broadened to include “remnants of edible plant cells, polysaccharides, lignin, and associated substances resistant to (hydrolysis) digestion by the alimentary enzymes of humans.” Included in dietary fiber were cellulose, hemicellulose, lignin, gums, modified celluloses, mucilages, oligosaccharides, and pectins and associated minor substances, such as waxes, cutin, and suberin.
After collaborative studies, AOAC Method 985.29 (1995) and AACC Method 32-05 (1995) were officially declared defining procedures for measuring dietary fiber. Modifications to separate total, soluble, and insoluble fiber were adopted as AOAC Method 991.43 (1995) and AACC Method 32-07 (1995). A reference standard with analytic values for those fractions is now available (Caldwell and Nelson, 1999). Total dietary fiber (TDF) (Prosky et al., 1985) is a more recent analytical method recognized as an official method of the AOAC, which has taken on an important role and is used extensively in human nutrition. Concentrations of TDF in human foods are included in the food composition tables in Chapter 12.
Despite that progress, analytic problems in defining dietary carbohydrates and fiber persist (Delcour and Eerlingen, 1996). Starch that is resistant to hydrolysis by digestive enzymes has physiologic effects in humans that make it comparable with dietary fiber. The formation, structure, and properties of enzyme-resistant starch have been reviewed (Eerlingen and Delcour, 1995), and its physiologic properties have been described (Annison and Topping, 1994). Type I resistant starch is trapped in the food matrix. For example, starch granules in cell contents can be physically separated from amylolytic digestive enzymes by an unbroken cell wall, and enzymatic digestion will proceed if the cell wall is ruptured by chewing or by food processing, such as grinding. Type II resistant starch is native granular starch that is resistant to enzymatic digestion because of its compactness and partially crystalline structure; this resistance can be overcome by gelatinization (heating in the presence of water to disrupt hydrogen bonding and destroy crystallinity). Type III resistant starch is formed during retrogradation (recrystallization), primarily of amylose, although retrogradation of amylopectin can also be involved.
The implications of the preceding paragraph for “accuracy” of the current AOAC and AACC methods depend on the intent to include or not include resistant starch in the dietary-fiber residue. Type I resistant starch generally would not be included in the residue, because type I resistance is destroyed during grinding of the sample in preparation for the analysis. Type II resistant starch would not appear in the residue, because the temperature to which it is exposed during the analysis (100°C) results in gelatinization, and it would be hydrolyzed by the added heat-stable a-amylase. Type III resistant starch consisting of retrograded amylopectin generally would not be included in the residue, because heating to 100°C would destroy most or all of the enzyme resistance. Retrograded amylose would be included in the residue because its enzyme resistance would not be destroyed until it reached a temperature of about 150°C, which is above the temperature used in the analysis.
Progress is being made in defining the physiologically functional components of dietary fiber in human foods, but few TDF determinations have been made on the foods consumed by nonhuman primates in natural ecosystems or on the complete primate foods consumed in captivity. Except for crude-fiber values required by regulatory agencies on commercial feed labels, most measurements of fiber in the foods have been expressed as neutral-detergent fiber (NDF), acid-detergent fiber (ADF), and/or acid-detergent lignin (ADL), commonly using the procedures described by Van Soest et al. (1991) with the modifications described by Robertson and Horvath (1992). Although this detergent system of analysis does not quantify soluble fibers, quantification of insoluble fibers is comparable to that of the TDF system just described (Lee et al., 1992; Popovich et al., 1997), and soluble fiber concentrations may be estimated by subtracting NDF from TDF (Baer et al., 1997).
The scheme shown in Figure 3-1 illustrates plant cell components that one would expect to find in the various analytic fractions of the commonly used sequential detergent system devised by Robertson and Van Soest (1981). NDF includes the total insoluble fiber in plant cell wall, primarily cellulose, hemicelluloses, and lignin. ADF is primarily cellulose and lignin, and the quantity of hemicelluloses may be estimated by subtracting ADF from NDF. When ADF is treated with sulfuric acid, cellulose is dissolved, leaving a residue designated acid-detergent lignin (ADL) or acid lignin (AL). Lignins are polyphenols that not only are themselves indigestible and unfermentable but interfere with the fermentability of other fractions in the cell wall by physically and chemically entrapping them (Southgate and Englyst, 1985; Cummings and Branch, 1986; Van Soest, 1994), especially lignin-bound proteins (Pichard and Van Soest, 1977). The various fiber fractions also may include tannins, waxes (such as cutin and suberin), and latexes (Van Soest, 1994; Conklin and Wrangham, 1994).
Chitin (an unbranched polymer of ß-1,4-linked N-acetyl-D-glucosamine), found in the cell walls of bacteria and fungi and in the exoskeletons of insects and crustaceans (Vonk and Western, 1984), is similar in structure and chemical behavior to cellulose and can be measured in the ADF fraction when analyzing chitin-containing foods of omnivorous or insectivorous primates (Allen, 1989). Chitin can be