Lactose intolerance, also called lactase deficiency and hypolactasia, is the inability to digest lactose, a sugar found in milk and to a lesser extent milk-derived dairy products. Lactose intolerant individuals have insufficient levels of lactase, the enzyme that metabolizes lactose into glucose and galactose, in their digestive system. In most cases this causes symptoms such as abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi (rumbling stomach), or vomiting after consuming significant amounts of lactose. Some studies in the U.S. and elsewhere suggest that milk consumption by lactose intolerant individuals may be a significant cause of irritable bowel syndrome. Most mammals normally become lactose intolerant after weaning, 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.
Contributions by Filll, Davidruben, and Bunchofgrapes.