Whether you can digest milk comfortably after childhood is a genetic fluke. For many people, the ability to produce lactase--the enzyme that allows the body to break down lactase, the sugar in milk--disappears after childhood, when we no longer need to survive on our mother's milk.
Lactase persistence--the gene that allows about a third of adults to drink milk without major digestive pains--tends to break down geographically, as you can see in this infographic from Nature's history of milk tolerance. It's largely a European phenomenon, evolving from a single genetic mutation that occurred less than 10,000 years ago.
As Nature explains:
Researchers estimate that the allele for lactase persistence might have popped up as recently as 7,500 years ago, starting in Hungary. The small pockets of milk tolerance in the Middle East, West Africa and southern Asia are thought to be part of different genetic mutations. (The data for this infographic came from a 2012 paper on the evolution of lactase persistence in Europe, which explains why it leaves North and South America out.)
Read more about the history of milk tolerance here.
I am baffled by lack of info people making these maps have when they make them. Whenever I see a map of lactase tollerance made by western scientists they show Balkans as a place where about half of people can't drink milk, when in reality for the 20 years of my life I have never met a person that is lactase intollerant. Science is good, but only with right informations!
As a Dutchman I've drunk a quart of full milk every day all my life. I'm 65 years old now and have never had any problem with it. Together with a hefty chunk of 40+ Dutch cheese at least twice a week. Milk is an important source of protein, vitamins and, above all, calcium and other trace elements. Every Dutchman (the original ones, not the imported) drinks milk with their lunch. Hey, we're the tallest people in the world. On the other hand, Latins (I've been living in Mexico for over 30 years) are infamously lactose intolerant. They get bloated, have cramps and diarrhea when consuming dairy products.
I'm Chinese and consume dairy product regularly up to nearly a 1/2 gallon or every other day. Can consume 3/4 with cereal without any side effect except taking a pee. Is this normal?
or have I just built a tolerance to drinking milk and eating cheese?