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TABLE 1 Dietary Reference Intakes for Total Fibera by Life Stage Group DRI values (g/1,000 kcal) [g/day]b AIc males females Life stage group NDd 0 through 6 mo ND 7 through 12 mo ND ND 1 through 3 y 14  14  4 through 8 y 14  14  9 through 13 y 14  14  14 through 18 y 14  14  19 through 30 y 14  14  31 through 50 y 14  14  51 through 70 y 14  14  > 70 y 14  14  Pregnancy < 18 y 14  19 through 50 y 14  Lactation < 18 y 14  19 through 50 y 14  a Total Fiber i s the combination of D ietary Fiber , the edible, nondigestible carbohydrate and lignin components as they exist naturally in plant foods, and Functional Fiber, which refers to isolated, extracted, or synthetic fiber that has proven health benefits. b Values in parentheses are example of the total g/day of total fiber calculated from g/1,000 kcal multiplied by the median energy intake (kcal/1,000 kcal/day) from the Continuing Survey of Food Intakes by Individuals (CSFII 1994–1996, 1998). c AI = Adequate Intake. If sufficient scientific evidence is not available to establish an Estimated Average Requirement (EAR), and thus calculate a Recommended Dietary Allowance (RDA), an AI is usually developed. For healthy breast-fed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover the needs of all healthy individuals in the group, but a lack of data or uncertainty in the data prevents being able to specify with confidence the percentage of individuals covered by this intake. d ND = Not determined.
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PART II: FIBER 111 FIBER T he term Dietary Fiber describes the carbohydrates and lignin that are intrinsic and intact in plants and that are not digested and absorbed in the small intestine. Functional Fiber consists of isolated or purified car- bohydrates that are not digested and absorbed in the small intestine and that confer beneficial physiological effects in humans. Total Fiber is the sum of Di- etary Fiber and Functional Fiber. Fibers have different properties that result in different physiological effects, including laxation, attenuation of blood glucose levels, and normalization of serum cholesterol levels. Since data were inadequate to determine an Estimated Average Require- ment (EAR) and thus calculate a Recommended Dietary Allowance (RDA) for Total Fiber, an Adequate Intake (AI) was instead developed. The AIs for Total Fiber are based on the intake levels that have been observed to protect against coronary heart disease (CHD). The relationship of fiber intake to colon cancer is the subject of ongoing investigation and is currently unresolved. A Tolerable Upper Intake Level (UL) was not set for fiber. DRI values are listed by life stage group in Table 1. Dietary Fiber is found in most fruits, vegetables, legumes, and grains. Di- etary and Functional Fibers are not essential nutrients; therefore, inadequate in- takes do not result in biochemical or clinical symptoms of a deficiency. As part of an overall healthy diet, a high intake of Dietary Fiber will not cause adverse effects in healthy people. DEFINITIONS OF FIBER Dietary Fiber, Functional Fiber, and Total Fiber This publication defines Total Fiber as the combination of Dietary Fiber, the edible, nondigestible carbohydrate and lignin components as they exist natu- rally in plant foods, and Functional Fiber, which refers to isolated, extracted, or synthetic fiber that has proven health benefits. Nondigestible means that the material is not digested and absorbed in the human small intestine (see Box 1 for definitions). Fiber includes viscous forms that may lower serum cholesterol concentrations (e.g., oat bran, beans) and the bulking agents that improve lax- ation (e.g., wheat bran). Dietary Fiber in foods is usually a mixture of the polysaccharides that are integral components of plant cell walls or intracellular structures. Dietary Fiber
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 112 BOX 1 Definitions of Fibera • Dietary Fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants. • Functional Fiber consists of isolated nondigestible carbohydrates that have beneficial physiological effects in humans. • Total Fiber is the sum of Dietary Fiber and Functional Fiber. a In the United States, dietary fiber is defined for regulatory purposes by a number of analytical methods that are accepted by the Association of Official Ana- lytical Chemists International (AOAC). In Canada, a distinction is made between dietary fiber (defined as the endogenous components of plant material in the diet that are resistant to digestion by enzymes produced by man) and novel fibers, whose definition is similar to functional fiber. Novel fibers must be demonstrated to have beneficial effects to be considered as fiber for the purposes of labeling and claims. sources contain other macronutrients (e.g., digestible carbohydrate and pro- tein) normally found in foods. For example, cereal brans, which are obtained by grinding, are anatomical layers of the grain consisting of intact cells and substantial amounts of starch and protein. Other examples include plant nonstarch polysaccharides (e.g., cellulose, pectin, gums, and fibers in oat and wheat bran), plant carbohydrates (e.g., inulin, fructans), lignin, and some resis- tant starch. Functional Fiber may be isolated or extracted using chemical, enzymatic, or aqueous steps, such as synthetically manufactured or naturally occurring iso- lated oligosaccharides and manufactured resistant starch. In order to be classi- fied as a Functional Fiber, a substance must demonstrate a beneficial physiologi- cal effect. Potential Functional Fibers include isolated nondigestible plant (e.g., pectin and gums), animal (e.g., chitin and chitosan), or commercially produced (e.g., resistant starch, polydextrose) carbohydrates. FIBER AND THE BODY Function Different fibers have different properties and thus varying functions. They aid in laxation and promote satiety, which may help reduce energy intake and there- fore the risk of obesity. They can also attenuate blood glucose levels, normalize serum cholesterol levels, and reduce the risk of CHD. For example, viscous
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PART II: FIBER 113 fibers can interfere with the absorption of dietary fat and cholesterol, as well as the enterohepatic recirculation of cholesterol and bile acids, which may result in reduced blood cholesterol concentrations and a reduced risk of CHD. Absorption, Metabolism, and Excretion Once consumed, Dietary Fiber and Functional Fiber pass relatively intact into the large intestine. Along the gastrointestinal tract, the properties of different fibers result in varying physiological effects: Gastric emptying and satiety: Viscous fiber delays gastric emptying, thereby slowing the process of absorption in the small intestine. This can cause a feeling of fullness, as well as delayed digestion and absorption of nutrients, including energy. Delayed gastric emptying may also reduce postprandial blood glucose concentrations and potentially have a beneficial effect on insulin sensitivity. Fermentation: Microflora in the colon can ferment fibers to carbon dioxide, methane, hydrogen, and short-chain fatty acids. Foods rich in hemicellulose and pectin, such as fruits and vegetables, contain Dietary Fiber that is more completely fermented than foods rich in celluloses, such as cereals. The con- sumption of Dietary and certain Functional Fibers, particularly those that are poorly fermented, is known to improve fecal bulk and laxation and ameliorate constipation. Contribution of fiber to energy: When fiber is anaerobically fermented by micro-flora of the colon, the short-chain fatty acids that are produced are ab- sorbed as an energy source. Although the exact yield of energy from fiber in humans remains unclear, current data indicate that the yield is between 1.5 and 2.5 kcal/g. Physiological effects of isolated and synthetic fibers: Table 2 summarizes the beneficial physiological effects of certain isolated and synthetic fibers. Note that the discussion of these potential benefits should not be construed as endorse- ments of the fibers. For each fiber source listed, evidence relating to one of the three most commonly accepted benefits of fibers is presented: laxation, nor- malization of blood lipid levels, and attenuation of blood glucose responses. DETERMINING DRIS Determining Requirements There is no biochemical assay that can be used to measure Dietary Fiber or Functional Fiber nutritional status. Blood fiber levels cannot be measured be-
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 114 TABLE 2 The Physiological Effects of Isolated and Synthetic Fibers Potential Effect on Attenuation Normalization of Blood Other of Blood Lipid Glucose Physiological Laxation Levels Responses Effects Cellulose Increases stool No effect on blood Did not decrease — weight; may lipid levels or a postprandial decrease transit slight increase glucose response. time. in them. Chitin and There was no Numerous animal No known reports Some animal studies Chitosan evidence for a studies suggested in humans. have shown that laxative effect in that chitin and chitosan reduces fat humans. chitosan may absorption and may decrease lipid promote weight absorption. loss. However, However, this has human studies have not always been found no effect of observed in chitosan controlled human supplementation studies. More on weight. research is needed. Guar Gum Little effect on Numerous studies Viscous fibers, — fecal bulk or have shown an including guar laxation. 11–16 percent gum, produced reduction in blood significant cholesterol levels reductions in with guar gum glycemic response supplementation. in 33 of 50 studies. In addition, guar gum has been shown to decrease triacylglycerol concentrations and blood pressure.
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PART II: FIBER 115 TABLE 2 Continued Potential Effect on Attenuation Normalization of Blood Other of Blood Lipid Glucose Physiological Laxation Levels Responses Effects Inulin, A few studies Studies with inulin Some, but not all, Numerous human Oligofructose, have shown a or oligofructose studies suggest studies show that and Fructooligo- small increase have provided that inulin and the ingestion of saccharides in fecal bulk mixed results. fructooligo- fructooligo- and stool saccharides saccharides frequency with reduce fasting increases ingestion of insulin fecal Bifidbacteria. inulin or concentrations or This bacteria strain oligofructose. fasting blood has been shown to glucose. have beneficial health effects in animals, but the potential benefits to humans are not well understood. Extracted b- Oat Products In a large study of Some research — and b-Glucans glucans have adults with multiple suggests that oat minimal effects risk factors for heart bran reduces on fecal bulk. disease, including postprandial rises Oat bran high LDL cholesterol in blood glucose increases stool levels, oat cereal levels. weight by consumption was supplying rapidly linked to a dose- fermented dependent reduction viscous fiber to in LDL cholesterol. the colon for Other research also bacterial growth. suggests that oat products help lower LDL cholesterol. continued
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 116 TABLE 2 Continued Potential Effect on Attenuation Normalization of Blood Other of Blood Lipid Glucose Physiological Laxation Levels Responses Effects Pectin A meta-analysis Pectin has been Viscous fibers, — of about 100 shown to lower including pectin, studies showed cholesterol to have significantly that pectin is not varying degrees. reduced glycemic an important There was some response in 33 of fecal-bulking evidence that this 50 studies. agent. effect was due to increased excretion of bile acids and cholesterol. Polydextrose Polydextrose In one study, — — was shown to polydextrose increase fecal lowered HDL (high mass and density lipoprotein) sometimes stool cholesterol levels. frequency. Findings on the effect of polydextrose on fecal bacterial production are mixed. Psyllium There is A number of studies When added to a — extensive have shown that meal, psyllium has literature on the psyllium lowers been shown to laxative effect of total and LDL decrease the rise psyllium, which cholesterol levels of postprandial is the active via the stimulation glucose levels ingredient in of bile acid and to reduce the some over the production. glycemic index counter of foods. laxatives.
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PART II: FIBER 117 TABLE 2 Continued Potential Effect on Attenuation Normalization of Blood Other of Blood Lipid Glucose Physiological Laxation Levels Responses Effects Resistant No evidence to One study showed One animal study — Dextrins support a that resistant and two human laxative effect. maltodextrin helps studies suggest reduce blood that resistant cholesterol and maltodextrins triacylglycerol reduce fasting levels. and postprandial blood glucose levels. Resistant Increased fecal Several animal In one study, — Starch bulk due to studies have adding resistant increased starch shown that starch to bread intake has been resistant starch at various levels reported. lowers blood was shown to Because cholesterol and reduce the resistant starch triacylglycerol glycemic index in is partly levels. In humans, a dose-dependent fermented in the resistant starch manner. colon, intake does not appear may lead to an to provide the increased cholesterol- production of lowering effects of short-chain viscous fiber, but fatty acids. rather acts more like nonviscous fiber.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 118 cause fiber is not absorbed. Therefore, the potential health benefits of fiber consumption have been considered in determining DRIs. Since information was insufficient to determine an EAR and thus calculate an RDA, an AI was instead developed. The AIs for Total Fiber are based on the intake level observed to protect against CHD based on epidemiological, clini- cal, and mechanistic data. The reduction of risk of diabetes can be used as a secondary endpoint to support the recommended intake level. The relationship of fiber intake to colon cancer is the subject of ongoing investigation and is currently unresolved. Recommended intakes of Total Fiber may also help ame- liorate constipation and diverticular disease, provide fuel for colonic cells, re- duce blood glucose and lipid levels, and provide a source of nutrient-rich, low energy-dense foods that could contribute to satiety, although these benefits were not used as the basis for the AI. There is no AI for fiber for healthy infants aged 0 to 6 months who are fed human milk because human milk does not contain Dietary Fiber. During the 7- to 12-month age period, solid food intake becomes more significant, and so Dietary Fiber intake may increase. However, there are no data on Dietary Fiber intake in this age group and no theoretical reason to establish an AI. There is also no information to indicate that fiber intake as a function of energy intake differs during the life cycle. Criteria for Determining Fiber Requirements, by Life Stage Group Life stage group Criterion NDa 0 through 6 mo 7 through 12 mo ND 1 through 70 y Intake level shown to provide the greatest protection against coronary heart disease (14 g/1,000 kcal) ¥ median energy intake level from CSFII (1994-1996, 1998) (kcal/1,000 kcal/day) Pregnancy and Intake level shown to provide the greatest protection against coronary heart disease (14 g/1,000 kcal) ¥ median energy Lactation intake level from CSFII (1994–1996, 1998) (kcal/1,000 kcal/day) a Not determined.
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PART II: FIBER 119 The UL The Tolerable Upper Intake Level (UL) is the highest daily nutrient intake that is likely to pose no risk of adverse effects for almost all people. Although occa- sional adverse gastrointestinal symptoms are observed when consuming some of the isolated or synthetic fibers, serious chronic adverse effects have not been observed. A UL was not set for Dietary Fiber or Functional Fiber. Due to the bulky nature of fibers, excess consumption is likely to be self-limited. DIETARY SOURCES Dietary Fiber is found in most fruits, vegetables, legumes, and grains. Nuts, legumes, and high-fiber grains typically contain fiber concentrations of more than 3 percent Dietary Fiber, or greater than 3 g/100 g of fresh weight. Dietary Fiber is present in the majority of fruits, vegetables, refined grains, and miscel- laneous foods such as ketchup, olives, and soups, at concentrations of 1 to 3 percent or 1 g/100 g to 3 g/100 g of fresh weight. Dietary Supplements This information was not provided at the time the DRI values for fiber were set. Bioavailability Fiber is not absorbed by the body. Dietary Interactions Foods or diets that are rich in fiber may alter mineral metabolism, especially when phytate is present. Most studies that assess the effect of fiber intake on mineral status have looked at calcium, magnesium, iron, or zinc (see Table 3). INADEQUATE INTAKE AND DEFICIENCY Dietary and Functional Fibers are not essential nutrients, so inadequate intakes do not result in biochemical or clinical symptoms of a deficiency. A lack of these fibers in the diet, however, can cause inadequate fecal bulk and may detract from optimal health in a variety of ways depending on other factors, such as the rest of the diet and the stage of the life cycle.
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DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 120 TABLE 3 Potential Interactions of Dietary Fiber with Other Dietary Substances Substance Potential Interaction Notes FIBER AFFECTING OTHER SUBSTANCES Calcium Decreased calcium absorption Some types of fiber have been shown to significantly when ingested with Dietary increase fecal excretion of calcium. However, most Fiber human studies have reported no effect. Magnesium Decreased magnesium Studies report no effect on magnesium balance or absorption when ingested absorption. with Dietary Fiber Iron Reduced iron absorption In one study, the addition of 12 g/day of bran to a meal when ingested with Dietary decreased iron absorption by 51–74 percent, which Fiber was not explained by the presence of phytate. Other studies suggest that the effect of bran on iron absorption is due to phytate content rather than fiber. Zinc Reduced zinc absorption Most studies also include levels of phytate that are when ingested with Dietary high enough to affect zinc absorption. Metabolic Fiber balance studies in adult males consuming 4 oat bran muffins daily show no changes in zinc balance. ADVERSE EFFECTS OF CONSUMPTION Although occasional adverse gastrointestinal symptoms were observed with the consumption of Dietary and Functional Fibers, serious chronic adverse effects have not been observed. The most potentially deleterious effects may arise from the interaction of fiber with other nutrients in the gastrointestinal tract. Addi- tionally, the composition of Dietary Fiber varies, making it difficult to link a specific fiber with a particular adverse effect, especially when phytate is also present. It has been concluded that as part of an overall healthy diet, a high intake of Dietary Fiber will not cause adverse effects in healthy people. In addi- tion, the bulky nature of fiber tends to make excess consumption self-limiting.
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PART II: FIBER 121 KEY POINTS FOR FIBER A new set of definitions for fiber has been developed for 3 Dietary Fiber, Functional Fiber, and Total Fiber. The term Dietary Fiber describes the nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Functional Fiber consists of the isolated nondigestible carbohydrates that have beneficial physiological effects in humans. Total Fiber is the sum of Dietary Fiber and Functional Fiber. Nondigestible means not digested and absorbed in the human small intestine. There is no biochemical assay that reflects Dietary Fiber or 3 Functional Fiber nutritional status. Blood fiber levels cannot be measured because fiber is not absorbed. Since data were inadequate to determine an EAR and thus 3 calculate an RDA for Total Fiber, an AI was instead developed. The AI for fiber is based on the median fiber intake level 3 observed to achieve the lowest risk of CHD. A UL was not set for Dietary Fiber or Functional Fiber. 3 Dietary Fiber is found in most fruits, vegetables, legumes, and 3 grains. Dietary and Functional Fibers are not essential nutrients, 3 therefore inadequate intakes do not result in biochemical or clinical symptoms of a deficiency. As part of an overall healthy diet, a high intake of Dietary Fiber 3 will not cause adverse effects in healthy people.