Digestion and absorption Carbohydrates The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen. This digestion is rapid and is catalyzed by enzymes known as glycoside hydrolases (glycosidases) that hydrolyze glycosidic bonds. Because there is little monosaccharide present in diets of mixed animal and plant origin, the enzymes are primarily endoglycosidases that hydrolyze polysaccharides and oligosaccharides, and disaccharidases that hydrolyse tri- and disaccharides into their reducing sugar components (Figure 7.8). Glycosidases are usually specific for the structure and configuration of the glycosyl residue to be removed, as well as for the type of bond to be broken. The final products of carbohydrate digestion are the monosaccharides, glucose, galactose and fructose, which are absorbed by cells of the small intestine.

Whenever Zachariah went into the sanctuary where she was, he found that she had food. He said: O Mary! Whence cometh unto thee this? She answered: it is from Allah. Allah gives without stint to whom he will. 37 the family of Imran Al-Qur’an

Carbohydrates diet

Dietary carbohydrates Largest source of calories 40-45% Plant starch ( grains, tubers, veggies ) sucrose glucose fructose dietary fiber Animal glycogen glycolipids lactose

Carbohydrates in our diet CH₂OH O OH CH₂OH O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-glucose α-D-glucose

Carbohydrates in our diet CH₂OH OH CH₂OH O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-galactose α-D-glucose

Carbohydrates in our diet CH₂OH OH CH₂OH O HOCH₂ O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-fructose α-D-glucose

Carbohydrates in our diet CH₂OH OH CH₂OH O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-glucose α-D-glucose

Carbohydrates in our diet CH₂OH OH CH₂OH O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-galactose α-D-glucose

Carbohydrates in our diet CH₂OH OH CH₂OH O HOCH₂ Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-fructose α-D-glucose

Carbohydrates in our diet CH₂ CH₂OH O OH OH α-D-glucose α-D-glucose

Carbohydrates in our diet So what’s our relationship? O α-D-glucose O O Its called α-1—2 glycosidic bond Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. β-D-fructose Sucrose

Carbohydrates in our diet Its called β-1—4 glycosidic bond So what’s our relationship? O O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Lactose

Carbohydrates in our diet Its called α-1—4 glycosidic bond So what’s our relationship? O O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Maltose

Carbohydrates in our diet Of course! It’s the α-1—4 glycosidic linkages So what’s our relationship? O O O O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Maltotriose

Carbohydrates in our diet So what’s our relationship? Its called α-1—6 glycosidic bond O O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Iso- maltose

Carbohydrates in our diet So what’s our relationship? Its called α-1—1 glycosidic bond , is’nt it cool? O O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Trehalose

Carbohydrates in our diet So what’s our relationship? O Its called β-1—2 glycosidic bond O O I don’t understand! Lactulose Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. O O O

Carbohydrates in our diet We’ve got a strong relashionship, the α-1—4 glycosidic linkage Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Amylose

Carbohydrates in our diet We are stronger, we have got α-1—4 glycosidic linkages as well α-1—6 branch points O O O O O O O Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. O O O O Amylo-pectin

Carbohydrates in our diet We are a team, we have got the β-1—4 glycosidic linkages! Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase. Cellulose

Carbohydrates digestion

Digestion of carbohydrates Endoglycosidases / exoglycosidases specific for sugar, type of bond, number of saccharide units Glycosidases Enzymes that hydrolyze glycosidic bonds b/w sugars

Mouth Salivary alpha amylase Endoglycosidase No activity against internal alpha 1,4 bonds random intervals No activity against Alpha 1,6 bonds Little or no activity against Alpha 1,4 bond at non reducing ends Digestion of carbohydrates begins in the mouth The major dietary polysaccharides are of plant (starch, composed of amylose and amylopectin) and animal (glycogen) origin. During mastication, salivary α-amylase acts briefly on dietary starch and glyco-gen, hydrolyzing random α(1→4) bonds. [Note: There are bothα(1→4)-and β(1→4)-endoglucosidases in nature, but humans donot produce the latter. Therefore, we are unable to digest cellulose—a carbohydrate of plant origin containing β(1→4) glycosidic bonds between glucose residues.] Because branched amylopectin and glycogen also contain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains a mixture of short, branched and unbranched oligosaccharides known as dextrins(Figure 7.9) [Note: Disaccharides are also present as they, too, are resistant to amylase.] Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase.

Shortened polysaccharides Salivary α-amylase O O O Shortened polysaccharides α-limit dextrins

Stomach Salivary alpha amylase Inactivated by gastric HCl

Small intestine Pancreatic alpha amylase Bicarbonate Intestinal disaccharidases Further digestion of carbohydrates by pancreatic enzymes occurs in the small intestine. When the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas, and pancreatic α-amylase continues the process of starch digestion. Final carbohydrate digestion by enzymes synthesized by the intestinal mucosal cells. The final digestive processes occur primarily at the mucosal lining of the upper jejunum, and include the action of several disaccharidases (Figure 7.10). For example, isomaltase cleaves the α(1→6)bond in isomaltose and maltase cleaves maltose and maltotriose, each producing glucose, sucrase cleaves sucrose producing glucose and fructose, and lactase(β-galactosidase) cleaves lactose producing galactose and glucose. Trehalose, an α(1→1)disaccharide of glucose found in mushrooms and other fungi, is cleaved by trehalase. These enzymes are secreted through, and remain associated with, the luminal side of the brush border membranes of the intestinal mucosal cells. [Note: The substrates for isomaltase are broader than its name suggests, as it hydrolyzes the majority of maltose.]

4-9 glycosyl residues and 1 or more 1,6 branches Pancreatic α-amylase Endoglycosidase Continues digestion of starch and glycogen Limit dextrins Maltotriose Maltose Duodenum 4-9 glycosyl residues and 1 or more 1,6 branches O O

Intestinal disaccharidases Glucoamylase Sucrase isomaltase complex Lactase glucosylceramidase/ β-glycosidase complex Trehalase

Glucoamylase Really an oligosaccharidase Highest activity in ileum Hydrolyses 1,4 bonds of dextrins Exoglucosidase Begins at nonreducing end and sequentially cleaves glycosyl units It cleaves dextrins down to isomaltose Highest activity in ileum O O O O O O O O O O O O O O O O

Sucrase-isomaltase complex Sucrase-maltase Splits sucrose, maltose and maltotriose Isomaltase-maltase Splits alpha 1,6 bonds in isomaltose and limit dextrins alpha 1,4 bonds in maltose and maltotriose Jejunum Sucrase and isomaltase are enzymic activities of a single protein which is cleaved into two functional subunits that remain associated in the cell membrane, forming the sucrase-isomaltase complex. Maltase forms a similar complex with an exoglucosidase (glucoamylase) that cleavesα (1→4) glycosidic bonds in dextrins.

Sucrase-isomaltase complex Sucrose

Sucrase-isomaltase complex Maltose

Sucrase-isomaltase complex Iso- maltose

Lactase glucosylceramidase/ β-glycosidase complex Splits β-glycosidic bonds b/w glucose/galactose and hydrophobic residues Lactase Splits β1,4 bonds b/w glucose and galactose Jejunum

β-glycosidase complex

Glycolipids Cerebrosides Globosides Ceramide-monosaccharides Ceramide-oligosaccharides GLU CERAMIDE GAL CERAMIDE GLU

Glycolipids Forssman antigen Ceramide-oligosaccharides GAL CERAMIDE GLU GALNAc

Trehalase Splits α1,1 bond in trehalose Found in mushrooms,insects and seafood O O O Trehalose

Carbohydrates Absorption

Absorption of monosaccharides Hi! Glucose Fructose Galactose Site Duodenum Upper jejunum O Absorption of monosaccharides by intestinal mucosal cells The duodenum and upper jejunum absorb the bulk of the dietary sugars. However, different sugars have different mechanisms of absorption. For example, galactose and glucose are transported into the mucosal cells by an active, energy-requiring process that requires a concurrent uptake of sodium ions; the transport protein is the sodium-dependent glucose cotransporter 1 (SGLT-1). Fructose uptake requires a sodium-independent monosaccharide transporter(GLUT-5) for its absorption. All three monosaccharides are trans-ported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2. (See p. 97 for a discussion of these transporters.) O

GLUT 5 O Na+ O 2K+ Na+ 3Na+ Na+ SGLT-1 O Na+ O GLUT 2 O Na+ O Na+ Na+

GLUT 5 GLUT 2 O Na+ O Na+ Na+ SGLT-1 O Na+ O GLUT 2 O Na+ O Na+ Na+

Carbohydrates Indigestible

Indigestible carbohydrates Cellulose Hemicelluloses Gums Mucilages Pectin Raffinose Lignin

Colon Dietary fibre and nondigested carbohydrates Bacterial action Gases H2, CO2, methane Lactate Short chain FA Acetic acid Propionic acid Butyric acid

What is the cause of excessive flatulence after having a meal containing beans? Oligosaccharides with (1,6) linked galactose residues

Carbohydrates Disorders

Abnormal degradation of disaccharides Disorders Lactose intolerance Disaccharide intolerance Isomaltase sucrase deficiency Defect in absorption of fructose Symptoms Osmotic diarrhea Bacterial fermentation 2 and 3 carbon compounds Gases Abnormal degradation of disaccharides The overall process of carbohydrate digestion and absorption is so efficient in healthy individuals that ordinarily all digestible dietary carbohydrate is absorbed by the time the ingested material reaches the lower jejunum. However, because it is mono saccharides that are absorbed, any defect in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine. As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhea. This is reinforced by thebacterial fermentation of the remaining carbohydrate to two- andthree-carbon compounds (which are also osmotically active) pluslarge volumes of CO2and H2gas, causing abdominal cramps, diar-rhea, and flatulence.

Lactose intolerance Etiology Clinical features Deficiency of enzyme lactase Clinical features Abdominal cramps GI bloating Intermittent diarrhea 45min-1hr after eating dairy products Lactose intolerance: More than three quarters of the world’s adults are lactose intolerant (Figure 7.11). This is particularly manifested in certain populations. For example, up to 90% of adults of African or Asian descent are lactase-deficient and, therefore, are less able to metabolize lactose than individuals of Northern European origin. The age-dependent loss of lactase activity represents a reduction in the amount of enzyme rather than a modified inactive enzyme. It is thought to be caused by small variations in the DNA sequence of a region on chromo-some 2 that controls expression of the gene for lactase, also on chromosome 2. Treatment for this disorder is to reduce consumption of milk while eating yogurts and cheeses, as well as green vegetables such as broccoli, to ensure adequate calcium intake; to use lactase-treated products; or to take lactase in pill form prior to eating. [Note: Because the loss of lactase is the norm for most of the world’s adults, use of the term “adult hypolactasia” for lactose intolerance is becoming more common.]

Lactose intolerance Treatment Reduce consumption of dairy products Consume Green veggies Yogurts Cheese Lactase treated products Lactase pills Lactose intolerance: More than three quarters of the world’s adults are lactose intolerant (Figure 7.11). This is particularly manifested in certain populations. For example, up to 90% of adults of African or Asian descent are lactase-deficient and, therefore, are less able to metabolize lactose than individuals of Northern European origin. The age-dependent loss of lactase activity represents a reduction in the amount of enzyme rather than a modified inactive enzyme. It is thought to be caused by small variations in the DNA sequence of a region on chromo-some 2 that controls expression of the gene for lactase, also on chromosome 2. Treatment for this disorder is to reduce consumption of milk while eating yogurts and cheeses, as well as green vegetables such as broccoli, to ensure adequate calcium intake; to use lactase-treated products; or to take lactase in pill form prior to eating. [Note: Because the loss of lactase is the norm for most of the world’s adults, use of the term “adult hypolactasia” for lactose intolerance is becoming more common.]

Yogurt is a dairy product, why is it recommended in lactose intolerance? Yogurt does not cause these problems because lactose is consumed by the bacteria that transform milk into yogurt.

Disaccharide intolerance Etiology Hereditary deficiency Injured Mucosa Disease, drugs, malnutrition Clinical features Osmotic diarrhea Abdominal cramps Flatulence Digestive enzyme deficiencies: Genetic deficiencies of the individual disaccharidases result in disaccharide intolerance. Alterations in disaccharide degradation can also be caused by a variety of intestinal diseases, malnutrition, or drugs that injure the mucosa of the small intestine. For example, brush border enzymes are rapidly lost in normal individuals with severe diarrhea, causing a temporary, acquired enzyme deficiency. Thus, patients suffering or recovering from such a disorder cannot drink or eat significant amounts of dairy products or sucrose without exacerbating the diarrhea.

Sucrase-isomaltase complex deficiency intolerance of ingested sucrose Sucrase-isomaltase complex deficiency: This deficiency results in an intolerance of ingested sucrose. The disorder is found in about 10% of the Inuit people of Greenland and Canada, whereas 2% of North Americans are heterozygous for the deficiency. Treatment includes the dietary restriction of sucrose, and enzyme replacement therapy. Diagnosis: Identification of a specific enzyme deficiency can be obtained by performing oral tolerance tests with the individual di -saccharides. Measurement of hydrogen gas in the breath is a reliable test for determining the amount of ingested carbohydrate not absorbed by the body, but which is metabolized instead by the intestinal flora (see Figure 7.11).

Defect in absorption of fructose Clinical features Gas and distented abdomen after eating fruit, sweets or juices

Carbohydrate disorders diagnosis

Disorders of carbohydrate digestion and absorption Diagnosis Oral tolerance tests H2 breath test

Carbohydrate disorders Treatment

Treatment Avoid foods containing specific disaccharide Tablets and capsules containing enzymes