Sugars, starches and fiber are the three basic types of carbohydrates. During digestion, the body turns sugars and starches into the sugar glucose. Glucose is the body's preferred energy source, especially for the brain and central nervous system. Fiber is the part of plants that the body does not digest. It is not converted to sugar and does not provide energy to the body.
Sugars and Starches
There are two types of sugars, or saccharides, in food. Monosaccharides are single sugars. They include fructose, galactose and glucose, or dextrose. Disaccharides are two sugars linked together. They include sucrose, maltose and lactose. Starches are polysaccharides, made up of many sugars linked together. During digestion, the body breaks carbohydrates down into monosaccharides so they can be absorbed into the bloodstream.
Digestion: The Mouth
Digestion begins in the mouth. Chewing causes mechanical digestion, and the salivary glands secrete an enzyme called salivary amylase. Amylase begins breaking down polysaccharides into smaller pieces. After you swallow, food passes through the esophagus into the stomach. In the stomach, mechanical digestion of food continues, but the secretion of stomach acids stops the action of salivary amylase.
Digestion: The Small Intestine
When food -- now a milky liquid called chyme -- passes into the duodenum of the small intestine, carbohydrate digestion continues. Amylase that the pancreas produces breaks polysaccharides down into disaccharides. The mucosa of the small intestine produce brush border enzymes that break disaccharides down into monosaccharides. Lactase, maltase and sucrase break down lactose, maltose and sucrose into glucose, galactose and fructose.
Metabolism
The monosaccharides glucose, galactose and fructose are absorbed into the liver, which converts fructose and galactose into glucose. The liver regulates blood glucose levels to provide sufficient energy for the body. Glucose may be distributed to cells throughout the body for immediate use or it may be stored. Liver and muscle cells can convert glucose to glycogen for storage through a process known as glycogenesis. When blood sugar levels fall, the glycogen can be broken down to glucose through glycogenolysis. Hormones such as insulin, glucagon and epinephrine regulate these processes.
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