Distinguishing Features of Dietary Fiber Compared with Functional Fiber

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Dietary Fiber consists of nondigestible food plant carbohydrates and lignin in which the plant matrix is largely intact. Specific examples are provided in Table 7-1. Nondigestible means that the material is not digested and absorbed in the human small intestine. Nondigestible plant carbohydrates in foods are usually a mixture of polysaccharides that are integral components of the plant cell wall or intercellular structure. This definition recognizes that the three-dimensional plant matrix is responsible for some of the physicochemical properties attributed to Dietary Fiber. Fractions of plant foods are considered Dietary Fiber if the plant cells and their three-dimensional interrelationships remain largely intact. Thus, mechanical treatment would still result in intact fiber. Another distinguishing feature of Dietary Fiber sources is that they contain other macronutrients (e.g., digestible carbohydrate and protein) 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; they would be categorized as Dietary Fiber sources.

TABLE 7-1 Characteristics of Dietary Fiber

Characteristic Dietary Fiber

Nondigestible animal carbohydrate No

Carbohydrates not recovered by alcohol precipitationa Yes

Nondigestible mono- and disaccharides and polyols No

Lignin Yes

Resistant starch Some

Intact, naturally occurring food source only Yes

Resistant to human enzymes Yes

Specifies physiological effect No a Includes inulin, oligosaccharides (3-10 degrees of polymerization), fructans, polydextrose, methylcellulose, resistant maltodextrins, and other related compounds.

342 DIETARY REFERENCE INTAKES

Resistant starch that is naturally occurring and inherent in a food or created during normal processing of a food, as is the case for flaked corn cereal, would be categorized as Dietary Fiber. Examples of oligosaccharides that fall under the category of Dietary Fiber are those that are normally constituents of a Dietary Fiber source, such as raffinose, stachyose, and verbacose in legumes, and the low molecular weight fructans in foods, such as Jerusalem artichoke and onions.

Functional Fiber consists of isolated or extracted nondigestible carbohydrates that have beneficial physiological effects in humans. Functional Fibers may be isolated or extracted using chemical, enzymatic, or aqueous steps. Synthetically manufactured or naturally occurring isolated oligosaccharides and manufactured resistant starch are included in this definition. Also included are those naturally occurring polysaccharides or oligosaccharides usually extracted from their plant source that have been modified (e.g., to a shorter polymer length or to a different molecular arrangement). Although they have been inadequately studied, animal-derived carbohydrates such as connective tissue are generally regarded as nondigestible. The fact that animal-derived carbohydrates are not of plant origin forms the basis for including animal-derived, nondigestible carbohydrates in the Functional Fiber category. Isolated, manufactured, or synthetic oligosaccharides of three or more degrees of polymerization are considered to be Functional Fiber. Nondigestible monosaccharides, disaccharides, and sugar alcohols are not considered to be Functional Fibers because they fall under "sugars" or "sugar alcohols" on the food label. Also, rapidly changing lumenal fluid balance resulting from large amounts of nondigestible mono- and disaccharides or low molecular weight oligosaccharides, such as that which occurs when sugar alcohols are consumed, is not considered a mechanism of laxation for Functional Fibers.

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