No disrespect intended, I am biologically challenged, but I do not understand how anyone can have a Vitamin D deficiency when 15 minutes of sunlight a day on the skin creates all the Vitamin D anyone needs, and too much Vitamin D is very bad. I suppose it makes sense in the Winter months when everyone is inside for months, but usually, the sun shines every day, even in Winter.
I have discussed at length regarding this with endocrinologists.
There is no shortage for the source of Vitamin D - As you mentioned - UV from Sun, certain food and of course supplements. But the destination should be able to activate it. So, in my case my body is not able to activate the Vitamin D in the system properly and so is the case with most Vitamin-D deficient people.
>Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol[1].
Further, Vitamin D is necessary for calcium absorption in the gut. Hence, the lack of Vitamin D causes bone problems among other issues.
[1]https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona...
https://jaoa.org/article.aspx?articleid=2635300
https://www.grassrootshealth.net/document/sunshine-calendar/
Serum levels of Vitamin D also do not plummet with the winter months - it's more of a slow dip, and the seasonal low levels do not seem to negatively effect most adults.
It simply does not follow from "the sun shines" that "everyone basks in it all the time".
Not in Vancouver! And even on the few sunny winter days, the full daylight hours might typically be spent in an office. Even the commute happens in the dark.
The further north, the less days of direct sunlight. Exposure would have to also be all over, but most are covered by clothing.