Fiber, or roughage, is not digested by humans and has 0 calories. It helps you digest food and helps you pooh easier, which is why people eat fiber when constipated. There are two kinds of Fiber, soluble and insoluble. Soluble fiber helps you digest food better because it slows down digestion. Oat Bran, barley, nuts, seeds, and beans all are good sources of soluble fiber. Wheat bran, vegetables and whole grains are good sources of insoluble fiber and your best bet when using fiber to help you pooh faster and more efficiently. Fiber should be increased in your diet slowly to avoid gas and stomach pains, unless you are really in need.
some info from www.slrhc.org