Dietary fiber is the stuff that puts the crunch in carrots, the bulk in salads, the chewiness in whole-meal bread, and the thickness in stewed prunes and pea soup. Fiber in the diet can help relieve constipation and promote bowel regularity. It is also considered helpful in controlling the "irritable bowel syndrome," preventing hemorrhoids and diverticular diseases. Other medical researchers and nutritionists believe high-fiber foods may help protect against adult-onset diabetes, cardiovascular disease, and colon cancer. Fiber may also help to lower blood cholesterol levels, reducing the risk of heart attack and stroke.

WHAT IS DIETARY FIBER?--Dietary fiber is that part of fruit, nuts, vegetables, grains, and other plant foods that's not digested or only partially digested by humans. Fiber is a diverse group of chemical compounds--cellulose, hemicelluloses, mucilages, microscopic polysaccharides, pectins, gums, and lignin. All are complex carbohydrates except lignin, a very tough substance found in all plants.

Cellulose, hemicelluloses, and lignin are components of wood as well as edible plants. They are tough, fibrous, and insoluble in water. Pectins (a substance used in jellies) and gums are water-soluble and form gel-like, or viscous, textures. All of the dietary fibers are found in varying combinations and amounts in plant leaves, stems, tubers, roots, flowers, and seeds. Cellulose, the most abundant fiber, forms the basic structural material of cell walls. Cotton is almost pure cellulose, and the outer layers of cereal grains contain large amounts of cellulose.

HOW FIBER WORKS--Aside from a few exceptions (corn being one), most fiber works on a level much more microscopic than the word "roughage" implies. Fiber's effects on the digestive system begins in the mouth. The considerable chewing that may be required for foods such as salad greens and whole-grain products stimulates saliva flow. This in turn starts the stomach's digestive juices flowing. Once swallowed, fiber contributes to bulk and some swelling of stomach contents as water is absorbed. Soluble forms of fiber such as pectins and gums increase the viscosity, or thickness, of the stomach contents. These effects contribute to a feeling of fullness and also slow down the emptying of the stomach.

Once past the stomach, the insoluble fibers, by increasing the bulk and weight of the food mass, cut down the transit time through the intestines. On the other hand, the increased viscosity resulting from pectin and other soluble fibers slows down the movement. This allows more time for digested food to be absorbed by the body, but the process also has its nutritional drawbacks: Some minerals such as calcium and zinc may be bound by fiber and, as a result, don't get absorbed; and fiber can bind bile acids, which aid in the digestive process.

Cellulose and other insoluble fibers are essentially unchanged as they pass through the intestines, but the pectins and gums are fermented by bacteria in the large intestine, producing gases and some fatty acids. Cellulose and lignin provide stool bulk and water-soluble fibers help relieve constipation by adding softness to stools.

Moderation is the key in adding fiber to your diet. It's better to eat a wide variety of foods rather than just adding a single high-fiber food, such as oat bran. Eating a wide variety of foods will furnish far better nutrition, since different kinds of dietary fiber provide different benefits. To ensure thorough digestion, be sure to drink plenty of liquids when you eat fiber.

Nutrition experts also advise against using dietary fiber supplements as they contend natural sources of fiber also contain many other nutrients. However, to minimize constipation while traveling it may be advisable to add packaged fiber to the diet--not as a substitute for eating high fiber foods, but in addition.

HOW TO GET FIBER--Most authorities agree that you should get about 20 to 30 grams of dietary fiber a day. They also recommend that you not go overboard in consuming fiber as fiber can bind some minerals (calcium, zinc), preventing absorption and use by the body, and leading to deficiencies in these trace minerals.

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