for it when creating zone-balanced meals frequently arises. This becomes a concern mainly for people who prefer to count grams of macro-nutrients in creating meals rather than rely on the appendices to Sears' book.