What is fiber?

Structure and Functions

Dietary fiber helps regulate the passage of food material through the gastrointestinal tract and influences the absorption of various nutrients. It represents the content of substances that cannot be broken down by human digestive enzymes or absorbed by the gastrointestinal tract. Nearly all dietary fiber content is contributed by the insoluble structural matter of plants. Cellulose is an insoluble unbranched glucose polymer that can absorb relatively large volumes of water. Hemicellulose is the name for a wide variety of polymers of five carbon sugars. Pectin is a water-soluble polymer that forms gels and binds water, cations, and bile acids. Gums and mucilages are highly branched polysaccharides that form gels and bind water and other organic material. Increased fiber intake may promote health by promoting the normal elimination of waste products of digestion, by promoting satiety, by helping control serum cholesterol, and by other mechanisms. Greatly increased fiber intake, however, may reduce the absorption of some nutrients.

Disorders and Diseases

The ingestion of too much fiber can result in the formation of an obstructing bolus in a narrowed intestinal or esophageal lumen. The purpose of a low-fiber or fiber-restricted diet is to help prevent this occurrence and to rest the gastrointestinal tract. In acute phases of ulcerative colitis, a fiber-restricted diet lessens the pain and stress of defecation by decreasing the weight and bulk of the stool and delaying intestinal transit time.

A low-fiber diet contains approximately two grams of crude fiber. Foods included are refined bread and cereal products, cooked fruits and vegetables that are low in fiber, and juices. Nuts, legumes, and whole-grain bread and cereal products are restricted. Minimal-fiber diets consist of strained fruit and vegetable juices and white potatoes without skins. Milk is limited to two cups per day, as it indirectly contributes to fecal residue even though it contains no fiber. Continued use of a low-fiber diet in refined carbohydrates, however, is believed to cause diverticular disease of the colon. Reduced bulk causes the colonic lumen to narrow.

A high-fiber diet contains increased amounts of foods containing cellulose, hemicelluloses, lignin, and pectin, and reduced amounts of refined carbohydrates. Insoluble fibers increase the volume and weight of the residue to maintain the normal size of the colonic lumen and to increase gastrointestinal mobility. Soluble fibers, such as gums and pectins, reduce the rate of intestinal absorption, altering the metabolic effects. High-fiber intake necessitates increased fluids.

Certain individuals should not be encouraged to increase the amount of fiber in their diet. Those who have had gastric
surgery, vagotomy, pyloroplasty or Roux-en-Y, and some diabetics with gastroparesis diabeticorum have less acid secretion or decreased gastrointestinal motility and may encounter bezoar formation, a compacted mass that does not pass into the intestine. The high-fiber diet has been recommended, however, in the treatment or prevention of dumping syndrome, hyperlipidemia, gallstones, diabetes, and many other diseases and disorders.

Bibliography

"Dietary Fiber." MedlinePlus, May 8, 2013.

"Digestive System." MedlinePlus, January 14, 2013.

Dudek, Susan G. Nutrition Essentials for Nursing Practice. 6th ed. Philadelphia: Lippincott, Williams and Wilkins, 2010.

Kirschmann, John D. Nutrition Almanac. 6th ed. New York: McGraw-Hill, 2007.

Nix, Staci. Williams’ Basic Nutrition and Diet Therapy. 14th ed. New York: Elsevier Health Sciences, 2013.

Sizer, Frances, and Ellie Whitney. Nutrition: Concepts and Controversies. 13th ed. New York: Cengage Learning, 2013.

Whitney, Eleanor Noss, and Sharon Rady Rolfes. Understanding Nutrition. Updated 12th ed. New York: Cengage Learning, 2011.