Vitamin D deficiency is a widespread problem

The article suggests drinking 1 quart of milk a day to get the minimum requirement. That's almost a litre (as per google). In most 3rd world countries, people don't get that much, and we don't have Vit D deficiency.

The article makes me wonder -- if you have to take supplements to reach the recommended daily amount, are we sure the RDA is correct ? How have people managed for thousands of years ?

From what I've been reading this is more of a problem in northern latitudes where the Sun comes out for a few hours only, for half the year.