Digestion and absorption of carbohydrates

Disaccharides (section 4.2.1.3) are formed by condensation between two monosaccharides to form a glycoside bond. The reverse reaction, cleavage of the glycoside bond to release the individual monosaccharides, is a hydrolysis. 

Oligosaccharides consist of three or four monosaccharide units (trisaccharides and tetrasaccharides), and occasionally more, linked by glycoside bonds. Nutritionally, they are not particularly important, and indeed they are generally not digested, although they may be fermented by intestinal bacteria and make a significant contribution to the production of intestinal gas. 

Nutritionally, it is useful to consider sugars (both monosaccharides and disaccharides) in two groups  

A complication in the classification of sugars as intrinsic (which are considered desirable in the diet) and extrinsic (which are considered undesirable in the diet) is that lactose (section 4.2.1.3) occurs in free solution in milk, and hence is an extrinsic sugar. However, lactose is not a cause of dental decay, and milk is an important source of calcium (section 11.15.1), protein (see Chapter 9) and vitamin (section 11.7). It is not considered desirable to reduce intakes of milk, which is the only significant source of lactose, and extrinsic sugars are further subdivided into milk sugar and nonmilk extrinsic sugars. 

The classes of monosaccharides are named by the number of carbon atoms in the ring, using the Greek names for the numbers, with the ending -ose to show that they are sugars. The names of all sugars end in -ose . 

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Digestion and absorption in the small intestine. Most digestion and absorption of carbohydrates, proteins, and lipids occurs in the small intestine. A summary of the digestive enzymes involved in these processes is found in Table 18.3. 

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