Don't forget seasonal affective disorder, or just the general influence on affects that cloudy weather has. Imagine waking up to streaming sunlight every morning. I can see that making a huge change in people's lives.
My understanding is that (a) utilization of dietary vitamin D is significantly lower than naturally produced D (from sun exposure) and (b) vitamin D is one of several micronutrients produced from sun exposure.
> My understanding is that (a) utilization of dietary vitamin D is significantly lower than naturally produced D (from sun exposure)...
The end product of UVB irradiated skin (i.e., sunlight) is vitamin D3. (D3 is "activated" locally in tissues utilizing it.) Ingested D3 supplements effectively add to the body's supply, though the optimum intake is controversial and varies among individuals.
One difference between sunlight and supplements is that D3 production via UVB exposure has an upper bound, only so much can be made because there's a limited amount of substrate available in skin cells to convert to vitamin D. The upshot is sunlight is "safer" re: potential toxicity of excessive vitamin D intake.
AFAIK vitamin D is unique among nutritional factors in its reliance on UVB exposure. Don't know of any other micronutrients directly related to sunlight, but possible there's something I've overlooked.
It is unfortunate that "vitamin D" is called "vitamin" since it is in fact a hormone. As with most hormones, their effect depends on many factors including the number of receptors on target cells that can change significantly increasing or decreasing sensitivity to D. That itself is poorly understood and it is unknown how long-term consumption of D supplements in what ever form with little exposure to real Sun affects sensitivity.
So the advise is to take D supplements only if recommended by a doctor.
Vitamin D (for lack of better official term) indeed has an apparent role in an amazingly broad array of body systems. I suppose calling it a "vitamin" is appropriate in environments with insufficient sunlight which includes northerly latitudes, say above the 45th parallel (where I happen to live).
Presumed deficiency of D (per lab studies) is quite common here and supplementation is usually recommended for optimum health. Besides location, lifestyle factors often reduce sun exposure increasing chances of low D level.
There are tradeoffs involved with most recommendations. The common advice to avoid UV exposure re: melanoma risk has propelled manufacture of sunscreens into a big industry. Sunscreens filter out nearly all UVB, and interfere with forming D. Ironically, low body stores of D may increase cancer risk according to some studies. BTW my dermatologist knows about that, nonetheless advocates minimizing cancer risk, worrying less about vitamin D deficiency or risks of supplementation.
Indigenous Alaskan populations were always vulnerable to D deficiency yet it was uncommon, probably associated with their diet, rich in the oils of sea-going animals, known sources of D. Out of curiosity, a while back based on published info I calculated their intake of vitamin D was roughly 1500-2000IU/day. This seems consistent with opinions of some researchers re: appropriate intake level (don't have the references at hand--I can locate them if there's interest).
The jury's still out on vitamin D in terms of its functions and dietary requirement. It is a fascinating story that with any luck will be at least better resolved in my lifetime.
Source: Me in wintertime Sweden.