FIBER, DIETARY. In 1972 British physician Hugh Trowell defined dietary fiber as "that portion of the food which is derived from cellular walls of plants which is digested very poorly by human beings" (Revue Européenne d'Etudes Cliniques et Biologiques ). Most of the current interest in dietary fiber stems from the efforts of Trowell and other researchers in the 1960s and 1970s to examine the differences in disease patterns between populations consuming diets high in refined foods (typical of developed countries) and populations consuming diets high in unrefined foods (typical of less developed or undeveloped countries). Populations with a higher intake of unrefined food, and thus a higher intake of dietary fiber, had lower risk of chronic diseases, such as heart disease, intestinal cancers, and gastrointestinal disorders, as compared to populations consuming highly refined, low-fiber diets. These observations stimulated a large number of research studies and, while the ability of fiber to prevent chronic disease is difficult to prove, the data gathered since the 1960s strongly supports the importance of dietary fiber for the health of the gastrointestinal tract and thus its importance in the general diet.
Definition of Dietary Fiber
In 2001 the U.S. National Academy of Sciences Institute of Medicine (IOM) recommended that "dietary fiber" be defined as the nondigestible carbohydrates and lignin that are intrinsic and intact in plants and that the term "added fiber" be used to characterize isolated, nondigestible carbohydrates that are added to foods or supplements. Defined as such, dietary fiber includes nonstarch polysaccharides (NSP) and oligosaccharides that cannot be digested in the small intestine by alpha amylase or any of the sugar-hydrolyzing enzymes in the gut. The most commonly consumed NSPs include cellulose, pectins, glucans, hemicelluloses, and gums. Inulin, a nondigestible carbohydrate of lower molecular weight than NSP, also is included in this definition of dietary fiber. The only noncarbohydrate component of dietary fiber is lignin; however, this is probably a minor component of most edible portions of plant foods since it is associated with tough or woody tissues. An important distinction of the IOM definition is that dietary fiber is the term applied to plant foods in which the nondigestible carbohydrates remain intact and part of the structure of the plant cells. Because of inconsistencies in the precise definition of dietary fiber within the research community, the methods to analyze the fiber fraction of foods have been controversial. Earlier food composition tables provided crude-fiber values; however, this value does not include most of the dietary fiber in foods and should not be used to estimate fiber intake. Food tables that contain data using the enzymatic total dietary fiber (TDF) method or that include analysis of NSP provide more accurate estimates of fiber intake.
Dietary fiber is characterized as soluble or insoluble depending on its extraction by one of the steps in the TDF analytical method. Originally it was believed that this characterization might predict physiological effects of different fibers; however, this has not been the case, and more detailed descriptions of chemical-physical properties are needed to understand the metabolic response to diverse sources of fiber. Some fibers that are categorized as soluble fibers, such as hydrolyzed gums or oligosaccharides, are not very effective in lowering cholesterol. On the other hand, fibers vary in their physical properties, which appears to be important in understanding how fiber can affect metabolism. Certain polysaccharides can become viscous or thick when mixed with water. Viscosity is associated with slowing gastric emptying, lowering plasma cholesterol, and reducing the increase in blood glucose due to consumption of digestible carbohydrates. NSPs that can become viscous include glucans, pectins, and gums. Nondigestible carbohydrates can be fermented by the microflora in the large intestine, and these carbohydrates are the primary substrates for their growth and metabolism. To be fermented by microorganisms, polysaccharides must have water-holding capacity (WHC) so that microbes can penetrate the fiber matrix. Those polysaccharides with relatively high WHC are degraded to a larger extent than those with low WHC. In addition to viscosity, fermentability, and WHC, dietary fiber provides bulk in the intestines, and some fibers may bind bile acids and increase their excretion. Most foods contain a mixture of different types of polysaccharides; however, certain foods are good sources of particular types of fiber. For example, oats and barley contain mixed beta-glucans, fruits provide pectins, wheat bran is high in cellulose and hemicelluloses, and dry beans are a source of viscous polysaccharides. Cellulose is the most abundant NSP in foods.
Fiber Content of Food
The table that follows provides values for some foods that are sources of fiber, including fruits, vegetables, cereals and grains, and dry beans and nuts. Based on the proposed definition from IOM, animal products contain no dietary fiber; it is derived only from plant foods. Some food products that are formulated with animal products could be a source of fiber if plant foods with dietary fiber or isolated polysaccharides are added in the preparation. The values in the table are reported as grams per 100 grams of edible portion so that it is easy to compare foods. Information is given in the conversion column so that the value can be converted to the foods as normally eaten. The table helps to illustrate how the handling and preparation of foods influence fiber content. As the data show,
|Fiber content of some foods|
|Food description||Dietary fiber||Water||Conversion information|
|g/100 g of edible portion|
|Bananas||2.4||74.3||1 medium = 118 g|
|Apples||2.7||83.9||1 medium = 138 g|
|Oranges||2.4||86.8||1 medium = 140 g|
|Orange juice||—||88.4||8 ounces = 248 g|
|Grapes||1.0||80.6||1 grape = 5 g|
|Plums, dried||7.1||32.4||1 dried plum = 8.4 g|
|Tomatoes||1.1||93.8||1 medium = 123 g|
|Broccoli, cooked||2.9||90.7||1 spear = 37 g|
|Corn, cooked||2.4||76.7||1/2 cup = 82 g|
|Snap beans, cooked||2.8||89.9||1 cup = 124 g|
|Lettuce, romaine||1.7||94.9||1/2 cup = 28 g|
|Potatoes, baked without skin||1.5||75.4||1/2 cup = 61 g|
|Potatoes, French-fried, oven-baked||3.2||57.1||10 pieces = 50 g|
|Peas, cooked||5.5||77.9||1 cup = 160 g|
|Cereals and Grains|
|Bread, whole wheat||4.3||37.1||1 slice = 25 g|
|Bread, white||2.3||36.7||1 slice = 25 g|
|Bread, rye||5.8||37.3||1 slice = 32 g|
|Rice, white, cooked||0.4||68.4||1 cup = 158 g|
|Rice, brown, cooked||1.8||73.1||1 cup = 195 g|
|Bran flakes||14.1||2.5||1 cup = 49 g|
|Corn flakes||2.8||3.2||1 cup = 28 g|
|Dry Beans and Nuts|
|Kidney beans, canned||3.5||77.9|
|Garbanzo beans, canned||4.4||69.7||1 cup = 240 g|
|Almonds, dry-roasted||11.8||2.6||1 cup = 138 g|
|Walnuts, English||6.7||4.1||1 cup chopped = 120 g|
|Peanuts, dry-roasted||8.0||1.5||1 cup = 146 g|
|SOURCE: Data obtained from the USDA Nutrient Database, release 13 at www.nal.usda.gov/fnic/foodcomp/|
changing the water content of a food will change its fiber content. Dried plums have the highest fiber content for the fruits listed but also the lowest water content, due of course to the drying process. Likewise, the fiber content of potatoes is higher when the water content is lower. Cooking per se does not generally alter the fiber content of foods unless the water content is changed in the process. The cooking of vegetables will break down the cell wall structure and hence soften tissues; however, this does not remove the polysaccharides associated with the cell wall.
Food preparation methods that remove or separate parts of the food also can alter fiber content. Raw oranges and orange juice have similar water content, but no fiber is present in the juice because the process of squeezing the fruit extracts the watery fraction and leaves behind the fiber associated with the pulp. The milling of whole grains removes fiber, which is concentrated in the outer bran layers of the cereal grain. White rice has a lower fiber content than brown rice due to milling. Likewise, breads made with whole grains that have the bran layer intact will have higher fiber content than breads made with refined grains such as white wheat flour. In some starchy products the presence of resistant starch can contribute to the amount of fiber measured by the TDF method. Starch is digested normally by alpha-amylase in the intestine; however, some starch, because of retrogradation or the structure of the starch polymers, cannot be digested and passes into the large intestine as does NSP. Bran fractions are used in food product formulations to increase the fiber content of cereal products. A breakfast cereal made with wheat bran has a much higher fiber content than one made with a whole grain or a refined grain. Consumers can make quick assessments of such differences, as most food products are labeled with nutrition facts that give the per-serving fiber content.
Fiber Intake and Health
Because fiber is nondigestible by enzymes in the mammalian small intestine, it mediates its effects on metabolism by its impact on the functioning of the gastrointestinal tract. Although fiber has not been considered a nutrient from a traditional perspective, it appears to be essential for the normal function of the small and large intestines. Fiber-containing foods take longer to masticate and involve more chewing than do highly refined foods. Because of the WHC of certain polysaccharides, their presence will increase the amount of water in, and therefore the volume of, the gut contents. The presence of viscous polysaccharides, as well as an increased volume of contents, will slow the rate of gastric emptying. The rate of gastric emptying determines the rate at which nutrients are exposed to the digestive enzymes and absorptive surface in the small intestine and hence the rate of nutrient absorption. Within the contents of the small intestine, the presence of nondigested carbohydrates will expand the bulk phase of the contents; this type of dilution affects the mixing of gut contents and the process of digestion and absorption.
The net effect of these factors is that a diet high in fiber-rich foods is likely to slow the rate at which nutrients are digested and absorbed and cause digestion and absorption to occur along a greater length of the small intestine. Extending the period in which nutrients are available for absorption might result in a prolonged feeling of fullness and improve the satiety effects of meals. Because fiber is not digested in the small intestine, it passes into the large bowel, where it can remain undigested or serve as a substrate for the microflora that are normally present. Fiber is the only dietary component that increases stool weight. Its ability to improve laxation in this manner is either direct, by providing bulk as nonfermented carbohydrates, or indirect, by allowing growth of the microflora, which contribute to stool bulk. Scientific reviews have estimated that at least eighteen to twenty grams of NSP per 2,000 kilocalories should be consumed daily for adequate stool formation. Thus the properties of fiber result in a slower transit of food through the stomach and small intestine; however, transit through the large intestine may be shorter due to a high fiber intake. The net effect of fiber is to shorten total transit time through the gastrointestinal tract since residence time in the large bowel accounts for more than 90 percent of the total transit time.
Epidemiological or population-based studies have demonstrated that diets rich in plant foods are associated with a lower risk of chronic diseases such as cardiovascular disease, certain cancers, obesity, and Type 2 diabetes. The clinical and experimental studies to examine this relationship indicate that dietary fiber is one of the components of plant foods associated with reducing this risk. The ability of fiber to be protective relates to its functions within the gastrointestinal tract. Sources of viscous polysaccharides such as glucans, pectins, and gums reduce plasma cholesterol (and specifically LDL-cholesterol) levels by reducing absorption of cholesterol and bile acids, which are made from cholesterol. Viscosity helps to blunt the increase in blood glucose after a meal, which reduces the amount of insulin needed to clear glucose from the blood. Diets high in fiber-containing foods such as whole grains, fruits, vegetables, and dry beans tend to be lower in total fat and saturated fatty acids, which is a dietary pattern associated with a lower risk of cardiovascular disease and cancer. These foods also appear to be more filling and may help to regulate short-term appetite. Although fiber has not been proven to facilitate weight loss, its potential effects on appetite may help with weight maintenance. Because of fiber's importance to the microflora in the large bowel, there is considerable interest in its ability to protect against colon cancer. However, the primary evidence for this role of fiber is from epidemiology or experimental animal studies rather than from clinical studies. When microbes metabolize fiber, short chain fatty acids (SCFA) are produced. These SCFA are used by cells in the colon and have been associated with maintaining a healthy mucosal layer in the gut. Advancing our understanding of the role that these compounds play in the health of the gut and in prevention of bowel diseases continues to be an active area of research.
In summary, dietary fiber has specific attributes that promote the normal functioning of the gastrointestinal tract. In addition, actions of fiber contribute to the ability of plant foods to lower the risk of chronic disease; however, it is difficult to isolate the effects of fiber from the overall response to a diet rich in plant foods, which provides many compounds that contribute to a lower risk of disease. As a consequence most recommendations of dietary fiber emphasize the importance of consuming foods high in fiber rather than relying on isolated fiber supplements.
Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes: Proposed Definition of Dietary Fiber. Washington D.C.: National Academies Press, 2001.
Gallaher, Daniel D., and Barbara O. Schneeman. "Dietary Fiber." In Present Knowledge in Nutrition. 8th ed., edited by Barbara A. Bowman and Robert Russell. Washington, D.C.: ILSI Press, 2001.
Schneeman, Barbara O. "Fiber, Inulin and Oligofructose: Similarities and Differences." Journal of Nutrition 129 (1999): 1424S–1427S.
Trowell, Hugh. "Dietary Fibre and Coronary Heart Disease." Revue Européenne d'Etudes Cliniques et Biologiques 17 (1972): 345–349.
Barbara O. Schneeman