Fiber, which is found in all plant-based foods, is composed of a group of compounds that makes up the framework of plants. Although fiber cannot be digested, it is an essential nutrient for good health. The health benefits of a diet rich in fiber include lower cholesterol and a reduced risk of heart disease and certain cancers. Also referred to as roughage, fiber is made up of many compounds, mostly carbohydrates . It can be found in a variety of foods, including wheat, potatoes, and certain fruits and vegetables. Although the recommended amount of fiber is 20 to 35 grams a day, the average American consumes only 12 to 15 grams on a daily basis. Asians, on average, consume three times as much fiber as Americans do.
Types of Fiber
Complex carbohydrates, which are a major source of energy for the body, are comprised of two main classes: starch, which is digestible, and fiber, which is generally not digestible. There are also two kinds of fiber: insoluble and soluble. Insoluble fiber, found in wheat bran and some fruits and vegetables, cannot be dissolved in water. This type of fiber is made up of cellulose and hemicellulose, substances that offer rigidity to plant material (e.g., the peels and skins of fruits and vegetables, wood, stems, and the outer coverings of nuts, seeds, and grains). Insoluble fiber acts as a natural laxative, giving stool the bulk necessary to move quickly through the gastrointestinal tract. In addition to preventing constipation and hemorrhoids , insoluble fiber may also reduce the risk of colon cancer by speeding the passage of food through the digestive tract.
Soluble fiber, found in beans, oats, and some fruits and vegetables, is fiber that can be dissolved in water. This type of fiber is made up of pectins, gums, and mucilages. Marie Boyle notes that, because it reduces the level of cholesterol in the blood, soluble fiber can reduce the risks of heart and artery disease and atherosclerosis . When consumed in large amounts, soluble fiber also slows glucose absorption from the small intestine, which can be helpful in treating diabetes . Finally, a diet high in fiber may also promote weight control and reduce the risk of developing obesity .
How Much Fiber Is Necessary?
According to the American Dietetic Association, the daily goal for fiber intake is between 20 and 35 grams. However, the average intake in the United States is only 12 to 15 grams. In contrast, people in China consume as much as 77 grams of fiber per day. Children also need fiber, although in different
|SOURCE: Adapted from Edlin et al., 2002.|
|Whole-wheat bread||1 slice||1.6|
|Rye bread||1 slice||1.0|
|White bread||1 slice||0.6|
|Brown rice (cooked)||½ cup||2.4|
|White rice (cooked)||½ cup||0.1|
|Spaghetti (cooked)||½ cup||0.8|
|Kidney beans (cooked)||½ cup||5.8|
|Lima beans (cooked)||½ cup||4.9|
|Apple (with skin)||Medium||2.6|
amounts than adults. For children up to age 18, the recommended daily dose (in grams) is determined by adding five to a child's age. For example, a seven-year-old child would need 12 grams of fiber a day.
The recommended daily amount of fiber can be consumed by eating a diet high in fiber-rich fruits, vegetables, and whole grains. There are several ways to ensure one consumes enough fiber. First, it is important to read food labels. Although they do not distinguish between the two types of fiber, the labels of almost all foods will provide the amount of dietary fiber in each serving. Raw or slightly cooked vegetables will also provide an excellent source of fiber. However, overcooking vegetables may reduce the fiber content. Whole-grain cereals, whole-wheat bread, fresh or dried fruit, beans, rice, and salad are all good sources of fiber. The table presents the fiber content of various foods.
Problem with High-Fiber Diets
Including fiber in one's daily diet has definite benefits. However, although very uncommon, fiber has the potential to cause harm if taken in excess of 60 or 70 grams daily. "Since fiber carries water out of the body, taking too much can cause dehydration and intestinal discomfort or gas," (Boyle, p. 84). Large amounts of fiber require a high fluid intake. Therefore, as one increases fiber in the diet, water intake must also be increased. If one does not consume enough fluid, then one's stool could become very hard, resulting in difficult and painful elimination.
Fiber speeds the movement of foods through the digestive system. Since iron is mainly absorbed early during digestion, high amounts of fiber may limit the opportunity for the absorption of iron, calcium , and other nutrients. Finally, large amounts of fiber can also cause deficiencies of nutrients and energy by causing one to feel full before enough nutrients have been consumed. Children and elderly persons are especially vulnerable to these concerns, since they eat smaller portion sizes.
In conclusion, fiber is an important element of the diet and provides several health benefits. Eating balanced meals containing whole grain and fresh fruits and vegetables will ensure meeting the proper recommended allowances.
see also Cancer; Carbohydrates; Heart Disease; Nutrients.
Elissa M. Howard-Barr
Boyle, Marie, A. (2001). Personal Nutrition, 4th edition. Belmont, CA: Wadsworth/Thomson Learning.
Edlin, Gordon; Golanty, Eric; and McCormick Brown, Kelli, eds. (2002). Health and Wellness, 7th edition. Sudbury, MA: Jones and Bartlett.
Wardlaw, Gordon M. (2000). Contemporary Nutrition, Issues and Insights, 4th edition. Boston: McGraw Hill.
American Dietetic Association. "Health Implications of Dietary Fiber—Position of ADA." Available from <http://www.eatright.com>
Food and Drug Administration. "Why Is Fiber Important to Your Diet?"Available from <http://www.cfsan.fda.gov>
Nutrition Newsletter (1997). "China Project Monograph Named China's Best Scientific Publication." Available from <http://www.nutrition.cornell.edu/news>
Fiber is found only in foods of plant origin. It occurs in the skins, seeds, leaves and roots of fruits and vegetables, and in the germ and bran layers of grains. Pectins, lignans, cellulose, gums and mucilages are all different forms of fiber found in these foods. Because humans lack the digestive enzymes to break down fiber, it passes through the digestive tract largely unchanged.
Depending on the type, fiber may either slow down or speed up the passage of food through the digestive tract. It contributes to stool bulk and stimulates the colon walls to contract. Foods rich in soluble fiber are often recommended to help improve blood glucose and cholesterol levels, while diets containing high amounts of insoluble fiber are known to contribute to bowel regularity and the prevention of diverticular disease. Since high-fiber diets tend to be satisfying but relatively low in calories, they are often promoted for weight management.
Dietary fiber belongs to one of two types, depending on whether or not it is able to dissolve in water Fiber that dissolves in water is called soluble, while fiber that cannot be dissolved in water is known as insoluble Upon ingestion, soluble fiber dissolves in the fluids secreted by the digestive tract, forming a gel. This gel moves slowly through the digestive tract, thus slowing the rate of digestion and absorption. Diets containing large amounts of soluble fiber have been shown to stabilize blood sugar levels in people with diabetes, and have been shown to reduce blood levels of unhealthy (LDL) cholesterol. Foods high in soluble fiber include beans, lentils, oats, psyllium, citrus fruits, barley and apples. In contrast, insoluble fiber acts as roughage. It contributes to stool bulk and promotes regularity. Foods rich in insoluble fiber include wheat bran, whole grains, dried beans, nuts, seeds, and those fruits and vegetables with an edible outer skin or seeds.
In 2001, the Food and Nutrition Board of the Institute of Medicine established its first recommendations for fiber intake. The recommendations are based on the findings of numerous studies showing a reduced risk of heart disease and type 2 diabetes with a daily fiber intake of approximately 14 grams for every 1,000 calories consumed. For adults who are 50 years
|Age||Recommended Intakes (g/day)|
|Children 1<yr||Not established|
|Children 1–3 yrs||19|
|Children 4–8 yrs||25|
|Boys 9–13 yrs||31|
|Girls 9–13 yrs||26|
|Boys 14–18 yrs||38|
|Girls 14–18 yrs||26|
|Men 19–50 yrs||38|
|Women 19–50 yrs||25|
|Men 50> yrs||30|
|Women 50> yrs||21|
|Beans, lima, fresh, cooked, ½cup||6.6|
|Beans, baked, canned, plain, ½cup||6.3|
|Beans, black, cooked, ½cup||6.1|
|Beans, kidney, fresh, cooked, ½cup||5.7|
|Winter squash, cooked, 1 cup||5.7|
|Spaghetti, whole wheat, plain, 1 cup||5.6|
|Cereal, bran flake, ¾cup||5.3|
|Cereal, shredded wheat, 1 cup||5.2|
|Pear, raw, 1 med||5.1|
|Turnips, cooked, ½cup||4.8|
|Rice, brown, cooked, 1 cup||3.5|
|Apple, raw, with skin, 1 med||3.3|
|Oatmeal, plain, cooked, ½||3.0|
|Broccoli, fresh, cooked, ½cup||2.6|
|Summer squash, cooked, 1 cup||2.5|
|Carrot, fresh, cooked, ½cup||2.3|
|Potato, fresh, cooked, 1||2.3|
|Spinach, fresh, cooked, ½cup||2.2|
|Brussels sprouts, fresh, cooked, ½cup||2.0|
|Bread, whole-wheat, 1 slice||1.9|
|Tangerine, raw, 1 med||1.9|
|Cauliflower, fresh, cooked, ½cup||1.7|
|Cabbage, fresh, cooked, ½cup||1.5|
|Peach, raw, 1 med||1.5|
|Asparagus, fresh, cooked, 4 spears||1.2|
|Romaine lettuce, 1 cup||1.2|
|Peanuts, dry roasted, 1 tbsp||1.1|
|Tomato, raw, 1||1.0|
|Rice, white, cooked, 1 cup||0.6|
|Almonds, slivered, 1 tbsp||0.6|
|g = gram,||0.6|
(Illustration by GGS Information Services/Thomson Gale.)
of age and younger, the recommended fiber intake is 38 g/day for men and 25 g/day for women. For adults over 50 years of age, the recommendation is 30 g/day for men and 21 g/day for women.
On average, North Americans consume less than 50% of the dietary fiber recommended for good health.
- A high fiber intake promotes bowel health by preventing constipation and diverticular disease.
- High-fiber diets may assist with weight management because they tend to be satisfying without being calorie-dense.
- Soluble fiber has been shown to help lower blood cholesterol by binding to cholesterol molecules in the digestive tract, thus encouraging their elimination from the body.
- High consumption of fiber-rich whole grains is associated with a lower risk of developing type 2 diabetes.
- Soluble fiber slows the emptying of food from the stomach to the small intestine, thus causing a gradual release of glucose into the bloodstream after a meal. For this reason, a diet high in soluble fiber may promote better blood sugar management in those with diabetes.
- A low-fat, high-fiber diet combined with daily exercise appears to be associated with a reduced risk of developing breast cancer.
- Studies investigating whether a high-fiber diet is protective against colon cancer are inconclusive. Those that support the protective effect of fiber suggest that fiber encourages the movement of food waste through the bowel, possibly reducing the body’s exposure to carcinogens in the waste products.
- High-fiber foods tend to be rich in phytochemicals that have been linked to cancer protection.
- High-fiber diets that are comprised of large amounts of fruits, vegetables and whole grains are associated with better blood pressure control.
Fiber supplements such as psyllium may reduce the absorption of certain medications when taken at the same time. In general, medications should be taken at least one hour before or two hours after fiber supplements.
Saturday Evening Post278 (March-April 2006):74-79.
Ward, Elizabeth. “The Incredible Bulk: Fiber is a Nutritional Scouring Pad. Here are 31 Ways to Keep it From Tasting Like One.” Men’s Health18(June 2003):102-103.
American Dietetic Association.120 South Riverside Plaza,Suite 2000, Chicago, IL 60606-6995. (800) 877-1600.<http://www.eatright.org>
American Heart Association. 7272 Greenville Avenue,Dallas, TX 75231. (800) 242-8721. <http://www.americanheart.org>
Jackson Gastroenterology. High Fiber Diet <http://www.gicare.com/pated/edtgs01.htm>
Linus Pauling Institute. Micronutrient Information Center(Fiber) <http://lpi.oregonstate.edu/infocenter/phyto chemicals/fiber/>
Mayo Clinic. Dietary Fiber: An Essential Part of a Healthy Diet <http://www.mayoclinic.com/health/fiber/NU00033>
Medline Plus. Dietary Fiber <http://www.nlm.nih.gov/medlineplus/dietaryfiber.html>
Marie Fortin, M.Ed., RD
Denis Burkitt (1911–1993), a British surgeon and medical researcher, is usually credited with popularizing the idea that dietary fiber may protect against the development of chronic diseases such as diabetes, hypercholesterolemia, heart disease, diverticular disease, and colon cancer that are prevalent in Western countries. Writing in the 1970s and 1980s, Burkitt described the relationship between large stools, which reflect a high intake of plant foods rich in dietary fiber, and a lack of "Western diseases," as he called them.
Dietary fiber is plant cell material that resists digestion by the endogenous enzymes of humans. It is not really an accurate term, as many of its components are not fibrous. Gums and mucilages, for example, are classified as dietary fiber because mammalian enzymes or secretions do not digest them. Only one component of dietary fiber, cellulose, is truly fibrous; yet "dietary fiber" is the accepted term for describing the roughage in the human diet.
Dietary fiber is found only in plant products, including fruits, vegetables, nuts, and grains. The most concentrated sources of dietary fiber are the bran layers of grains, such as wheat bran. Because of their higher water content, fruits and vegetables provide less dietary fiber per gram of ingested material than grains and cereals.
Recommendations for adult dietary fiber intake generally fall in the range of 20 to 35 grams per day. For children, the general rule is to add five to a child's age to determine the number of grams of fiber to be consumed daily. Thus, a ten-year-old child should consume 15 grams of dietary fiber a day. Usual intakes of dietary fiber in the United States average only 11 grams per day, so few people get the recommended amount. Most of the popular foods Americans consume contain little dietary fiber. For example, most servings of grains, fruits, and vegetables contain 1 to 3 grams of dietary fiber. Thus, to get the recommended amounts of dietary fiber one would need to consume at least ten servings of fiber-containing foods per day. Dietary fiber content of foods is listed on the Nutrition Facts panel on food packages. Foods particularly high in dietary fiber include bran cereals, which contain up to 13 grams of dietary fiber per serving, and beans and legumes, which contain more than 5 grams of dietary fiber per serving.
Several epidemiologic studies indicate that a high intake of dietary fiber protects against most chronic diseases. This is true even when confounding variables such as fat and calorie intake are accounted for. Dietary fiber may protect against large bowel cancer by enhancing the environment of the large intestine. Dietary fiber escapes digestion in the small intestine and is fermented in the large intestine by intestinal microflora. This fermentation yields short-chain fatty acids and gases. Short-chain fatty acids, including butyrate and propionate, have interesting physiological properties. Butyrate is a preferred gut fuel for the cells in the colon. Additionally, propionate may be involved in the cholesterol-lowering effects of certain dietary fiber. Dietary fiber fermentation may also enhance the number of beneficial microflora, such as bifidobacteria and lactobacillus. However, two recent large intervention studies did not find any protection in polyp prevention, which has led to questions about whether fiber should be recommended to prevent colon cancer (Goodlad, 2001).
Dietary fiber is an accepted therapy for gastrointestinal disorders such as constipation and diarrhea, and is often consumed as bulk laxatives or high-fiber breakfast cereals. Fiber may also protect against other cancers. International comparisons show an inverse correlation between breast cancer death rates and consumption of fiber-rich foods.
Dietary fiber has also been shown to be effective in reducing serum cholesterol, and it may decrease the risk of coronary heart disease by decreasing serum lipids, lowering blood pressure, improving glucose metabolism, and aiding in weight control. Soluble fibers appear to be the most effective in lowering serum cholesterol. The U.S. Food and Drug Administration (FDA) has accepted health claims for the cholesterol-lowering ability of oat bran and psyllium fiber. A significant reduction in serum cholesterol by soluble fiber was observed in sixty-eight of the seventy-seven human studies reviewed in a meta-analysis. Often, soluble fibers also decrease low-density lipoproteins (LDL) while maintaining high-density lipoproteins (HDL). Multiple mechanisms appear to be involved in the hypocholesterolemic response, and mechanisms for lowering cholesterol may vary considerably among the various sources of dietary fiber.
Some clinical research suggests that dietary fiber may also play a role in improving blood-sugar control in diabetes. Dietary fiber, especially soluble fiber, can delay glucose absorption and reduce insulin requirements in both insulin-dependent and non-insulin-dependent diabetes mellitus. Obese persons with diabetes often respond to a high-fiber diet with weight loss and decreased insulin requirements.
The best way to get dietary fiber in the diet is to consume a wide range of grains, legumes, fruits, and vegetables. Concentrated fiber sources such as bulk laxatives, fiber supplements, and foods fortified with fiber may be useful in the prevention and treatment of bowel disorders and as lipid-lowering therapies. Fiber supplements should be taken under medical supervision, since bowel obstructions, dehydration, and other medical contra-indicators have been reported with their use.
(see also: Chronic Illness; Coronary Artery Disease; Foods and Diets; HDL Cholesterol; LDL Cholesterol; Nutrition )
Burkitt, D. P.; Walker, A. R.; and Painter, N. S. (1972). "Effect of Dietary Fiber on Stools and the Transit-Times, and its Role in the Causation of Disease." Lancet 2(7792):1408–1412.
Goodlad, R. A. (2001). "Dietary Fiber and Risk of Colorectal Cancer." GUT 48:587–589.
Schatzkin, A.; Lanza, E.; Corle, D.; Lance, P.; Cann, B.; Shike, M.; Weissfeld, J.; Burt, R.; Cooper, M. R.; Kikendall, J. W.; Cahill, J.; and the Polyp Prevention Trial Study Group (2000). "Lack of Effect of a Low-Fat, High-Fiber Diet on the Recurrence of Colorectal Adenomas." New England Journal of Medicine 342:1149–1155.
fiber, threadlike strand, usually pliable and capable of being spun into a yarn. Many different fibers are known to be usable; some 40 of these are of commercial importance, and others are of local or specialized use. Fibers may be classified as either natural or synthetic. The natural fibers may be further classed according to origin as animal, vegetable, or inorganic fibers.
Animal fibers are composed chiefly of proteins; they include silk, wool, and hair of the goat (known as mohair), llama and alpaca, vicuña, camel, horse, rabbit, beaver, hog, badger, sable, and other animals. Vegetable fibers are composed chiefly of cellulose and may be classed as short fibers, e.g., cotton and kapok; or long fibers, including flax, hemp, Manila hemp, istle, ramie, sisal hemp, and Spanish moss. The chief natural inorganic fiber is asbestos. Fibers are also derived from other inorganic substances that can be drawn into threads, e.g., metals (especially gold and silver). Artificial fibers can be produced either by the synthesis of polymers (nylon) or by the alteration of natural fibers (rayon).
Fibers are classified according to use as textile, cordage, brush, felt, filling, and plaiting fibers. The largest volume is used for textiles and cordage. The chief textile fibers used for clothing and domestic goods are cotton, wool, rayon, nylon, flax, and silk. Coarse-textured fibers (principally jute) are used for burlap, floor covering, sacks, and bagging materials. Cordage fibers include most of the long vegetable fibers and cotton. Brush fibers include istle, sisal, broomcorn, palmyra, and animal hairs. The chief felt fibers are rabbit and beaver hair. Filling fibers include horsehair, wool flock, kapok, cotton, and Spanish moss. Plaiting fibers are used for braided articles (e.g., hats, mats, and baskets) and include Manila hemp, sisal, rushes, and grasses.
Flax, hemp, and wool have been used extensively from remote times; cotton, however, became the leading commercial fiber c.1800. The demand for fibers was greatly increased by the invention of spinning and weaving machinery during the Industrial Revolution. The artificial fibers (see synthetic textile fibers) have rapidly grown in diversity and extent of use since the development of rayon in 1884.
fi·ber / ˈfībər/ (Brit. fi·bre) • n. 1. a thread or filament from which a vegetable tissue, mineral substance, or textile is formed. ∎ a substance formed of such threads or filaments: ordinary synthetics don't breathe as well as natural fibers high strength carbon fiber. ∎ a threadlike structure forming part of the muscular, nervous, connective, or other tissue in the human or animal body: muscle fibers fig. she wanted him with every fiber of her being. ∎ fig. strength of character: a weak person with no moral fiber. 2. dietary material containing substances such as cellulose, lignin, and pectin, which are resistant to the action of digestive enzymes: cereals high in fiber.