Abstract
The carbohydrate in milk is lactose, which is hydrolyzed to its monosaccharides principally by lactase. The topics reviewed are: location of lactase enzyme; animal feeding experiments; human lactase; hypolactasia in man; terminology and diagnosis of hypolactasia [lactose malabsorption, milk intolerance, lactose intolerance]; etiology of hypolactasia; human feeding experiments; family studies; and factors affecting decline in jejunal lactase activity.