Lactose is a type of sugar found in human and cow's milk. The official chemical name for lactose is 4-O-β-D-galactopyranosyl, D-glucopyranose. This sugar is important to mammals since it serves as primary energy source during infancy. The ability to digest lactose decreases with age and in some people this results in a digestive condition known as lactose intolerance.
Chemical Structure
Lactose sugar is a disaccharide, meaning it is can be broken down into two subunits of smaller sugars. One molecule of lactose consists of a molecule of galactose and a molecule of glucose linked together. There are two forms of the lactose molecule in milk, isomeric forms called α- and β-lactose.
Lactose Digestion
In the body lactose is broken down into its two component sugars, glucose and galactose, by an enzyme known as lactase. Lactase cuts the beta-glucosidic linkage that bonds the molecule together. This enzyme is produced by the cells lining the small intestine. People with lactose intolerance are deficient in lactase and unable to digest lactose efficiently.
Lactose Fermentation
Lactose that remains undigested in the small intestine passes through to the large intestine or colon. There it becomes a food source for the normal bacteria that colonize the large intestine. The bacteria convert the sugar to lactic acid, a process called fermentation. Carbon dioxide gas is released as a by-product of fermentation, causing the bloating and abdominal pain that affect people with lactase deficiency.
Lactose Sources
Lactose is found in milk and all dairy products. It is present at lower levels in yogurt and hard cheeses and some people with lactose intolerance are able to consume small amounts of these foods. Lactose is added to many processed foods to improve shelf life. Breads, soups, salad dressing, lunch meats and other processed meat and breaded foods all contain lactose.
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