A 2016 study of over 60,000 participants from 89 countries found regional prevalence of lactose malabsorption was "64% (54–74) in Asia (except Middle East), 47% (33–61) in eastern Europe, Russia, and former Soviet Republics, 38% (CI 18–57) in Latin America, 70% (57–83) in the Middle East, 66% (45–88) in northern Africa, 42% (13–71) in northern America, 45% (19–71) in Oceania, 63% (54–72) in sub-Saharan Africa, and 28% (19–37) in northern, southern and western Europe." According to Johns Hopkins Medicine, lactose intolerance is more common in Asian Americans, African Americans, Mexican Americans, and Native Americans. Analysis of the DNA of 94 ancient skeletons in Europe and Russia concluded that the mutation for lactose tolerance appeared about 4,300 years ago and spread throughout the European population.Some human populations have developed lactase persistence, in which lactase production continues into adulthood probably as a response to the benefits of being able to digest milk from farm animals. 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 lactase production when it is not needed in cultures in which dairy products are not an available food source. Although populations in Europe, India, Arabia, and Africa were first thought to have high rates of lactase persistence because of a single mutation, lactase persistence has been traced to a number of mutations that occurred independently.
Lactose intolerance is a common condition caused by a decreased ability to digest lactose, a sugar found in dairy products. Those affected vary in the amount of lactose they can tolerate before symptoms develop. Symptoms may include abdominal pain, bloating, diarrhea, flatulence, and nausea. These symptoms typically start thirty minutes to two hours after eating or drinking milk-based food. Their severity typically depends on the amount a person eats or drinks. Lactose intolerance does not cause damage to the gastrointestinal tract.Lactose intolerance is due to the lack of the enzyme lactase in the small intestines to break lactose down into glucose and galactose. There are four types: primary, secondary, developmental, and congenital. Primary lactose intolerance occurs as the amount of lactase declines as people age. Secondary lactose intolerance is due to injury to the small intestine. Such injury could be the result of infection, celiac disease, inflammatory bowel disease, or other diseases. Developmental lactose intolerance may occur in premature babies and usually improves over a short period of time. Congenital lactose intolerance is an extremely rare genetic disorder in which little or no lactase is made from birth. The onset of primary lactose intolerance, the most common type, is typically in late childhood or early adulthood, but prevalence increases with age.Diagnosis may be confirmed if symptoms resolve following eliminating lactose from the diet. Other supporting tests include a hydrogen breath test and a stool acidity test. Other conditions that may produce similar symptoms include irritable bowel syndrome, celiac disease, and inflammatory bowel disease.