This paper traces the complete physiological pathway of carbohydrate digestion and absorption through the human gastrointestinal tract. Beginning with mechanical and chemical breakdown in the oral cavity via salivary amylase, the paper follows the bolus through the esophagus and into the stomach, where acidic conditions halt enzymatic activity. It then details the critical role of the small intestine — including pancreatic amylase, brush-border disaccharidases, and villous absorption — in converting complex carbohydrates into monosaccharides. Finally, the paper describes how absorbed sugars travel via blood capillaries to the liver for storage or redistribution as glycogen or free glucose.
The mechanical and chemical digestion of carbohydrates begins in the mouth. Chewing — also termed mastication — ensures that carbohydrates are broken down into smaller pieces. Salivary glands within the oral cavity secrete saliva, which coats the food. Saliva contains the enzyme salivary amylase, which breaks the bonds found between monomeric sugars such as disaccharides, oligosaccharides, and starches. It also breaks down amylose and amylopectin into small glucose chains referred to as dextrin and maltose. Approximately 5% of starches are broken down within the mouth. Mucus cells within the salivary glands also produce mucus, which helps the food stick together, lubricates it, and aids in swallowing. At this stage the food is known as a bolus, and it is forced into the pharynx with the help of the tongue (Swartz, 2012).
During swallowing, food passes through the esophagus — a straight, collapsible tube that provides a passage from the pharynx to the stomach. The bolus is moistened and lubricated by mucus produced by mucous glands within the esophagus. It then passes through the cardiac sphincter and into the stomach.
The bolus carries some salivary amylase from the mouth; however, enzymatic breakdown does not continue within the stomach. This is because the stomach environment is highly acidic, and salivary amylase cannot function under acidic conditions. The stomach has several divisions: the cardiac section, body section, fundic section, and pyloric region. Within the stomach, food is mixed and churned, facilitating further digestion of carbohydrates. Mechanical breakdown occurs through strong peristaltic contractions, converting the bolus into a semifluid paste combined with gastric juice — a mixture referred to as chyme. This chyme then moves through the pyloric sphincter into the first part of the small intestine (Swartz, 2012).
The small intestine has three distinct sections: the duodenum, jejunum, and ileum. Most carbohydrate digestion occurs here. As chyme enters the duodenum, an enzyme called pancreatic amylase is released from the pancreatic duct and begins breaking dextrin down into shorter carbohydrate chains. Enzymes are also secreted by the intestinal cells lining the villi; these enzymes are collectively referred to as disaccharidases and include sucrase, maltase, and lactase. Sucrase breaks sucrose into molecules of glucose and fructose. Maltase breaks the bond between the two glucose units found in maltose, while lactase breaks the bond between glucose and galactose. After the chemical breakdown of carbohydrates into single sugar units, the monosaccharides are transported into the small intestine for absorption (Swartz, 2012).
"Villi absorb sugars; liver stores or releases glucose"
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