Re: Why children under 3 do not get lactase deficiency

Hi Chen,

Your assumptions are basically correct. Human babies (and all baby mammals) express the lactase enzyme. This enzyme allows them to digest the lactose sugar in milk. But mammary glands, lactation and milk did not evolve as a food source for adult mammals, so the expression of lactase is down-regulated after the age of nursing has passed. As a result, many adult humans are lactose intolerant.

So ... if this is true, then why are there adult humans who can drink milk with no discomfort? In modern humans, lactose tolerance is generally found in the descendants of pastoralists -- people who started raising sheep and cattle after the innovation of agriculture about 10,000 years ago. These pastoralists experienced a sort of natural selection for lactose tolerance (allowing them to consume milk as adults)*, and as a result, those of us who are descended from them express lactase as adults. Interestingly, there are a few modern pastoral populations that remain lactose intolerant, but these groups ferment or otherwise process the milk that they get from their herds so that they don't have to contend with lactose.

There is a nice discussion of this in Richard Dawkins' new book, "The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution". In addition, it is also discussed in these answers from our archives : 986144654.Ge and 901562541.Bc.

Keep asking questions!

*The mutations that arose and were selected for in these populations have been recently identified (See: OMIM #22310). One in particular results in the creation of a novel enhancer element that cancels out the developmental suppression of the lactase gene, allowing its expression to persist into adulthood.