Lactose intolerance is an impaired ability to digest lactose. Lactose is normally broken down by lactase, an enzyme which is produced by cells in the lining of the small intestine. Lactose intolerance in adults is different from congenital lactase deficiency, which is a disorder in which infants are unable to break down lactose. Lactose intolerance in adulthood is caused by reduced production of lactase after infancy, called lactase non-persistence or adult type hypolactasia and is the most common type of lactose intolerance. The expression of the enzyme lactase, encoded by the LCT gene, is regulated by a DNA sequence located nearby, MCM6. Changes in this element have led to sustained lactase production in the small intestine and the ability to digest lactose throughout life. People without these changes have a reduced ability to digest lactose as they get older, resulting in the signs and symptoms of lactose intolerance. Most people with lactase non-persistence retain some lactase activity, resulting in different capabilities of digesting lactose. Often there is a difference seen between digesting fresh milk and other dairy products, as these have been processed and a large part of the lactose has been already broken down. Real-time PCR detection methods have been proposed for the detection and typing of the 13910C/T and 13915T/G mutations, which are associated with lactase persistence.
Moreover, ‐14010 G/C and ‐14009 T/G variants were later shown to also influence lactase persistence.