Lactose: The sugar found in milk. Lactose is a large sugar molecule that is made up of two smaller sugar molecules, glucose and galactose. In order for lactose to be absorbed from the intestine and into the body, it must first be split into glucose and galactose. The glucose and galactose are then absorbed by the cells lining the small intestine. The enzyme that splits lactose into glucose and galactose is called lactase, and it is located on the surface of the cells lining the small intestine.
Lactose intolerance is a common medical condition that results in diarrhea, abdominal pain, and gas (flatulence) and is caused by reduced or absent activity of enzyme lactase. When there is a deficiency of lactase, the lactose in the intestine cannot be split for digestion.