lactase deficiency, an inherited abnormality in which the amount of the digestive enzyme lactase is inadequate for the normal digestion of milk products, resulting in lactose intolerance, the inability to digest lactose (except for the bacterial breakdown of lactose in the large intestine). In adults a relative deficiency may appear as a natural process of aging; it occurs more frequently in persons of Asiatic, Native American, and African heritage. A lactase deficiency also may result from subtotal gastrectomy and may be secondary to any disease of the small intestine in which structural changes occur, such as tropical sprue, ulcerative colitis, infectious hepatitis, and kwashiorkor; severe malnutrition; or some types of antibiotic therapy. See also lactose intolerance.
