There is not, and never has been a culture that eats a vegan diet.

Correct. Veganism started around 100 years ago by a monastery of monks. The leaders noticed that eliminating meat from the diet made the males less interested in sex, which is what they wanted in a monastery. Kellogg would later learn about this, and implement veganism into his religious cult because he thought that all sex was a sin. Kellogg was a really weird mofo.

Maybe some people have trouble with milk

Milk has a sugar, called lactose. When we are born, we have an enzyme in our body called lactase. Lactase is the enzyme needed to break down the sugar lactose.

Raw milk includes lactase in it, but the pasteurization process kills the lactase.

As adults, some people lose the lactase enzyme, while others retain it. This is why the "lactose intolerant" people have a hard time digesting pasteurized milk in the stores. They don't have the lactase enzyme in their body, and the pasteurization process has killed off the lactase that was in the raw milk.

But they can ususally drink raw milk. They might have to start with a very small amount and work their way up to larger amounts, but they should be able to digest raw milk.