I have personally found vitamin D supplementation to help with my mood and energy level, at least in winter.
If you get a lot of sun on a summer day, your body can make as much as 10kIU. So supplementing at the level of around 1kIU/day, which is what I do, seems reasonable.
A friend on mine who has Multiple Sclerosis mentioned to me that it gets much worse for him in the winter, and then gets better heading into the spring and summer. IIRC low vitamin D is believed to make MS worse. He was going to try tanning beds this winter to see if it helps (usually tanning beds are bad news due to cancer risk, but in his medical case his doctor recommended it). Disclaimer: All anecdotical.
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Vitamin D is synthesized by your skin from sunlight using UVB rays. This is interesting to note, indicating that being far from the equator does not mean you cannot get enough UVB rays from sunlight during the winter:
The assumption that vitamin D levels in the population follow a latitude gradient is especially questionable in view of surveys which have shown that UVB penetrating to the earth's surface over 24 hours during the summer months in northern Canada equals or exceeds UVB penetration at the equator. Accordingly, there is sufficient opportunity during the spring, summer, and fall months at high latitude for humans to form and store vitamin D3.
Perhaps if true, that means when it's cold out people stay indoors and wear lots of clothing when outside, effectively limiting sunlight exposure, meaning less vitamin D production in the winter...
Living in the southern hemisphere (the one with the bigger ozone hole) and being an import from dark cold places I turn into a lobster with moderate sun exposure. It doesn't always get better heading away from the equator here due to ozone depletion.
Adding computers, air conditioned comfort and a complete disinterest in outdoor activities to the mix and I doubt if I get enough vitamin D to last me through the winter. Skin cancer avoidance is well understood here and drilled into kids from pre-school age but lifestyle changes may be putting some people into other risk areas.
Curiously, I have a skin condition that is treatable by UV exposure, topical vitamin D analogues and generally improves in summer yet as far as I know there is no clinical evidence that vitamin D supplements have any benefit.
If you get a lot of sun on a summer day, your body can make as much as 10kIU. So supplementing at the level of around 1kIU/day, which is what I do, seems reasonable.