Sunnier countries can still have large vitamin D deficient populations. I read a study regarding the correlation between mental illness and vitamin D deficiency a few years ago, and Italian patients were still often very deficient. If people don't go out in the sun enough, or wear too much sun cream, it doesn't matter how sunny it is.
That's misleading because Europe is on the western edge of the Eurasian landmass, meaning it gets the benefit of warm air heated up over the ocean. By the time the air gets to the eastern edge of a landmass it's a lot colder, which is why London has a milder average temperature than NY and Vladivostok, despite being almost 10° further north than both.
Yes, but vitamin D production relates to direct sunlight ... I think the angle of the sun and amount of atmosphere it has to go through affects UV amounts so a warmer European country does have less direct sun therefor more potential for vitamin D deficiency ...
TBH I am not super solid on every one of these details.
That's a good point, but this paper seems to indicate that the amount of sunlight required over the summer in order to build enough vitamin D to last over winter isn't much for white Caucasians in the UK - nine minutes each day at lunchtime between March and September, assuming shorts and t-shirts are worn during the summer:
Vitamin D deficiency can still occur in sunny countries. To get the adequate amount of Sun from Vitamin D takes a day out in the sun entirely as opposed to basic supplementation.