Congenital lactose malabsorption.

A breastfed female infant is described who developed from the third day of life a severe watery diarrhoea, with weight loss. This ceased when lactose was omitted from the feeds and returned when it was resumed. Intolerance to this carbohydrate was confirmed by a lactose tolerance test which showed inability to hydrolyse the disaccharide. Absorption of glucose and sucrose was normal. No other cause for the diarrhoea was established. Congenital lactose malabsorption (alactasia) was confirmed by examination of the duodenal mucosa. The lactase activity was virtually absent, and the maltase, isomaltase, and sucrase activities were normal. The differential diagnosis of the congenital and acquired forms is discussed, and the importance of intestinal mucosal enzyme assays for final confirmation of the diagnosis is stressed.