Carbohydrates (Fig, 3,1) thus exist as a vast family of naturally occurring compounds and derivatives of these compounds. Fortunately, only a small number of them are commercially significant and used in the food industry, while a similar number are of metabolic importance. Dietary carbohydrate is a major nutrient for both man and omnivorous animals. Human adults in the Western world obtain approximately half their daily caloric requirements from dietary carbohydrates; in the developing countries, it is the major source. Of this ingested carbohydrate, some 60% is in the form of polysaccharides, mainly starch and glycogen, but the disaccharides sucrose and lactose represent 30% and 10%, respectively (I§ble...3,1). More recently, in a few Western countries, a significant intake of monosaccharide sugars (glucose and fructose) can be obtained from manufactured foods and drinks. Some oligosaccharides, such as raffinose and stachyose, are found in small amounts in various legumes. They cannot be broken down by the enzymes of the pancreas and small intestine, but they are digested by bacterial enzymes, especially in the colon.
The important dietary polysaccharides, starch and glycogen, have to be broken down into their constituent monosaccharides before they can be used for metabolic purposes. This breakdown is initially carried out by the carbohydrase a-amylase secreted by the salivary glands and the pancreas and completed by the barrier of disaccharidases in the brush border membrane of the mature enterocytes covering the villi of the small intestine (see Table 3.2 for the major intestinal glycosidases) (1).
Table 3.2 Major Glycosidases of the Mammalian Enterocyte Brush Border
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