Lactose intolerance, also called lactase deficiency and hypolactasia, is the inability to digest lactose, a sugar found in milk and to a lesser extent milk-derived dairy products. Lactase deficiency is the most common form of disaccharidase deficiency. Enzyme levels are highest shortly after birth and decline with aging, despite a continued intake of lactose. Some populations of the human species, including those of Asian, South American, and African descent, have a propensity for developing lactase deficiency. By contrast, races descended from northern Europe or from the northwestern Indian subcontinent are likely to retain the ability to absorb lactose into adulthood.