More evidence for the recessive inheritance of selective adult type lactose malabsorption.

Selective adult type lactose malabsorption appears in childhood or adolescence because of the great decline in jejunal lactase activity. There is strong evidence that this is a genetically determined disorder. Specifically, selective adult type lactose malabsorptions seem to be inherited by a single autosomal recessive gene. In the present prospective study the transition from the state of lactose absorption to that of lactose malabsorption was documented for the first time in two Finnish boys who were at risk for selective adult type lactose malabsorption because the parents of both boys had the disorder. At the age of 14 and 9 years, respectively, the boys had normal lactose absorption. Three years and 7 months and 4 years and 5 months later, respectively, the boys were shown to have lactose malabsorption. The period of documented transition averaged less than 4 years. These manifestations clearly strengthen the genetic model proposed.