Center for Human Genetics and Laboratory Diagnostics, Dr. Klein, Dr. Rost and Colleagues

Lactose Intolerance, hereditary, adult-onset [E73.1]

OMIM numbers: 223100, 603202 (LCT)

Dipl.-Biol. Birgit Busse

Scientific Background

Lactose intolerance can be an inherited or acquired disorder of the lactose metabolism. Lactose taken in from food is not broken down sufficiently due to reduced production of the digestive enzyme lactase. Undigested lactose accumulates in the colon; osmotic processes and fermentation in the intestinal flora cause abdominal complaints. Generally this form of lactose intolerance phenotypically manifests itself no earlier than from the age of six years and older.

In hereditary lactose intolerance the C/T polymorphism in the position -13910 (g.-13910C>T) in the regulatory region of the LCT gene has been strongly associated with the lactase activity. Approx. 25% of the Central and Northern European population are homozygous carriers of the inactivating C allele and are therefore affected by this form of lactose intolerance. The disorder has a considerably higher frequency in Southern Europe and other parts of the world. Inheritance is autosomal recessive, i.e. for the intolerance to cause any signs or symptoms, both alleles of the LCT gene have to be affected.

Regarding differential diagnosis, the fact that disorders such as celiac disease and Crohn’s disease may cause secondary lactose intolerance that occurs independantly from the lactase genotype should be considered. An impaired lactase secretion from the impaired intestinal epithelium causes the signs and symptoms in this form of lactose intolerance. Lactase intolerance in infants and toddlers represents no indication for testing for this hereditary form of lactose intolerance. Severe signs (uncontrollable diarrhea, failure to thrive) may indicate congenital lactose intolerance, which is caused by mutations in the LCT gene.