Sweden here. My partner is a doctor, she claims with assurance that you can't make a carpet decision like this. People who come from norther latitudes are used to the lack of sun, and their bodies don't need more vitamin D than is provided. Depending on your heritage, ymvm.

Swedes need as much vitamin D as anyone, but usually get it from their diet. Herring is a vitamin D gold mine, for example :)

That seems implausible. Obviously there was evolutionary pressure on northern Europeans to tolerate lower levels of sunlight and no doubt adaption happened. However, I expect even Swedes evolved in an environment that included more time outdoors than a modern Swedish office worker gets. So, in the absence of any actual evidence, I'd still expect a problem. On top of that, there is some evidence. Basically all the papers found by this search show some evidence of vitamin D deficiency in Swedish people: https://scholar.google.com/scholar?start=10&q=sweden+vitamin...

The first two articles I get from that search looks at vitamin d and the older population, and the third[1] suggests that even in sub-arctic elevations "Adequate levels were found in 79.2%, more often in women (82.7%) than in men (75.6%). Only 0.7% of the population were vitamin D3–deficient but 23.1% of men and 17.1% of women had insufficient levels." That doesn't seem too damning.

At the end of the day I don't know enough, so I'm not making the claim about Vitamin D quantities, but isn't the evidence that there was at least some form of adaptation evident based upon skin tone alone?

https://www.tandfonline.com/doi/full/10.3402/ijch.v74.27963

I already agreed there was adaption. I just can't see how there can have been adaption for an _indoor_ life at northern latitudes. That's doubly hard, compared to a traditional life in northern latitudes, right? Although, to argue against myself, I guess if you go far enough north, people won't be getting any sunlight in the winter even if they are outdoors all the time. People there must have adapted to that. I guess by eating things with lots of vitamin D.

I wonder, have any northern populations developed an instinctive winter hibernation adaptation?

I love the phrase "carpet decision". Although we don't have it in English, I'm guessing it's roughly equivalent to "blanket statement", something that applies in all cases without exception?

Very much what I meant. Happy the meaning went through.

It may still be worth taking a blood test, especially if you have some symptoms that match vitamin D deficiency.

Most people in North America are not from northern latitudes genetically unless we are talking about the UK and Germany and even then..

Huh? Europe is generally populated further north than North America is.

It doesn't work in the binary fashion you described. Low sun angles have two results: Cosine effect and greater atmospheric absorption. The cosine effect decreases the watts per square meter you receive, if your skin is not normal (90 degrees) to the incident sunlight. Greater atmospheric absorption means more UVB is absorbed by the atmosphere before it gets to you, but some still gets through, as anyone who has ever been sunburned in the winter can attest. Higher altitudes and snow on the ground both increase the amount of UVB you receive, even when the sun is low.

Do you have a source for that? Even at a lower angle you still get some UV B and energy from the sun.

Yep - I learned that a few years ago.