Digestion Of Carbohydrates In The Gastrointestinal Tract

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In the human diet, the most common carbohydrates are starch, sucrose, lactose, fructose, glucose, and dietary fibers. Most (±50-60% of daily intake) carbohydrates are starch, which is a mixture of linear (amylose) and branched (amylopectin) polymers of glucose with D-1, 4 and D-1, 4 + D-1, 6 linkages, respectively. Starch, as well as the disaccharides lactose and sucrose, is hydrolyzed in the upper part of the gastrointestinal system (Figure 4.1), essentially the oral cavity and the small intestine, whereas the dietary fibers are not. The monosaccharides that preexist in the diet (fructose and glucose) and that are produced by the hydrolysis of starch and disaccharides (lactose and sucrose) are absorbed and reach the systemic circulation via the portal vein. But the oligo- and monosaccharides that reach or are produced in the large bowel, essentially by bacterial hydrolysis of dietary fibers and, in some populations, lactose, are not absorbed but fermented. Strictly speaking, the digestion process concerns only starch, lactose, and sucrose, and the absorption process in the small intestine concerns fructose and galactose but mainly glucose.

4.1.1 Carbohydrate Hydrolysis in the Oral Cavity and the Stomach

Digestion of amylose but not amylopectin starts in the oral cavity because of the presence of salivary ptyalin, a D-amylase that specifically splits the □!, 4 glucose-

Digestion Carbohydrates
FIGURE 4.1 Fate of dietary carbohydrates in upper organs of the gastrointestinal system.

glucose linkages in linear but not in branched chains. The products of amylose hydrolysis in the oral cavity are maltose (D-1, 4-D-glucopyranosyl-D-D-glucopyra-nose) and maltotriose (D-1, 4-D-glucopyranosyl-D-D-glucopyranosyl-D-D-glucopy-ranose). In the stomach, due to the acidic pH, free ptyalin is rapidly inactivated. However, in the presence of oligosaccharides (DP 3-8), the enzyme is protected from acidic inactivation, and it remains the only carbohydrate-hydrolase that is active in the stomach, an organ in which no such enzyme is produced. But in addition and because of the low pH, acid hydrolysis does occur more or less extensively depending on the residence time, the concentration of the carbohydrates in the stomach, and on the granulometry of starch (after chewing and mastication). In the chyme that leaves the stomach and enters the duodenum, dietary lactose, sucrose, and glucose-oligosaccharides are thus not digested, whereas starch is only partly hydrolyzed (amylose) or practically not hydrolyzed (amylopectin).

4.1.2 Carbohydrate Hydrolysis in the Small Intestine

The only carbohydrate hydrolase in the lumen of the small intestine is a pancreatic □-amylase that hydrolyses both amylose and amylopectin but cannot hydrolyze □1,4 glucose-glucose linkages close to D-1,6 branching points. It does not hydrolyze all types of starches because the extent of starch digestion in the small intestine is variable depending on physical form of starch-containing food and of starch itself.1 Moreover, it does not produce free glucose from starch. Indeed, the end products of □-amylase hydrolysis of starch in the small intestinal lumen are maltose, maltotriose, and D-limit dextrins (glucose oligomers with DP 5-10 and D-1, 4 and D-1, 6 glucose-glucose linkages). These short chain glucose oligomers as well as the dietary disaccharides (lactose and sucrose) are rapidly split up by hydrolases that are located in the outer portion of the brush border, the membrane of the microvilli of the small intestine. The human brush border hydrolases are glucoamylase or maltase, D-limit dextrinase, lactase, and sucrase that hydrolyze D-limit dextrins, maltose and maltot-riose, lactose, and sucrose, respectively, to produce glucose, fructose, and galactose. The hydrolytic activity is relatively low in the duodenum; it reaches its maximum activity in the jejunum and then falls off to a relatively low level in the ileum. Consequently, the hydrolysis of carbohydrates is nearly completed in the mid-jejunum in normal physiological conditions. In humans, high dietary intake of fructose and sucrose, but not of glucose, stimulates the biosynthesis of the sucrase-isomaltase complex, but it has no effect on lactase activity that is also not stimulated by lactose intake.

4.1.3 Methods to Study the Digestibility of Oligo- and Polysaccharides2

The methodologies to demonstrate that oligo/polysaccharides resist digestion in the upper part of the gastrointestinal system are:2

• Chemical analysis to confirm the configuration (□) of the osidic linkages

• In vitro incubation in fresh saliva or gastric juice or in the presence of pancreatic or small intestinal tissue homogenate

• In vivo disappearance of ingested carbohydrate from the intubated rat small intestine

• In vivo recovery in feces of germ-free or antibiotic treated rats to suppress the intestinal microflora

• In vivo aspiration of residual digesta from the ileum using intubation

• Recovery in the pouch of ileostomy

• Ingestion of the carbohydrate resulting in change in glycemia and insuline-mia

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