Dude, where’s my lactose?

Those who looked at the previous post carefully may have found themselves wondering: Well now if the bacteria eat most of the lactose in the milk, does that mean people who are lactose intolerant can eat yogurt? Why yes it does. And in fact most people who are lactose intolerant do indulge freely in it. For yogurt is, in a sense, “pre-digested”, with lactic acid bacteria preemptively applying the lactase enzyme that’s absent in many humans.

Today many of us tend to think of lactose intolerance as just one more malady-du-jour, something fussy people who aren’t really sick get to complain about over brunch. But the inability to digest milk as an adult is real, and is in fact normal if you consider the human race as a whole. Among the vast majority of humans, lactase production falls off sharply around the time we’re weaned from our mother’s milk, then vanishes entirely after the age of four. Only a very few people (say one or two in ten) continue to produce lactase into adulthood.

Yet somehow, suddenly, around 7,000 years ago, something happened — at least among animal herding peoples in Eurasia. Some of them spontaneously evolved the ability to produce lactase on an ongoing basis, which meant they could go on consuming milk throughout their lives. In time this genetic mutation spread to the point where now it’s only about 10% of Eurasian adults (or people of Eurasian descent) who can’t digest milk. It’s all a pretty amazing coincidence when you stop to think about it: a random genetic mutation that allows adults to digest milk just happened to occur among people who herded milk-producing animals. Or maybe it wasn’t an accident at all. Maybe that “random” mutation is in fact concrete evidence of evolution in humans. Many scientists believe just that.

But the neat thing about yogurt is that you need not be genetically disposed to digesting dairy in order to digest it. Any human can do it. Which is why you find (and have always found, at least on the Asian continent) peoples who eat yogurt but who don’t otherwise consume much dairy.