Why are some people lactose intolerant?

Lactose is the main sugar in milk, and the enzyme lactase breaks it down in the small intestine. In most mammals, including most humans, lactase production drops after weaning. Scientists estimate around 65 to 70 percent of the world’s adults have reduced ability to digest lactose. Some populations, especially in northern Europe and parts of Africa and the Middle East, carry genetic mutations for lactase persistence that spread after dairy farming began several thousand years ago. When undigested lactose reaches the large intestine, bacteria ferment it, causing gas, bloating and discomfort.