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People like me who live in the northern part of the world, often get a bit depressed at this time of the year, when it's essentially dark when you leave for work and when you get home from work. This has been blamed on vitamin D deficiency.

Are there any studies on supplementation for this? I imagine it would be quite simple to do a double blind placebo experiment comparing vitamin D supplement with a control, checking for general mood.

What I'm looking for is something that actually "proves" the whole sequence so to speak. From intake of vitamin D to better mood. If I end up raising my vitamin D levels without any effect on mood, that's kind of useless.

Compro01
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Vitamin D supplementation at the level of 800 IU per day does not appear to be an effective treatment for Seasonal Affective Disorder in older women.

"Winter depression" is properly known as "Seasonal Affective Disorder" (SAD) or "major recurrent depressive disorder with seasonal pattern". As the name suggests, it's recurrent depression that occurs and remits in sync with the seasons, typically occurring in the winter and remitting in the summer.

Typical treatments include light therapy (involving the daily use of a bright, broad-spectrum light to simulate summertime sunlight. Related question/answer here), cognitive-behavioural therapy, and/or anti-depressant medication, though possible links between vitamin D deficiency (which typically occurs due to lack of UVB exposure, which is common at high latitudes in the winter) and SAD have been investigated.

In a (admittedly small, only 30 participants) study comparing Seasonal Affective Disorder patients against normal controls, no difference in serum vitamin D levels was found.

Another study (though again, small, 29 participants) found the same and also compared the serum vitamin D levels in the SAD group against the same group's levels during the summer. No difference was found there either, suggesting that vitamin D deficiency is not linked to SAD.

A larger study (over 3000 participants) in China also found no link between depressive symptoms and serum vitamin D levels.

Another large (over 2000 participants) study in the UK investigating vitamin D supplementation's effect on preventing SAD symptoms found no significant effect.

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Compro01
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    The first two studies used people who are at risk for SAD (northern latitudes), so their serum vitamin D levels could be all-around low, not just dipping during the winter months. They also didn't check whether or not supplementation would help. The third study looked at depressive symptoms in general, not focusing on SAD. The last link seems the only one directly relevant to this question, testing the effects of supplementation on preventing SAD symptoms. –  Nov 07 '13 at 16:44
  • @Articuno - Yes, but the 2nd study compared the SAD group against itself during summertime remission and there was no difference between them. If D deficiency was the cause of the SAD, a consistently-low serum level should have resulted in year-round depression, not merely during the winter months. – Compro01 Nov 07 '13 at 17:40
  • If D deficiency was *the* cause of the SAD, a consistently-low serum level should have resulted in year-round depression. But, if D deficiency was *one of several jointly necessary causes*, a consistently low serum level might not result in year-round depression, and supplementation could reduce the risk of SAD. The question doesn't suppose that D deficiency is *the* cause, but only asks if supplementation is beneficial. –  Nov 07 '13 at 17:43
  • Also, the last study (which I do believe is relevant) supplemented only at a level of 800 IU, which is starting to be considered by many to be a low level of supplementation. [For example](http://www.mayoclinic.com/health/vitamin-d/NS_patient-vitamind/DSECTION=dosing): "*For deficiency, at least 1,000 IU (25 micrograms) of vitamin D has been taken by mouth daily (or 8,400 IU of vitamin D3 weekly)*". –  Nov 07 '13 at 18:02
  • To improve this answer, I would not make such a strong conclusion as you do in your first sentence, and I would point out that only the last study is relevant to the question that was asked. I've edited as an example, but don't mean to hijack the answer. Feel free to revert. –  Nov 07 '13 at 18:09
  • @Articuno - Ah, I wasn't aware of the lowness of the dose. Thanks for the edit in that case. – Compro01 Nov 07 '13 at 19:29