nal cells. The other volatile fatty acids are absorbed and enter metabolic pathways (Cummings, 1981; Cummings and Branch, 1986; Bourquin et al., 1992). Wheat bran is an example of a food source of cellulose.
Hemicelluloses are a heterogeneous group of single and mixed polymers of arabinose, xylose, mannose, glucose, fucose, galactose, and glucuronic acid closely associated with cellulose and lignin. Examples are xyloglucans, xylans, glucomannans, arabinoxylans, and glucuronoxylans (Taiz and Zeiger, 1998). Most hemicelluloses are water insoluble, but a few will form a viscous or gel-like solution (Gaillard, 1962). Like cellulose, hemicelluloses cannot be digested by endogenous mammalian enzymes, although they can be partially hydrolyzed in the acid stomach. Anaerobic fermentation is required for effective use of the energy that hemicelluloses contain, and the products of fermentation are essentially the same as those of cellulose. Humans and chimpanzees ferment hemicelluloses somewhat more completely than they do cellulose (Keys et al., 1970; Wiggens and Cummings, 1976; Milton and Dement, 1988).
Soluble non-starch polysaccharides do not dissolve in water completely but swell to form a gel or a gummy solution. Nevertheless, they are referred to as soluble fiber. They are nonstructural polysaccharides, some of which serve as plant energy reserves, but they are not as digestible as starch, although fermented quite completely by ruminal and intestinal bacteria (Salyers et al., 1977; Van Soest, 1994; Bourquin et al., 1996).
Included among the non-starch plant energy reserves are fructans, mannans, and galactans. Fructans (also known as fructosans, and including inulin) are polymers of fructose that are stored in grasses and composites (Smith, 1969), as well as in parts of some food crops (Ernst and Feldheim, 2000). Fructans are broken down in an acid environment (Smith, 1969), so passage through the acid stomach may result in release of some fructose monomers that can be absorbed in the small intestine (Ernst and Feldheim, 2000). Mannans are polymers of mannose found in sea weeds, algae, nuts, and seeds (Buckeridge et al., 2000; Sachslehner et al., 2000). Galactans are polymers of galactose found in sea weeds, algae, and with pectin in fruit pulps (Femenia et al., 1998).
Pectic substances are not plant energy reserves but are associated with the plant cell wall. Despite this association, their relative solubility results in their inclusion among the soluble non-starch polysaccharides along with soluble β-glucans and other gums. They are closely related to hemicelluloses, but have no covalent linkage with lignin, and occur as protopectin, pectin, and pectic acid. They are heterogeneous polysaccharides, characteristically containing galacturonic acid, rhamnose, galactose, and arabinose bound by α-1→4 linkages (Taiz and Zeiger, 1998), that cannot be digested by endogenous mammalian enzymes. Like cellulose and hemicelluloses, however, they can be degraded by fermentation, and microbial degradation of pectic substances is often quite complete (Cummings et al., 1979; Stevens et al., 1988 ).
Gumsand mucilagesare related to pectic substances, with which they share the property of swelling in water. Gums include β-glucans (soluble relatives of cellulose found in cereals, especially oats and barley), xyloglucans, and mannoglucans. Gums appear in plant exudates mainly as a result of physiologic or pathologic disturbances that induce breakdown of cell walls and cell contents. Mucilages occur in gelatinous or mucilaginous cell walls of aquatic plants and in seed coats. Sources are gum arabic, tragacanthic acid, locust bean gum, guar gum, xanthan, and tamarind. The algal polysaccharides are agar, alginates, and carrageenins. Psyllium or isopaghula is an indigestible mucilage used as a laxative by humans.
Fermentation of cellulose, hemicelluloses, and pectic substances is quantitatively important in meeting the energy requirements of herbivorous primates that have specialized pregastric (Colobinae) or postgastric (howlers) digestive compartments. Even in primates that have no specialized compartments, anaerobic fermentation of dietary carbohydrates in the colon and cecum can account for up to 28% or more of the total metabolizable energy supply, based upon natural dietary habits and analogies with simple-stomached animals, such as the pig (Parra, 1978). Many soluble fibers tend to ferment faster than do insoluble fibers and may be more energetically important to simple-stomached animals and hind-gut fermenters (Cork et al., 1999). Marmosets and tamarins may derive some of their nutrient and energy requirements by digesting or fermenting plant exudates, including gums, sap, and latex. Some callitrichids, notably the pigmy marmoset (Cebuella pygmaea) and Callithrix spp., have relatively large lower incisors adapted for tree-gouging and intestinal tract structure adapted for digesting the exudates released (Coimbro-Filho et al., 1980; Rylands and de Faria, 1993). Other callitrichids, such as Saguinus spp. and Leontopithecus spp., do not have specialized incisors for tree-gouging but feed on exudates opportunistically when they are available because of insect or mechanical damage to plants (Garber, 1993; Rylands, 1993).
Microbial fermentation of carbohydrates in the gastrointestinal tracts of some primates appears to support biosynthetic production of protein from recycled urea and some vitamins, such as vitamin B12 (Bauchop, 1978). On the basis of field observations by Jay (1965), it is probable that urea recycling and its associated water conservation contribute to colobines’ tolerance of climates that include an extended dry season.
A classification of common dietary carbohydrates and associated digestive enzymes or digestive processes is shown in Table 3-1.