Lactose intolerance is the inability of adults to digest lactose, a sugar found in milk and to a lesser extent dairy products, causing side effects. It is due to a lactase deficiency, or hypolactasia. Congenital lactase deficiency prevents babies from drinking even human milk. Lactose intolerant individuals have insufficient levels of lactase, an enzyme that catalyzes the 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 after consuming significant amounts of lactose. It is common for patients with inflammatory bowel disease to experience gastrointestinal symptoms after lactose ingestion, although the prevalence of lactase deficiency in this population has not been well studied. Most mammals normally cease to produce lactase, becoming lactose intolerant, after weaning, but some human populations have developed lactase persistence, in which lactase production continues into adulthood. Research reveals intolerance is more common globally than lactase persistence, and that the variation is genetic. A study of 21 African American girls showed that 75% had some decrease in lactase activity during adolescence. The frequency of lactose intolerance ranges from 5% in Northern European to more than 90% in some African and Asian countries. 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. Although populations in Europe, India, Arabia, and Africa were first thought to have high frequencies of lactase persistence because of a single mutation, lactase persistence has been traced to a number of mutations that occurred independently.