Also, a possible mechanism:
http://www.ncbi.nlm.nih.gov/pubmed/9792205It's hard to evaluate many studies as either the wrong test is used, the wrong type of D, or the levels are too low. For example, life guards in San Diego have levels of 81 ng/ml, but so far, the totalitarian medical establishment :
only recognizes much lower levels, less than 50ng/ml.
I have read studies reflecting gene expression requiring D in the brain that has influenced levels:
Increases serotonin production. The role of serotonin in mental health is now well established. One study found that summer sunlight increased brain serotonin levels twice as much as winter sunlight8
"One theory for this is that vitamin D stimulates the brain to produce more serotonin. In a wintertime experiment, serum vitamin D levels doubled in six months through supplementation and dramatically increased their scores on a wellbeing assessment when two groups were given either 1,000 IU or 4,000 IU of vitamin D daily." And although both groups improved, the higher dose produced better results. In another investigation, researchers studied the association between vitamin D levels and the risk of mood disorders in the elderly. The results were impressive."
- Marc Sorenson, Solar Power For Optimal Health
Hope this helps.
If you're really interested, scroll down to where it talks about Vitamin D and the Brain:
http://books.google.com/books?id=oSl8VY33634C&pg=PA156&lpg=PA156&dq=vitamin+d+brain+gene+expression&source=bl&ots=E51raGP6s_&sig=0FG6dN5viBz_NDcPMFd2M3pLxlo&hl=en&ei=GZpgTfydEIqisAPWkf3fCA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwADgK#v=onepage&q=vitamin%20d%20brain%20gene%20expression&f=false