What Is Dietary Fiber

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Burkitt and Trowell's definition of DF, the sum of polysaccharides and lignin that are not hydrolyzed by human alimentary enzymes, has gained a wide acceptance (4,5). This physiological definition allows the inclusion of other compounds that may differ in chemical structure but have fiber-like characteristics. On the other hand, analytical chemists prefer the definition proposed by Southgate in a a

Dietary Fiber Intake

1982 (6) where DF is the sum of lignin and non-starch polysaccharides (NSP). The NSP consist of cellulose, hemicelluloses, P-glucans, pectin, gums, and mucilages. Lignin, the noncarbohydrate constituent of DF, is a complex three dimensional polymer of phenyl propane units. Resistant starches, formed by retrogradation of amylose that escapes digestion in the small intestine (7,8), are also included in the definition of DF because they behave like NSP in the gut.

Two international surveys were conducted by the AOAC International in order to fulfill two objectives: 1) to determine if a consensus could be reached on the definition of DF and associated methodologies; and 2) to consider appropriate classification of oligosaccharides that are not hydrolyzed by human alimentary enzymes (9). Results of the survey should help international harmonization of acceptable definitions of DF and its components. The survey results will also provide insight for chemists seeking to modify analytical methods and develop reference materials to meet the accepted definition.

The first survey was initiated in December 1992 and 144 professionals participated (9). A large majority of participants (70%) supported that the DF definition should reflect both chemical and physiological perspectives. The survey results indicated that 65% of people supported the current DF definition as polysaccharides and lignin that are not hydrolyzed by human alimentary enzymes. However, 59% supported the future expansion of DF definition to include oligo-saccharides that are not hydrolyzed by human alimentary enzymes.

In December 1993, a follow-up survey was sent out, specifically addressing the issue of a new definition that may include oligosaccharides that are not hy-drolyzed by human alimentary enzymes, along with the results from the first survey for confirmation (9). The second time, 65% of the participants supported the inclusion of these oligosaccharides. Eighty (80) % supported the inclusion of resistant starches and lignin in the DF definition beyond NSP. It is noteworthy that only 6% believe DF includes only NSP or plant cell wall components, suggesting a general agreement that the chemical entity of DF is not limited to these components.

Based on these survey results, Cho (formerly Lee) and Prosky (9) have g proposed the expansion of the definition of DF to include resistant oligosaccha-

rides, in addition to the currently included NSP, resistant starch, and lignin. Resis- t?

tant oligosaccharides, defined as oligosaccharides that are resistant to hydrolysis by human alimentary enzymes, can be used synonymously with the term ''unavailable oligosaccharides.'' This proposal was adopted at the AOAC Workshop I on Complex Carbohydrates held in Nashville, TN, USA in October, 1995. 2

The addition of resistant oligosaccharides to the DF definition may open up new avenues in the nutritional, analytical, and food technology research areas. Along this line, a common understanding on resistant oligosaccharides should be developed. Certain types of oligosaccharides may be claimed for DF in the future if the unavailability in the human upper gastrointestinal tract and the re- u

Cho et al.

suiting physiological actions similar to other DF components are fully proven. It should be emphasized that most of the foods in today's market do not contain resistant oligosaccharides, except a few formulations using resistant oligosaccharides as ingredients. Thus this new definition would not affect the DF values of most food products. Nevertheless, the inclusion of resistant oligosaccharides would allow new product development efforts as well as nutrition research to more actively evolve. Subsequently, analytical methodology should be fine tuned to fully recover these resistant oligosaccharides as well as resistant starch in the DF analysis. Until a reliable methodology is developed to meet this new definition, current DF methods should continue to be used for DF labeling and dietary assessment studies (5).

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