Carbohydrates are foods containing sugars and starches, including the healthful dietary fiber found in whole grains and the swiftly-absorbed simple sugars found in candy. Carbohydrate digestion begins in the mouth and proceeds through the digestive tract as most carbohydrates are broken down into their simplest forms for use as fuel. Insoluble fiber forms are eliminated through excretion.
Breaking up the long polysaccharide chains of complex carbohydrates begins in the mouth, with the enzyme salivary amylase. Chewing physically breaks carbohydrates into smaller pieces, creating more surface area for salivary amylase to begin working. After chewing, the carbohydrates are swallowed and pass through the esophagus into the stomach, where stomach acid stops the effectiveness of salivary amylase. No additional carbohydrate digestion occurs until the mixture moves from the stomach into the small intestine, according to the University of Wisconsin Online Advanced Anatomy and Physiology program.
In the small intestine, an enzyme called pancreatic amylase, released from the pancreas, continues breaking down long saccharide chains from complex carbohydrates into disaccharides, or dual-molecule sugars such as sucrose. Additional enzymes -- lactase, maltase and sucrase -- then break disaccharides down into monosaccharides, or single molecule sugars such as glucose. Refined carbohydrates such as white flour and sugar are digested swiftly and mostly absorbed rapidly in the upper end of the small intestine, according to the United Nations Food and Agricultural Organization. More complex, less easily digested carbohydrates such as whole grains are digested toward the lower end of the small intestine, near the ileum. The villi, or small finger-like protrusions in the intestine and ileum that absorb digested materials, vary depending on whether the diet is rich in refined or whole grain carbohydrates.
Once carbohydrates are broken down by digestive enzymes into monosaccharides, these monosaccharides are absorbed by the liver and stored as fuel to be used by the body. A molecule called a sodium-dependent hexose transporter moves a single glucose molecule along with two sodium ions into the epithelial cells of the intestinal lining. Sodium is then exchanged out of the cell and into the bloodstream by potassium, and the released glucose is moved into the bloodstream by a glucose transporter, according to Colorado State University. From there, the glucose moves to the liver until needed as energy for body functions.
Dietary fiber is a complex form of carbohydrate that passes through the body essential undigested. Although dietary fiber consists of polysaccharides, the human body does not contain enzymes that break it down into absorbable components. Dietary fiber draws water into the intestines, softening stool for easier excretion while helping to avert constipation and hemorrhoids, according to the Cornell University Gannett Health Services. Dietary fiber can also slow the digestion of carbohydrates, allowing sugars to enter the bloodstream more slowly; this helps prevent hyperglycemia.
- Wisconsin Online; Advanced Anatomy and Physiology--Carbohydrate Digestion; Wendy Dusek
- United Nations Food and Agricultural Organization: Digestion, Absorption and Energy Value of Carbohydrates
- Colorado State University; Absorption of Monosaccharides; R. Bowen; 2006
- Colorado State University; Dietary Polysaccharides; R. Bowen; 2006
- Cornell University Gannett Health Services: Fiber, Digestion and Health