Lactose intolerance results from the loss of an enzyme called lactase from the lining of the small intestine. Lactose is the sugar found in milk. It is made of two molecules of sugar. The enzyme lactase cuts the bond between the two sugar molecules in lactose. This allows the sugar molecules to be absorbed. When the enzyme lactase is no longer produced by the body, the sugar lactose cannot be absorbed. When lactose is not absorbed, it results in excess gas, bloating and often loose stool or diarrhea.
Lactose intolerance is treated by avoiding dairy products or using lactose-reduced products. Pills containing the enzyme, lactase, are also used.