Dietary Reference Intakes: Proposed Definition of Dietary Fiber (2001)

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^a E-GC = enzymatic-gas chromatographic, E-C = enzymatic-colorimetric, E-HPLC enzymatic-high performance liquid chromatographic. ^b Yes, if molecular weight is 12,000 daltons or more.

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In 1987, the U.S. Food and Drug Administration (FDA) adopted AOAC method 985.29 for regulatory purposes to identify dietary fiber as a mixture of nonstarch polysaccharides, lignin, and some resistant starch (USFDA, 1987) ( Table 1). Related methods that isolated the same components as AOAC method 985.29 were developed independently (AOAC methods 991.42, 991.43, 992.16, 993.19, 993.21, and 994.13; see Table 2) and accepted by AOAC in subsequent years. These methods are also accepted by FDA. The 1976 Trowell definition was the basis for FDA accepting the AOAC methods for isolating dietary fiber. These methods exclude all oligosaccharides (3 to 9 degrees of polymerization) from the definition and include all polysaccharides, lignin, and some of the resistant starch that is resistant to the enzymes (protease, amylase, and amyloglucosidase) used in the AOAC methods. However, FDA did not and still does not have a written definition of dietary fiber for the purposes of food labeling and health claims.

Similar to the United States, there is no official definition of dietary fiber in Japan. A standard method for measuring dietary fiber in Japan is based on AOAC method 985.29 plus a chromatographic method that isolates low molecular weight maltodextrins (Gordon and Ohkuma, in press) ( Table 1). Dietary fibers can also be approved in Japan as effective ingredients in foods for specific health use; these include indigestible maltodextrin, hydrolyzed guar gum, chitosan, polydextrose, psyllium, wheat bran, and depolymerized sodium alginate (DeVries, 2001). For many Asian countries, dietary fiber intake tables have been based on AOAC methods 985.29 and 991.43, although the definition used by China since 1995 does not identify a specific method (Jian-xian, 1995) ( Table 1).

The Expert Panel on Dietary Fiber of the Life Sciences Research Office (LSRO) proposed a definition of dietary fiber in 1987 similar to the one identified by Health and Welfare Canada in 1985. This definition included nonstarch polysaccharides and lignin and excluded fiber-associated substances found in the plant cell wall such as phytates, cutins, saponins, lectins, proteins, waxes, silicon, and other inorganic components (LSRO, 1987). Other substances not considered to be dietary fiber according to the LSRO definition include indigestible compounds formed during cooking or processing (e.g., resistant starch, Maillard reaction products), oligosaccharides and carbohydrate polymers of less than 50 to 60 degrees of polymerization that are not recovered in dietary fiber analysis, nonplant-derived compounds (e.g., chitin, chitosan), and synthetic carbohydrate polymers.

In 1988, Health Canada published guidelines for novel fiber sources and food products containing them that can be labeled as a source of fiber in addiion to those included in their 1985 definition (Health Canada, 1988) ( Table 1). The rationale for these guidelines was that there were safety issues unique to novel sources of fiber, and if a product was represented as containing fiber, it should have the beneficial physiological effects associated with dietary fiber that

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the public expects. The guidelines indicate that both safety and efficacy of the fiber source have to be established in order for the product to be identified as a source of dietary fiber in Canada, and this has to be done through experiments using human subjects. Three measures of efficacy were identified: (1) laxation, (2) normalization of blood lipid levels, and (3) attenuation of blood glucose responses. Detailed guidelines were later produced for the clinical studies required to assess laxation effects, as this was the physiological function most often used by industry when seeking approval for a novel fiber source (Health Canada, 1997a).

In 1995, a definition for dietary fiber appeared in the Codex Alimentarius Guidelines on Nutrition Labelling (FAO/WHO, 1995) ( Table 1). The Codex allows the analytical methods AOAC 985.29 and AOAC 991.43 ( Table 2) for measurement of dietary fiber in special foods and infant formula. There have been recent attempts to revise the Codex definition; however, there has not been a consensus on the inclusion of animal and other chemically characterized substances (FAO/WHO, 2000).

Several countries in Europe published definitions for dietary fiber in the late 1980s and early 1990s, including Germany (Anonymous, 1989), Belgium (Anonymous, 1992), and Italy (Anonymous, 1993) ( Table 1). For labeling purposes, Denmark, Finland, Norway, and Sweden have defined dietary fiber as edible material that cannot be degraded by human endogenous enzymes, as measured by AOAC method 985.29 ( Table 1). The issue regarding inclusion or exclusion of inulin and fructooligosaccharides has been handled somewhat differently by these countries in the absence of European Union regulation. In Denmark and Norway, fructans have been allowed to be included as dietary fiber on the food labels since 1995 and 1998, respectively (i.e., before the approval of AOAC method 997.08). Sweden made a similar decision in 1999, specifying AOAC method 997.08. In 1998, the Food Administration of Finland recommended that inulin and oligofructose be labeled separately and not be included as dietary fiber. In 2001, however, AOAC method 997.08 was added to 985.29 for analysis of dietary fiber, implying that inulin and oligofructose can now be labeled as dietary fiber in the four Nordic countries (N-G Asp, Division of Applied Nutrition, Lund University, personal communication, February 22, 2001).

In 1998, the Committee on Medical Aspects of Food and Nutrition Policy (COMA) of the United Kingdom formally adopted the Englyst nonstarch polysaccharide method for defining dietary fiber (COMA, 1998) ( Table 1). In September 2000, the U.K. Food Standards Agency recommended AOAC methods 991.43 and 997.08 ( Table 2) to ensure consistent labeling of food products (Hignett, 2000) ( Table 1). In November 2000, the U.K. Food Standards Agency acknowledged COMA's definition of dietary fiber as nonstarch polysaccharides yet recognized that the “European rules preclude insistence on a national definition”. AOAC method 985.29 and the Englyst method (Englyst and Cummings, 1984) are cur-

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rently accepted by the European Community to measure dietary fiber but there is no clearly written definition of the material that is measured by these methods.

In May 2000, the American Association of Cereal Chemists (AACC) adopted an updated definition of dietary fiber that was developed by a committee appointed to review, and if necessary, update the original AACC definition of dietary fiber (AACC, 2000) ( Table 1). This definition is similar to the ANZFA definition. The AACC definition recognizes that the primary characteristics of dietary fiber are resistance to digestion and absorption in the small intestine and fermentation in the large intestine; the rationale for including these characteristics is that it recognizes the key physiological impacts of fiber demonstrated in the past 30 years of research (AACC, 2000).

In November 2000, the recently formed Australia New Zealand Food Authority (ANZFA) concluded that relying on a prescribed analytical method as the sole means of defining dietary fiber for regulatory purposes was unsatisfactory since analytical methods do not take into consideration the physiological impact of new food forms or food ingredients that are part of the diet (ANZFA, 2000). Thus, a definition has been proposed ( Table 1) that includes the origin, chemistry, and physiology of dietary fiber, similar to the Codex Alimentarius Guidelines on Nutrition Labelling (FAO/WHO, 1995) and the earlier New Zealand Food Regulations definition (New Zealand, 1984). Furthermore, ANZFA has endorsed the use of AOAC method 985.29 or 991.43, and AOAC methods 997.08 or 999.03, which measure fructans (e.g., inulin) ( Table 2).

In conclusion, a variety of definitions for dietary fiber have been promulgated by scientific and regulatory agencies worldwide. Some definitions specifically state a physiological definition of dietary fiber, whereas others rely on more prescribed analytical methods as the sole determinant of dietary fiber. The majority of accepted analytical methods for the measurement of dietary fiber are based on a variety of AOAC accepted methods.

Since many definitions are based on methods to analyze dietary fiber, the evolution of the methodologies to measure fiber were reviewed (see Appendix C). Nonstarch polysaccharides are recovered by all methods designed to measure all components of dietary fiber, and only those methods developed to measure a specific fiber component (e.g., resistant maltodextrins, inulin, polydextrose) do not recover nonstarch polysaccharides ( Table 2). Most methods include the non-carbohydrate lignin as a component of dietary fiber. Only the methods of Englyst and the methods developed to measure a specific type of polysaccharide exclude lignin. In addition, the methods of Englyst and of Mongeau and Brassard, which were designed to measure all fiber components, do not include resistant starch as fiber.

Dependence on ethanol precipitation as a means of recovering polysaccharides excludes polydextrose, resistant maltodextrin, and oligosaccharides, and most inulin, which are soluble in ethanol. These saccharides also are lost if ethanol is used at the beginning of an analytical procedure to remove mono- and

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disaccharides. Measurement of polysaccharides from animal sources (e.g., chitin, chitosan, or chondroitin sulfate) has not been systematically studied, but methods developed to measure total fiber do recover a portion of these types of polysaccharides.