Lactase persistence is the ability to digest milk that some people retain into adulthood.
Lactase is the enzyme produced by the human body to break down lactose, the sugar molecule of milk. Lactase is produced in the small intestine and breaks down the lactose in milk and milk products into glucose and galactose, which are easily digested by humans. Lactase production normally ceases before adulthood, with the result that the lactose cannot be broken down, and this in turn often results in symptoms such as nausea, bloating, and diarrhea if milk is ingested.
A number of genetic mutations, however, result in this normal "switching off" of lactase production not occurring, with the result that these affected humans are able to continue digesting milk into adulthood. These mutations occur particularly in northern Europeans, where some 95 percent of people carry one of the mutations. Some African and Asian groups, on the other hand, generally do not carry the mutation, which only occurs in about ten percent of the population.
As the ability to digest milk was thought to be "normal", the inability to do so was referred to as "lactose intolerance", but now the term "lactase persistence" is used to describe those who have not had lactase production turned off.