Why we drink milk – and why some people can't

Our relationship with milk begins as soon as we're born. Baby mammals, including us humans, drink our mother's breast milk for nourishment. But after weaning, most mammals' bodies begin making less and less of a kind of digestive enzyme called lactase. Without enough lactase, we become lactose intolerant – unable to properly break down and digest lactose, milk's natural sugar.

In fact, a considerable percentage of people are lactose intolerant, which causes them to experience symptoms – including abdominal pain and bloating, diarrhea, gas, and nausea – within 30 minutes to 2 hours after consuming milk or milk products. So why do so many of us continue to drink milk?

Some theorize that the reasons why may be traced to our geographical origins. Rates of lactose intolerance are especially high among people of Asian, African, Indigenous, and Hispanic origin. And rates are quite low among those of Northern European ancestry. The thinking goes that sunny days were scarce in Northern Europe, depriving people of their main source of vitamin D. Run low on vitamin D, and your body is less efficient at absorbing the all-important calcium you consume. Milk provides a good deal of both calcium and vitamin D. So, it could be that over the generations, Northern Europeans adapted genetically to be able to digest milk so that they could benefit from its nutrients.

Experts wondered, too, why levels of lactose intolerance tended to be low in Northern Africa, when they were so high in other parts of the continent. As it turns out, this region is largely pastoral, meaning that the people keep domesticated mammals and take as much nourishment from their animals as possible. Milk is, after all, a plentiful source of protein. So, like the Northern Europeans, perhaps Northern Africans developed a tolerance to lactose in order to get maximum nutritional benefit from their animals.

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