Dietary Functional and Total Fiber

Dietary Fiber is defined in this report as nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Functional Fiber is defined

958 DIETARY REFERENCE INTAKES

as isolated, nondigestible carbohydrates that have beneficial physiological effects in humans. Total Fiber is the sum of Dietary Fiber and Functional Fiber. Fiber includes viscous forms that lower serum cholesterol concentrations (i.e., soluble fiber: oat bran, beans) and the bulking agents that improve laxation (i.e., insoluble fiber: wheat bran). The AI for Total Fiber is 38 and 25 g/day for 19- to 50-year-old men and women, respectively, based on a reduced risk of coronary heart disease for those within the highest quintiles of dietary fiber consumption (g/1,000 kcal) in several epidemiological studies and the median energy intake (Appendix Table E-1). Unlike the AI for some nutrients, this AI does not describe the median Total Fiber intake of a healthy population. Instead, it is based on health benefits associated with consuming foods that are rich in fiber. Based on CSFII data (Appendix Table E-4), the median Dietary Fiber intakes are 16.5 to 17.9 g/day for men and 12.1 to 13.3 g/day for women. Thus, it is evident that to meet the AI, most people will need to substantially increase their Total Fiber intake. Usual intakes that meet or exceed the AI can be assumed adequate, but the likelihood of inadequacy of usual intakes below the AI cannot be determined.

Fiber consumption can be increased by substituting whole grain or products with added cereal bran for more refined bakery, cereal, pasta, and rice products; by choosing whole fruits instead of fruit juices; by consuming fruits and vegetables without removing edible membranes or peels; and by eating more legumes, nuts, and seeds. For example, whole wheat bread contains three times as much Dietary Fiber as white bread, and the fiber content of a potato doubles if the peel is consumed. The soluble and insoluble fiber components of 228 U.S. foods have been published by Marlett and Cheung (1997).

Dietary fiber data are listed for a wide range of foods in the USDA Nutrient Database for Standard Reference (USDA, 2001). The dietary fiber values in the USDA database represent Total Fiber (including both dietary and functional fiber) as defined in this report. For most diets (those that have not been fortified with Functional Fiber that was isolated and added for health purposes), the contribution of Functional Fiber is minor relative to the naturally occurring Dietary Fiber. For example, the Functional Fiber content for foods such as fat-free yogurts and ice creams that contain added guar gums and carrageenan is so low that the USDA database generally indicates zero dietary fiber for these foods. Although the AI is set for Total Fiber, this AI is generally based upon the fibers present in foods, and until these terms are further incorporated into nutrient databases, it is appropriate to apply the Dietary Fiber data from the USDA database to the AI for Total Fiber.

Because there is insufficient evidence of deleterious effects of high Dietary Fiber as part of an overall healthy diet, a Tolerable Upper Intake Level has not been established.

APPLICATIONS OF DRIs FOR MACRONUTRIENTS 959

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