HR, if you want to get natural Vitamin D, there is ONLY ONE WAY TO GET IT. Sun exposure on your skin.
Sure, you can take supplements, but the discussion I am referring to was regarding why people's bodies produce less Vitamin D during the winter. It is because they expose less skin, and there is less sun to expose it too. Whether it's -25 or +25, it's still too cold to go out in a T-shirt for long enough even if the sun is out. You need to expose your entire torso and arms and head to direct sunlight for about 10 minutes (longer if you're darker skinned) to get a full day's dose. The less skin you expose, the longer you need to be out there. That doesn't happen in winter. Despite living in Arizona, I still take supplements, but that doesn't change the physical process by which natural Vitamin D is created in the body.