Digestion of Dietary Carbohydrates

DIGESTION OF DIETARY 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 (Figure 7.8 ). Because there is little monosaccharide present in diets of mixed animal and plant origin, the enzymes are primarily endoglycosidases that hydrolyze polysaccharides and oliosaccharides, and disaccharidases that hydrolyse tri- and disaccharides into their reducing sugar components. 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 that are absorbed by cells of the small intestine.

Figure 7.8 Hydrolysis of a glycosidic bond.

A. Salivary a-amylase

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 glycogen, hydrolyzing random α(1→4) bonds. [Note: There are both α(1→4)- and β(1→4)-endoglucosidases in nature, but humans do not 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 cannot hydrolyze, 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.

Figure 7.9 Digestion of carbohydrates. [Note: Indigestible cellulose enters the colon and is excreted.]

B. Pancreatic a-amylase

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.

C. Intestinal disaccharidases

The final digestive processes occur primarily at the mucosal lining of the upper jejunum and include the action of several disaccharidases (see Figure 7.9 ). For example, isomaltase cleaves the α(1→6) bond in isomaltose, and maltase cleaves the α(1→4) bond in maltose and maltotriose, each producing glucose. Sucrase cleaves the α(1→2) bond in sucrose, producing glucose and fructose, and lactase (β-galactosidase) cleaves the β(1→4) bond in lactose, producing galactose and glucose. [Note: The substrates for isomaltase are broader than its name suggests, and it hydrolyzes the majority of maltose.] Trehalose, an α(1→1) disaccharide of glucose found in mushrooms and other fungi is cleaved by trehalase. These enzymes are transmembrane proteins of the brush border on the luminal surface of the intestinal mucosal cells.

Sucrase and isomaltase are enzymic activities of a single protein (SI) which is cleaved into two functional subunits that remain associated in the cell membrane, forming the sucrase-isomaltase complex. In contrast, maltase is one of two enzymic activities of a single membrane protein maltase-glucoamylase (MGA) that does not get cleaved. Its second enzymic activity, glucoamylase, cleaves a(1→4) glycosidic bonds in dextrins.

D. Intestinal absorption of monosaccharides

The duodenum and upper jejunum absorb the bulk of the monosaccharide products of digestion. However, different sugars have different mechanisms of absorption ( Figure 7.10). For example, galactose and glucose are transported into the mucosal cells by an active, energy-dependent process that requires a concurrent uptake of sodium ions, and the transport protein is the sodium-dependent glucose cotransporter 1 (SGLT-1). Fructose utilizes an energy- and sodium-independent monosaccharide transporter (GLUT-5) for its absorption. All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2. (See : for a discussion of these transporters.)

Figure 7.10 Digestion of carbohydrates. [Note: Indigestible cellulose enters the colon and is excreted.]

E. 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 only monosaccharides are absorbed, any deficiency (genetic or acquired) 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 the bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO₂ and H₂ gas, causing abdominal cramps, diarrhea, and flatulence.

1. 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, and 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. Therefore, patients suffering or recovering from such a disorder cannot drink or eat significant amounts of dairy products or sucrose without exacerbating the diarrhea.

2. Lactose intolerance: More than 70% of the world’s adults arelactose 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 produced. It is thought to be caused by small variations in the DNA sequence of a region on chromosome 2 that controls expression of the gene for lactase, also on chromosome 2. Treatment for this disorder is to reduce consumption of milk and eat yogurts and some cheeses (bacterial action and aging process decrease lactose content) 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.] Rare cases of congenital lactase deficiency are known.

3. Congenital sucrase-isomaltase deficiency: This autosomal recessive disorder results in an intolerance of ingested sucrose. Congenital sucrase-isomaltase deficiency has a prevalence of 0.02% in individuals of European descent and appears to be much more common in the Inuit people of Greenland and Canada. Treatment includes the dietary restriction of sucrose and enzyme replacement therapy.

4. Diagnosis: Identification of a specific enzyme deficiency can be obtained by performing oral tolerance tests with the individual disaccharides. 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).

Figure 7.11 Abnormal lactose metabolism. H₂ = hydrogen gas.