People who are lactose intolerant cannot extract energy from milk because they are missing an enzyme. Lactose is the main sugar found in milk and other dairy products. Those who have lactose intolerance find it hard to digest the sugar, usually because their small intestine doesn’t make enough lactase, the enzyme which digests lactose. Around 60% of adult humans are lactose intolerant or have reduced lactose tolerance after infancy. But tolerance varies dramatically depending on geography, affecting some ethnic groups more than others. In Britain, the condition is most prevalent among the Asian and African-Caribbean communities, according to the NHS. Only one person in 50 of northern European descent has some degree of lactose intolerance, while around 90% of people from China have the condition.
Scientists believe this because places in Africa and Asia historically had little access to milk, so people may not have evolved the ability to digest lactose because there was no benefit in being able to do so.