It turns out that Vitamin D is not a 'vitamin', but an essential hormone that regulates many things in our bodies. We very rarely get Vit. D in our diet (cod liver oil) and recent discoveries show its importance to our health.
A very informative video (80 minutes) is free online and will shake your faith in the USDA's 'minimum daily requirements'.
From the University of Wisconsin, which is one of the very first research facilities in the world, "Vitamin D: The New Old Natural Wonder Drug", 3/7/06.
The first 30 minutes is about the history of dietary research (yawn) but it is actually none the less interesting.