Info

Maltotriose_25_50_25

Maltose _25_50_25

Trehalose

Lactose _

^[Glucose J

^[Glucose J

Maltotriose_25_50_25

Maltose _25_50_25

Trehalose

Lactose _

[Fructose J

[Fructose J

FIGURE 4 Substrate specificities of the enzymes involved in carbohydrate digestion and the resulting products. Numbers are percentages of each substrate broken down by a particular enzyme. Note that only three final products are formed.

dextrins, which contain the a-1,6 linkage branch points (Fig. 3) and a-1,4 linkages adjacent to the branch points. These 1,4 linkages are resistant to amylase. Free glucose is never produced by the action of amylase.

The digestion of the oligosaccharides produced by the action of amylase on starch and the digestion of other forms of dietary carbohydrates are carried out by enzymes located in the brush border membrane of the enterocytes (Fig. 4). Many of these enzymes hydrolyze more than one substrate. As shown in Fig. 4, gluco-amylase cleaves the a-1,4 bonds of the limit dextrins, freeing glucose molecules sequentially from the nonreduc-ing ends. Sucrase and isomaltase (also called a-dextrinase) break these bonds as well. When the a-1,6 branch point is reached, it is hydrolyzed by isomaltase. Isomaltase also accounts for approximately half the digestion of malto-triose and maltose. In addition to hydrolyzing some of the a-dextrins, glucoamylase hydrolyzes maltotriose and maltose. Sucrase aids in the digestion of maltose and maltotriose and accounts for 100% of the breakdown of sucrose. Two disaccharidases have specificity for only one substrate. Lactose hydrolyzes all of the lactose present, and trehalase is the only enzyme that digests trehalose, an a-1,1-linked dimer of glucose.

The rate-limiting step in carbohydrate assimilation is absorption. Both the luminal and membrane-bound enzymes are present in excess, so that digestion is normally complete in the proximal jejunum. The human digestive tract is incapable of digesting the ^-glucose bonds found in cellulose and hemicellulose. As a result, these carbohydrates make up undigestible fiber in the diet and are eliminated in the feces.

Absorption

Carbohydrate digestion produces the monosacchar-ides glucose, galactose, and fructose. These three hexoses are the only dietary sugars of any consequence that are absorbed. In general, the hexoses are too large to pass through the aqueous channels between the enterocytes or through the pores in the apical cell membranes. Thus, only a small fraction of sugar absorption takes place by passive diffusion. The amount passively absorbed is somewhat variable, influenced by bulk flow. The large majority of hexose uptake is by mediated transport.

Fructose is absorbed by facilitated diffusion. The carrier involved is GLUT-5, which is located in the apical membrane of the enterocyte (Fig. 5). Hence, it is not absorbed against a concentration gradient. However, it is taken up more rapidly than can be explained by simple diffusion.

Glucose and galactose are absorbed by an active transport process requiring the presence of Na+ in the lumen. This Na+-dependent carrier (SGLT-1)is identical for both sugars because each competes with the other for transport. The uptake of glucose and galactose depends on the electrochemical Na+ gradient generated by the (Na+,K+)-ATPase in the basolateral membrane, and is therefore a secondary active transport system. A model illustrating the basic principles of this system is

FIGURE 5 Model of the carriers involved in the absorption of glucose (Glu), galactose (Gal), and fructose (Fru). SGLT-1 is the Na+-dependent carrier for the active absorption of glucose and galactose. GLUT-5 is the facilitated diffusion carrier for fructose. All three sugars exit the cell via the facilitated process, GLUT-2, in the basolateral membrane.

FIGURE 5 Model of the carriers involved in the absorption of glucose (Glu), galactose (Gal), and fructose (Fru). SGLT-1 is the Na+-dependent carrier for the active absorption of glucose and galactose. GLUT-5 is the facilitated diffusion carrier for fructose. All three sugars exit the cell via the facilitated process, GLUT-2, in the basolateral membrane.

Clinical Note

Abnormalities of Carbohydrate Assimilation

Although predominant forms of dietary carbohydrate are polysaccharides and oligosaccharides, it is the monosaccharide products that are absorbed. Most problems of carbohydrate assimilation are due to enzyme defects and the failure to reduce one of the dietary forms to mono-saccharides. Undigested carbohydrates remaining in the lumen increase the osmotic pressure of the luminal contents, preventing the absorption of a comparable amount of water. In addition, these carbohydrates are fermented by bacteria in the distal ileum and colon, adding to this osmotic effect. The retention of water results in diarrhea. Symptoms of diarrhea include cramps and abdominal distension.

Lactose intolerance is the most common problem of carbohydrate digestion and is caused by the absence of the brush border enzyme lactase. The incidence and distribution of this condition were discussed earlier in this chapter in the section on developmental changes. The condition is diagnosed by feeding lactose and monitoring the appearance of glucose in the blood. If blood glucose levels do not increase, it is evidence of lactase deficiency. However, one must also show that the individual is capable of absorbing glucose when it is fed, because the absence of the carrier for glucose and galactose has been documented in rare instances. Lactase-deficient individuals do fine if they avoid lactose in their diets. Those deficient in the glucose carrier are, of course, diagnosed as infants, and will thrive if fed fructose. In this condition, enzyme activities are normal.

Maltose intolerance has also been described, but this is extremely rare. Because three different brush border enzymes have specificity for maltose, it is not likely that a genetic defect will eliminate all maltase activity.

Sucrase and isomaltase are subunits of a single protein, and an inherited absence occurs in about 0.2% of North Americans. These individuals cannot digest sucrose, but do well on diets low in this sugar.

Assimilation of dietary sugars may also be impaired by diseases that involve a large area of the intestinal mucosal surface. Conditions leading to general structural impairment of the mucosa and decreased brush border enzyme activities include celiac disease and some bacterial and parasitic infections. These diseases often alter the assimilation of all nutrients.

shown in Fig. 5. Within the cell, the Na+ concentration is maintained low by the (Na+,K+)-ATPase. Thus, inside the cell, the equilibrium is such that Na+ leaves the carrier. Glucose and galactose within the cell exit across the basolateral membrane and are carried away by the circulation; therefore, there is also a concentration gradient favoring the removal of the sugars from the carrier. Sugars leave the cell by an Na + -independent facilitated diffusion process (GLUT-2). The exit of sugars from the cell is rapid, and there is little intracellular accumulation. The carrier is then free to combine with more Na+ and glucose or galactose in the luminal compartment. Because of this mechanism, the addition of glucose or galactose to the lumen will increase Na+ absorption.

Atkins Low Carb Diet Recipes

Atkins Low Carb Diet Recipes

The Atkins Diet is here. Dr Atkins is known for his great low carb diets. Excluding, Dr Atkins carb counter and Dr Atkins New Diet Revolution.

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