If you Google Vitamin D Levels you'll find lots of great information about studies that have recently been done on this. Among the best took place at the Johns Hopkins Hospital.
http://www.sciencedaily.com/releases/2008/08/080811195629.htm

Good Luck,

Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)