Lactase deficiency: Lack of the enzyme lactase in the small intestine. Lactase is needed to digest lactose, a sugar found in milk and most other dairy products and also used as an ingredient in other foods. Although most people are born with the ability to make adequate amounts of lactase, the production of lactase normally decreases with age, and there are significant differences in lactase production among ethnic groups. People of African or Asian descent commonly have difficulty digesting products that contain lactose. The most common symptoms of lactase deficiency are diarrhea, bloating, and gas. Treatment usually involves avoiding lactose in the diet or taking over-the-counter lactase supplements before eating foods that contain lactose.