I think it’s an established fact that in the northern latitudes in winter Vitamin D creation by exposure of skin to sunlight is severely limited, even if you have light coloured skin. Thus cod-liver oil etc…
However, what about being in snowy areas where the sunlight is magnified and one can even get a tan skin. Is the tan skin a sign that Vitamin D is being created?
Sure, but that’s rare for winter conditions.
Part of the problem at northern latitudes is that it’s cold outside. People tend to cover up with insulating layers of clothes, minimizing the amount of skin exposed to the sun’s UV rays, and therefore minimizing the amount of vitamin D they produce. If you do get a tan, likely it’s only on your face. The skin on your face will produce vitamin D, but is that enough exposed area to meet your body’s vitamin D needs? IANAD, but I’d guess no, not unless you’re out skiing for a couple of hours every day.
As long as the sun is up, going outside in shorts and a t-shirt for 15 minutes twice a day will produce enough vitamin D, regardless of latitude. (no sunscreen, either)
it’s -3F out here right now. Ain’t gonna happen!
AS for me, personally, I get sunburned in about 10 minutes without sunscreen. I have avoided sun exposure for a long time. I take a vitamin D3 supplement instead.