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. In some (rare) cases, babies have congenital lactase deficiency, which prevents them from being able to digest 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 to be more common globally than lactase persistence, and that the variation has been tied to genetics, but that the largest source of variation has been shown to be based on exposure (e.g., cultures that consume dairy). However, it is not clear whether digestion needs to be complete to avoid symptoms. The frequency of lactose intolerance ranges from 5% in Northern European to more than 90% in some African and Asian countries. Some have argued that this links intolerance to natural selection favoring lactase-persistent individuals, but it is also consistent with a physiological response to decrease lactose production when it is not needed in cultures in which dairy products not an available food source. But this is highly confounded by the fact that polymorphisms are associated with natural non-selective variation in the human genome. Many studies show that individuals with a similar genetic background can have different tolerances to lactose based on exposure more so than genetics and that one can acquire tolerance from repeated exposure, but children who immigrate into dairy rich environments tend to consume dairy products at a rate that is close to that of that population's average. 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. It is important to recognize that the genetic definition of lactose intolerance is not the same as an operative one, in which one experiences symptoms.