Dietary fiber has been defined as the part of material in foods impervious to the degradative enzymes of the human digestive tract. The dietary fiber of plants is comprised of carbohydrate compounds including cellulose, hemicellulose, pectin, gums, mucilages, and a noncarbohydrate substance, lignin. These substances, which form the structure of plants, are present in the cell walls of all parts including the leaf, stern, root, and seed.1 Animal tissue also contains indigestible substances.
Crude fiber and dietary fiber are not the same thing. Crude fiber refers to the residue left after strong acid and base hydrolysis of plant material. This process dissolves the pectin, gums, mucilages, and most of the hemicellulose and mainly is a measure of the cellulose and lignin content. Clearly, this method tends to underestimate the total amount of fiber in the food.1 Most food composition tables give only crude fiber values.
Current interest in fiber was stimulated by the suggestion that it might help to prevent certain diseases common in the United States, namely diverticular disease, cancer of the colon, irritable bowel syndrome, obesity, and coronary heart disease.2-4 African blacks in rural areas where the fiber intake was high rarely had these diseases; however, during the past 20 years as this population moved to the cities and adopted Western habits (including a Western diet), they began to suffer from the same "Western-type" diseases.
A high-fiber diet increases fecal bulk, produces softer, more frequent stools, and decreases transit time through the intestine.5 These factors may be responsible for the supposed beneficial effects of fiber.
Evaluation of the food composition tables: Beyond the divergence of intakes
