Fiber, a form of carbohydrate that is not digestible, is a non-nutritive but essential component of a healthy diet. Fiber is not a single compound, but a mixture of several components found in complex carbohydrates: cellulose, hemicelluloses, and lignin. These substances make up the structural building materials in the plant cell wall. The other components of fiber, pectin and gums, are involved with plant cell structure and metabolism. The proportion of these fiber components varies from food to food.
Fiber is divided into two basic types: soluble and insoluble. Soluble fiber dissolves in water. Good sources include beans, fruits, vegetables, oats, and barley. Soluble fiber plays a role in lowering high serum cholesterol level and reducing the risk of heart attack by binding with cholesterol-rich bile acids in the intestinal tract. When the fiber and bile are excreted, cholesterol molecules are eliminated as well. Soluble fiber also helps to regulate the body's use of sugars, slowing their digestion and release into the bloodstream, thereby delaying the onset of hunger.
Insoluble fiber does not dissolve in water. Instead, it absorbs water and provides bulk in the diet, causing a feeling of fullness and aiding in bodily waste removal. Insoluble fiber also may play a role in reducing the risk of certain types of cancer, as well as possibly reducing the risk of type 2 diabetes. Sources of insoluble fiber include most fruits and vegetables, wheat bran, popcorn, nuts, and whole-grain flours and meals.
The Professional Chef, 7th Edition by The Culinary Institute of America