Disaccharides

A disaccharide consists of two monosaccharide units linked together, such as the disaccharide sucrose which is a plant energy reserve. Monosaccharides and disaccharides are known collectively as soluble sugars.

Sucrose (glucose + fructose) is present in high concentrations in sugar cane and sugar beets and in much lower concentrations in fruits, vegetables, seeds, and nuts (Matthews et al., 1987). Adults have no problem in digesting sucrose, but very young baby pigs show little ability to use dietary sucrose or fructose (Becker et al., 1954a, 1954b) unless gradually adapted to them (Manners and Stevens, 1972). Although apparently not studied in nonhuman primates, that finding suggests caution in the selection of carbohydrates for use in primate milk-replacers.

Lactose (glucose + galactose) is present in most mammalian milks. Some adult humans exhibit lactose intolerance associated with limited intestinal lactase activity; intolerance to lactose also has been reported in captive macaques (Hart et al., 1980; Streett and Jonas, 1980).

Maltose (glucose + glucose) is seldom present free in foods but is an intermediate formed during the digestion of starch to glucose.

Oligosaccharides

An oligosaccharide is a polymer of three or more monosaccharide units. Some are intermediates in the synthesis or degradation of polysaccharides. Oligosaccharides include raffinose (a trisaccharide: fructose + glucose + galactose), stachyose (a tetrasaccharide: fructose + glucose + two galactose molecules), and verbascose (a pentasaccharide: fructose + glucose + three galactose molecules) (Taiz and Zeiger, 1998). Raffinose and stachyose have been found, and their concentrations determined, in some grains, leguminous seeds, nuts, and vegetables (Matthews et al., 1987).

Polysaccharides

Polysaccharides are large, and often complex, polymers of multiple monosaccharide units. They can be divided into two categories, starch and starch-like compounds, which are the only polysaccharides directly digestible by mammals, and non-starch polysaccharides. Non-starch-polysaccharides can be further divided into two sub-categories, insoluble non-starch polysaccharides, also referred to as insoluble fiber, and soluble non-starch polysaccharides, or soluble fiber.

STARCH AND STARCH-LIKE POLYSACCHARIDES

Starch, a polymer of glucose, is a plant energy reserve and occurs in granules that consists of amylose and amylopectin in various proportions (Taiz and Zeiger, 1998). Amylose is primarily a straight-chain polymer of glucose units linked by α-1→4 glycosidic bonds. Amylopectin is a branched-chain polymer of glucose units linked by α-1→4 and α-1→6 glycosidic bonds. Starch solubility ranges from soluble to highly insoluble but tends to form a gel in water unless physical or enzymatic treatment is applied to promote dissolution (Lee et al., 1992; Van Soest, 1994). Starch digestion by endogenous mammalian enzymes involves salivary and pancreatic α-amylases and yields maltose, maltotriose, some glucose, and limit dextrin (three to five α-1,4-glucose units and one α-1,6-glucose unit). Further digestion to glucose is accomplished principally by maltase in the intestinal brushborder. Resistant starch escaping enzymatic digestion or foregut fermentation may undergo microbial fermentation in the hindgut. Starch concentrations in diets fed to captive primates are commonly higher than found in wild foods (Clutton-Brock, 1975; Hladik, 1977; McKey et al., 1981). When high-starch diets are fed, excessively rapid fermentation may lead to digestive upsets, characterized by signs of abdominal discomfort and poor stool quality. This is particularly serious when high-starch, low-fiber foods are consumed by foregut fermenting primates, and may result in death (Go öltenboth, 1976).

Glycogen is an animal energy reserve consisting only of amylopectin and is of little quantitative significance in the diets of most nonhuman primates.

Dextrins are polymers of glucose and are intermediates in the digestion of amylopectin (principally from starch).

NON-STARCH POLYSACCHARIDES

Insoluble non-starch polysaccharides do not dissolve in water, nor do they generally swell in water to form a gel. Cellulose and hemicelluloses are structural polysaccharides making up the bulk of plant cell wall and also are referred to as insoluble fiber. They are commonly included in measures of fiber, along with non-carbohydrate components of cell wall, such as the highly complex phenylpropanoid lignin and the fatty substances cutin, suberin, and waxes. Other non-carbohydrate substances variously associated with cell wall (but not usually a part of fiber) are silica, calcium carbonate, tannins, resins, volatile oils, and crystalline pigments (Esau, 1965; Taiz and Zeiger, 1998).

Cellulose is a polymer of 1,000 or more glucose molecules bound together by β-1→4 linkages that cannot be broken (digested) by endogenous mammalian enzymes. Symbiotic gastrointestinal anaerobes can release the energy of cellulose through microbial fermentation and the production of volatile fatty acids, although digestion may not be complete. The principal volatile fatty acids are acetic, propionic, and butyric acids (in descending order of usual abundance) plus small and variable amounts of isobutyric, valeric, and isovaleric acids. Much of the butyric acid (and some acetic acid) can be used directly for energy by intesti-



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