Congenital lactase deficiency is a severe gastrointestinal disorder characterized by watery diarrhea in infants fed with breast milk or other lactose-containing formulas.

In humans, the activities of lactase and most of the other digestive hydrolases are maximal at birth. The majority of the world's human population experiences a decline in production of the digestive enzyme lactase-phlorizin hydrolase during maturation, with the age of onset ranging from the toddler years to young adulthood. Due to the reduced lactase level, lactose present in dairy products cannot be digested in the small intestine and instead is fermented by bacteria in the distal ileum and colon. The fermentative products result in symptoms of diarrhea, gas bloat, flatulence, and abdominal pain. However, in a minority of adults, high levels of lactase activity persist in adulthood. Lactase persistence is a heritable autosomal dominant condition that results in a sustained ability to digest the milk sugar lactose throughout adulthood (Olds and Sibley, 2003).