Dietary fiber is comprised of polysaccharides and lignins of plant origin that are not digested by the endogenous secretions of the human digestive tract.38 This includes water-soluble fibers (pectins, gums, mucilages, algal polysaccharides, storage poly-saccharides, and some hemicelluloses) and insoluble fibers (cellulose, lignins, and certain hemicelluloses). The term crude fiber, which appears in some nutritional literature, refers to a fraction of dietary fiber (15–25%) that is resistant to vigorous treatment with acid and alkali.39 Recognition of this terminology is important for the correct interpretation of the reported fiber content of different foods. Important sources of dietary fiber include cereal grains, vegetables, and fruits, which differ considerably in their content of soluble and insoluble fiber. A summary of fiber sources in representative low-, medium-, and high-fiber diets is presented in Table 124-2. It has been difficult to establish quantitative recommendations for dietary fiber
intake because of analytic problems in determining the many contributors to total dietary fiber, as well as uncertainty about the physiologic effects of each dietary fiber fraction. As a result, accurate dietary fiber levels have been established for only a few foods.39,40 Daily dietary fiber intake of adults in the United States (age 20–59 years) was found to be 18.9 g for men and 13.1 g for women in 1980–1981.12 The American Dietetic Association recommends a daily fiber intake of 20 to 35 g.41