So how exactly does the diet of Mongolians prove lactase persistence is not fundamentally genetic? Lactose intolerance isn't something like celiacs where a dash of the substance causes adverse reactions. Most of Asia still sell non-lactose-free dairy in supermarkets. Look up any Youtube video of Korean street foods and you will see dairy products such as processed cheese, yogurt, mil, and ice cream are ubiquitous in Korea. That doesn't detract from the fact that the vast majority of the population are lactose intolerant in the definition that they no longer produce enzymes that can break down lactose once reaching adulthood.
> So how exactly does the diet of Mongolians prove lactase persistence is not fundamentally genetic?
It doesn't, but I think it indicates that genetic lactase persistence is not the only factor that affects lactose intolerance. Otherwise, how do you call a lactose-intolerant person that can tolerate significant amounts of lactose?
Koreans don't get a third of their calories from dairy products, though. Whereas if Mongolians get ~30% of their calories from dairy on average (which is credible), that would mean they are getting far more than that in summer, since dairy consumption is largely seasonal. We're talking prodigious amounts by the standards of any culture, not a dash. That consumption includes both significant unfermented dairy (milk, milk tea, clotted cream) and dairy products fermented in a wide variety of ways.
Are lactose intolerance symptoms widespread among Mongolians? If not, and if Mongolians are really genetically lactose intolerant, then it is worth asking whether the fermented dairy foods (as opposed to kimchi) they consume are mitigating that intolerance.
