Lactose intolerance is a condition caused by a lack of an enzyme called lactase. Inadequate amounts of lactase cause the body to be unable to digest lactose, a sugar found in milk products.Lactase is normally produced in the small intestine where it breaks lactose down into a form that can be absorbed by the blood.A lack of lactase can cause uncomfortable symptoms for some people.nadequate amounts of lactase cause the body to be unable to digest lactose, a sugar found in milk products. Lactase is normally produced in the small intestine where it breaks lactose down into a form that can be absorbed by the blood. A lack of lactase can cause uncomfortable symptoms for some people.