Lactose intolerance is a condition wherein an individual cannot digest lactose, a sugar found in milk. When a lactose intolerant individual ingests any form of milk, he or she will get a stomachache. Other symptoms include cramps and bloating. Such individuals do not produce an enzyme called lactase that breaks down lactose after weaning.[1]
The proportion of lactose intolerant individuals varies by region on the planet. 99% of Chinese are lactose intolerant, whereas only 10% of Americans are. Recent research indicates that lactose intolerance is high where cows were not domesticated. [2]At higher latitudes, lactose intolerance decreases. As average temperature increases, lactose intolerance increases.[3]
Lactose tolerance is determined by a recessive gene. Researchers have recently identified a DNA mutation that allows people to tolerate milk.[4] Lactose tolerance is often called lactose persistence.