Small intestine monosaccharide digestion

Acarbose is a nonabsorbable a-glucosidase inhibitor which blocks the digestion of starch, sucrose, and maltose. The digestion of complex carbohydrates is delayed and occurs throughout the small intestine rather than in the upper part of the jejunum. Absorption of glucose and other monosaccharides is not affected. Acarbose is adrninistered orally three times a day and chewed with the first mouthful of food. 

Source and kinds of disaccharidases The final digestive processes occur at the mucosal lining of the small intestine. Several disaccharidases [for example, lactase (p-galactosidase), sucrase, maltase, and isomal-tase] produce monosaccharides (glucose, galactose, and fructose). These enzymes are secreted by and remain associated with the luminal side of the brush border membranes of intestinal mucosal cells. Absorption of the monosaccharides requires specific trans porters. 

The intestinal oligo- and disaccharidases are fixed components of the cell membrane of the brush border region of the wall of the small intestine. These enzymes digest dietary carbohydrate to monosaccharides which are absorbed through the intestinal wall. They include sucrase, maltase, isomaltase, lactase, trehalase, and hetero-p-glucosidase. In the late 1970s, it was realized that inhibition of all or some of these activities could regulate the absorption of carbohydrate, and that these inhibitors could... 

Glycosidase inhibitors (Table 1.9) delay the digestion of dietary carbohydrate in the form of starch and sucrose into monosaccharides, which can be absorbed from the small intestine. By delaying absorption, these agents lower postprandial blood glucose and insulin levels and are used for this purpose in the treatment of type 2 diabetes. 

Glucose, galactose, and fructose formed by the digestive enzymes are transported into the absorptive epithelial cells of the small intestine by protein-mediated Na -dependent active transport and facilitative diffusion. Monosaccharides are transported from these cells into the blood and circulate to the liver and peripheral tissues, where they are taken up by facilitative transporters. Facilitative transport of glucose across epithelial cells and other cell membranes is mediated by a family of tissue-specific glucose transport proteins (GLUT I-V). The type of transporter found in each cell reflects the role of glucose metabolism in that cell. 

Suppose you have a peanut butter and jelly sandwich for lunch. Digestion of starch from the bread begins as you chew and continues in your stomach. It is completed in the small intestine, where sugars from the jelly are also converted to monosaccharides. The indigestible carbohydrates (dietary fiber) from the bread and peanut butter pass unchanged through the small and large intestines. 

Carbohydrate digestion begins in the mouth, where the enzyme amylase in saliva begins to break down polysaccharides. The food then passes through the esophagus, then the stomach, and into the small intestine. Here, more enzymes are secreted to complete the hydrolysis of carbohydrates to form glucose and other monosaccharides. 

In the digestive process, carbohydrates are broken down to monosaccharides which are absorbed from the small intestine either by diffusion or by a specific mechanism involving phosphorylation. Glucose, galactose, and fructose are absorbed by both processes. Many factors infiuence ab-... 

The enzymes that catalyse the hydrolysis of disaccharides (the disaccharidases) are located on the brush border of the intestinal mucosal cells the resultant monosaccharides return to the lumen of the small intestine, and are absorbed together with dietary monosaccharides and glucose arising from the digestion of starch (section... 

Catabolism can be divided into four stages (Figure 25.1). The, first stage of catabolism is called digestion. In this stage, the fats, carbohydrates, and proteins we consume are hydrolyzed to fatty acids, monosaccharides, and amino acids, respectively. These reactions occur in the mouth, stomach, and small intestine. 

BLOOD. Most of the low molecular weight (small) products of digestion are absorbed and transported by the blood. These nutrients include water, salts, glycerol, amino acids, short-chain fatty acids, monosaccharides, and certain vitamins. These materials are absorbed into the capillary system of the intestine. The capillary network drains into the venous system, eventually entering the portal vein of the liver. From the liver, the nutrients then travel through the hepatic veins which, in turn, enter the main systemic vein—the vena cava. 

Example 11.1-1 Infestinal uptake Starch in the human diet is digested in the stomach and small intestine into monosaccharides, which are the only form which can be absorbed. For lactose, enzymatic activity limits the rate of absorption. For other sugars, diffusion is rate limiting. For one set of experiments, the rate of uptake of glucose is 1.6 X 10 mol/ cm sec from a solution containing 2.7 X 10" M. What is the mass transfer coefficient ... 

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