Got Milk? Lactose Tolerance in Adults

May I have a glass of milk, please?

Lactose tolerance is a genetic trait, it depends on your DNA that you inherited from your parents. If you descend from a population that began domesticating cattle 10 000 years ago you probably can digest milk effectively into adulthood. Here's the story of humans and milk.

Once upon a time, before we had pet cows.

We could all drink milk when we were babies, since the day we began our journey as warm-blooded mammals.  But unless you were that creepy kid in Game of Thrones you stopped breastfeeding before you could remember breast feeding. If you're old enough to ask for booby, you're too old for breastfeeding, don't fact check that.

Consequenly, the LCT gene (the one that lets us drink milk without throwing up everywhere) was switched off before adulthood because we didn’t need it.  

Then things changed roughly 10 thousand years ago. Some human tribes become pastoralists  and kept cows. They discovered a regular source of calcium, vitamins and protein. It was easily tapped, the humans were literally milking the cows for all they were worth.  

If you were able to digest it you would have a better chance of surviving.  Specific individuals developed a mutation in the LCT gene that made them lactose persistent- geek talk for “can eat cheese and stuff". These humans had a selective advantage.  

Modern day populations like the Masai Mara and many Europeans descend from a pastoral society.  About 80% of the people in these groups have the lactose persistent allele.  Interestingly, they were evolved completely independently which is a fine example of convergent evolution.

Will my child be lactose intolerant?

If you’re a parent reading this you might be wondering what the chances of your child being lactose intolerant are.  Check out the table below as a guideline.  

Remember to look at the population you descend from and not where you currently reside.  If you’re a white guy teaching English in China that does not make you good at maths.  

If you want to be sure you could do a DNA test, we know where these genes are found (Chromosome 2).  Or you could just not worry about it now and wait.  Then one day when your pubescent teenager child starts blaming you because he can’t eat ice cream you can say “It’s not all my fault."  Even though it is.

*Disclaimer: The cis acting elements and not the genes themselves are largely responsible for lactose persistence but just keeping it simple.

Extra reading

Bersaglieri, T et a helluva lot of people (2004) ‘Genetic Signatures of Strong Recent Positive Selection at the Lactase Gene’ American Journal of Human Genetics 74 (6) p. 1111 - 1120 

Tishkoff, SA et a helluva lot of people (link to all the other names), 2007 ‘Convergent adaptation of human lactose persistence in Africa and Europe’. Nature 39(1) p. 31 - 40