IDENTIFICATION AND USE: 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.

HUMAN STUDIES: Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar of milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down the lactose, milk sugar, into glucose and galactose that can then be absorbed into the bloodstream. When there is not enough lactase to digest the amount of lactose consumed, produce some uncomfortable symptoms. Some adults have low levels of lactase. This leads to lactose intolerance. The ingested lactose is not absorbed in the small intestine, but instead is fermented by bacteria in the large intestine, producing uncomfortable volumes of carbon dioxide gas. While not all persons deficient in lactase have symptoms, those who do are considered to be lactose intolerant.

ADVERSE REACTIONS: nausea, cramps, bloating, gas, and diarrhea, which begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms varies depending on the amount of lactose each individual can tolerate. Lactose intolerance that occurs after age 21 (genetically-determined lactase deficiency usually occurs between ages 5-21) is rarely due to genetic lactase deficiency; it suggests another process is interfering with lactose digestion.