Lactose is the sugar in milk. Your body can’t use lactose directly, so lactose gets broken down into simpler sugars with the enzyme lactase. Mammals naturally produce lactase during infancy to nurse from their mothers, but then stop producing it as they grow older because originally they would not consume milk ever again. Which is part of the supporting evidence for those against drinking milk/consuming dairy – we’re the only mammal to continue to drink milk after infancy. But we’re also the one of the few things that uses tools, or are capable of speech. All courtesy of evolution… 😉
As we evolved, we also got into agriculture and domesticating milk-producing animals such as goats or cows – gaining the ability to consume milk as adults. Our original state did not allow us to do that, but some individuals possessed a mutation wherein they would continue producing lactase as adults and thus allowing them to consume milk. In some cultures that trait was very valuable, resulting in increased survival and reproduction, so the trait became very common and eventually the norm. In other cultures the trait had no net gain and therefore does not propagate, and so not take hold. What makes adult lactase production worthless for East Asians? It has to do with geography and available sources of calcium and vitamin D.
You need calcium in your diet. You can get that calcium from dairy products (milk, yogurt, cheese), or tofu, sardines and leafy green vegetables. To make use of calcium your body also needs either vitamin D or lactose. You can get vitamin D from salmon, sardines, milk, tuna, eggs, or Shiitake mushrooms …or your body can make it when exposed to sunlight. Without vitamin D, lactose assists with the use of calcium. So, cultures with easy access to leafy greens plus sunlight or fish, calcium is taken care of and milk has no advantage. Cultures without access to leafy greens, sunlight or seafood need dairy either as a source of calcium, lactose, or both.
There is a difference between dairy that’s fermented (yogurt or most cheese) or unfermented (straight up milk). Fermented dairy products still have the calcium, but the lactose is broken down into simpler sugars, so lactase is not necessary to digest it. I suggest a study with a sample of at least 10 lactose intolerant people to test various fermented dairy products… and only one bathroom. You could call the paper “Insane in the Methane”… A culture with access to fish or sunlight but not leafy greens would benefit greatly from keeping dairy animals, but don’t benefit from still creating lactase as adults. They consume the milk as yogurt or cheese to get all the calcium they need and make use of that calcium, thanks to vitamin D.
While most attribute lactose tolerance to Northern Europeans, they were not the first people to have cheese – nor were they the originators of western civilization. Agriculture and domestication began in the Near East (Ancient Iraq, Syria etc) and they utilized their sheep (the first domesticated herd animal) for milk and cheese, while creating massive cities and infrastructure while the Europeans were sitting in tents or mud huts hunting deer. Sure, groups like the Scythians who were a nuisance to the Assyrians existed, but they never had infrastructure and true “civilization”. Civilization (and cheese) originated in the Ancient Near East. The Greeks then borrowed civilization from the Ancient Near East, and later the Romans borrowed civilization from the Greeks – spreading civilization to Northern Europe. [insert Life of Brian reference here]
Only the Asians living current day China, specifically far inland and to the north, would have an issue with lactose. Those peoples who became the Mongols, who did consume dairy. Furthermore even fermented dairy never took hold in Chinese culture because of their trade networks – the Chinese were able to obtain their labor animals from other cultures (Tibetans, Mongols, etc). They did not breed their own cows or goats – pigs were the primary meat animal raised. With no need for dairy, and without really having it around in the first place, they developed into a culture with virtually zero dairy of any type. Chinese culture influenced many others in Asia.
TLDR: It’s not that surprising when you realize that roughly 25% of the world’s population is lactose tolerant. Most Asian populations are lactose intolerant.