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Q: Why do some people become lactose intolerant as they age?
A: All of us are born with the ability to make an enzyme called lactase, which helps our small intestines digest the otherwise unwieldy sugar lactose found in milk. Many people’s lactase levels decline when they’re a few years old, a hardwired phenomenon that depends on a person’s genetic background — most Northern Europeans maintain their lactase levels while most people of African and Asian descent don’t. “Most of the world’s population has low lactase levels as adults — this is definitely not a disease,” says Richard Grand, a gastroenterologist at Boston Children’s Hospital.