Lactose intolerant individuals have insufficient levels of lactase, an enzyme that catalyzes hydrolysis of lactose into glucose and galactose, in their digestive system. In most cases this causes symptoms which may include abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi (rumbling stomach), or vomiting^[2] after consuming significant amounts of lactose. Some studies have produced evidence that milk consumption by lactose intolerant individuals may be a significant cause of inflammatory bowel disease.^[3][4]

Most mammals normally cease to produce lactase, becoming lactose intolerant, after weaning,^[5] but some human populations have developed lactase persistence, in which lactase production continues into adulthood. It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood.^[6] The frequency of decreased lactase activity ranges from 5% in northern Europe through 71% for Sicily to more than 90% in some African and Asian countries.^[7] This distribution is now thought to have been caused by recent natural selection favoring lactase-persistent individuals in cultures in which dairy products are available as a food source.^[8] While it was first thought that this would mean that populations in Europe, India, Arabia and Africa had high frequencies of lactase persistence because of a particular mutation, it was later shown that lactase persistence is caused by several independently occurring mutations.^[9]