North America is quite a bit south of Europe. There are only three cities in all of North America with a population larger than 100,000 people north of London, UK.
I'm the opposite of you. I store vitamin D like crazy and overdose easily. My genetics come from very far north, all my ancestors come from north of the European continent. I sometimes take a low dose of vitamin D in the depth of winter, but getting too much sun in the summer can lead to overdose symptoms. On blood tests I test low on vitamin D, but I think that's the way my body likes it.
My partner's genetics are central American and Mediterranean. She take large doses of vitamin D to just remain in the normal range.
You raise a good point. Vitamin D is a fat soluble vitamin. So, it begs the question: do blood (mostly water) levels accurately reflect vitamin D sufficiency or deficiency?
Another snip from Google follows. Decent sources. What percentage of the US population is deficient? Is it 5%, or is it 42%? Why are these estimates, published a year apart, so different?
Well, one article may be talking about "severe" deficiency, you might say, and another about just "deficient". But what does that mean? Do all of those people with Vit D blood levels below 30 (nearly half) really have a problem?
And what is so magical about 30? Some say it is 20.
Or maybe between 20 and 50! Well, what's so magical about 20? or 30? Awfully round numbers, aren't they? Why not a normal of 18.3? Or 32.7?
Well, it's almost as though you can pick your favorite number and plug it in! Wow!
What I am getting at is, when we can't even decide on a threshold for normal, how can we determine a cause and effect association?
Now, back to Covid. Vitamin D deficiency "associated" with worse mortality and morbidity. Right? So we treat with supplementation. Does it work? Depends on who you ask.
Really, not so much. Again, supplementation doesn't seem to treat Covid in people who appear to be deficient.
Bottom line:
Experts don't agree on what blood levels constitute vitamin D deficiency.
Supplementation of people with "low" blood levels, keeps on disappointing.
I suspect that conditions that cause falls, immune deficiency, and so forth, also cause "low" blood levels of vitamin D, which may not actually reflect actual body stores.
Sorry for the rant! Cheers!