Lactose Intolerance

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. 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. This distribution is now thought to have been caused by recent natural selection favoring lactase persistant individuals in cultures that rely on dairy products. While it was first thought that this would mean that populations in Europe, India, and Africa had high frequencies of lactase persistence because of a particular mutation, it has now been shown that lactase persistence is caused by several independently occurring mutations.