Digestion of carbohydrates is a complex process, beginning in the mouth and proceeding through the stomach and then into the small and large intestines. Along this digestive path, a variety of enzymes and hormones are secreted, each designed for a specific task. Collectively they work to break down starch into polysaccharides, then into disaccharides and finally into simple sugars, or monosaccharides. Fiber is also classified as a carbohydrate, however it is impermeable to the digestive action of enzymes and hormones.
Carbohydrate digestion begins in the mouth when saliva is secreted as food is chewed. Salivary amylase, the first digestive enzyme, begins the work of cleaving starch into smaller molecules called polysaccharides. Maltose, a disaccharide, is also formed as a result. You can actually taste the sweetness that occurs from this enzymatic action if you hold a bite of starchy food, such as bread, in your mouth, rather than swallowing it immediately. Fiber is not responsive to enzymatic activity; it is only partially broken down by the physical act of chewing.
Gastrin and Gastric Juices
The resultant food mass, or bolus, then enters the stomach where gastric juice, secreted by glands in the stomach, inactivate amylase. This secretion is stimulated by gastrin, a hormone produced by the stomach. This stops further enzymatic activity, however there is some residual digestion of starch from stomach acids. Fiber is not digested at all in the stomach. Its slow release into the small intestine helps you feel full.
Cholecystokinin, Pancreatic Amylase and Disaccharidases
The small intestine is where the majority of carbohydrate digestion takes place. Here the presence of food stimulates the small intestine to secrete cholecystokinin, or CCK. CCK then signals the pancreas to secrete pancreatic amylase, another important hormone in carbohydrate digestion. Pancreatic amylase is released into the small intestine through the pancreatic duct, where it further degrades starch and polysaccharides into maltose. On the intestinal surface, several enzymes collectively called disaccharidases break the disaccharides down into monosaccharides. Their enzymatic action works only on a specific disaccharide. For example, maltase breaks down maltose into two glucose molecules, sucrase breaks down sucrose into a fructose and a glucose molecule and lactase breaks down lactose into its component monosaccharides, galactose and glucose. These monosaccharides are then absorbed into the intestinal cells. Unlike polysaccharides, fiber remains physically intact.
Fiber and Bacterial Enzymes
The undigested fiber now in the large intestine is acted upon by bacterial enzymes, which partially ferment it, producing short-chain fatty acids, gas and water. Fiber then passes outside the body as stool. This digestive process normally takes between one and four hours, according to the authors of the text, “Understanding Nutrition.”
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