Starch is the main form of dietary, digestible carbohydrates. The process of digestion involves the breakdown of a complex molecule into the simplest form the body can use. Once the starch molecule is broken down, the small intestine transfers it into the bloodstream, where it is shuttled to the cells that need it. Starch digestion is a multi-step process that begins in the mouth.
The goal of digestion is the breakdown of nutrients into their simplest, usable form by the body. Starch is the storage form of glucose inside plant matter. It is a polysaccharide, a molecule composed of many simple sugar molecules, called monosaccharides, linked together. During digestion, the body must break down the polysaccharides into a form that it can absorb. The three absorbable forms of nutrients that starch can provide are glucose, fructose and galactose.
Mouth and Stomach
The mouth does two things for digestion. It begins to mash up your food, which helps to expose macronutrients. In addition, the salivary glands secrete an enzyme known as salivary amylase. This enzyme begins the breakdown of starches. When you swallow, the food passes through your esophagus and into the stomach. The acidity of the stomach quickly extinguishes the work of salivary amylase on the outside of the food bulk. However, amylase that is on the inside of the food in the stomach is protected from the stomach secretions and continues to work. Salivary amylase continues to work for one to two hours while guarded against stomach acid.
The small intestine is where digestion of starch starts to take action. The brush border of the small intestine releases dextrinase and glucoamylase, both of which slowly break down polysaccharides, chains of saccharide polymers, into oligosaccharides. Pancreatic amylase works to further break down oligosaccharides, which are chains of monosaccharides containing more than two saccharides. Finally, oligosaccharides are broken down into disaccharides, or two monosaccharides, then further into monosaccharides, the simplest form of a carbohydrate. Maltase, another brush border enzyme, breaks down maltose into glucose. The pancreas secretes many enzymes into the small intestine that all work in concert to break down the starch molecules. Other pancreatic enzymes include sucrase and lactase, which break down sucrose and lactose, two disaccharides.
Absorption of glucose, fructose and galactose -- the products of starch digestion -- begins with movement in the absorptive cells of the small intestine. Glucose and galactose are moved into these cells via the SGLT facilitated transport mechanism, which uses sodium. Fructose is transported through another mechanism, GLUT5. GLUT2 moves all of the molecules from the cell and into the bloodstream, where the body can finally take advantage of these nutrients.
- National Digestive Diseases Information Clearinghouse; Your Digestive System and How It Works; April 2008
- "Nutrition: Concepts and Controversies"; Frances Sizer and Eleanor Whitney; 2004
- "Anatomy and Physiology"; Kenneth S. Saladin; 2004
- "Advanced Nutrition and Human Metabolism"; Sareen S. Gropper, et al.; 2009
- Colorado State University: Absorption of Monosaccharides; R. Bowen; April 2006