> A lot of people I know recently got sick after living in Seattle for a few years and it ended up being that at least a portion of the cause was severe Vitamin D deficiency.

But vitamin deficiency is a relatively simple test, and often done in northern climates. Both my wife and I got one done as a standard blood screen by our doctor; my wife was much more deficient^ than I was, and was told to take a fairly-high vitamin D supplement for awhile. (I was just told to go outside more often.)

^one of the few times I get to say that