The big study on vitamin D that got everyone's attention was this one from the summer of 2007: Vitamin D and Calcium Supplementation Reduces Cancer Risk: Resul

The big study on vitamin D that got everyone's attention was this one from the summer of 2007: Vitamin D and Calcium Supplementation Reduces Cancer Risk: Results of a Randomized Trial (http://www.ajcn.org/cgi/content/abstract/85/6/1586 http://www.ajcn.org/cgi/content/abstract/85/6/1586) There is also a lot of information about vitamin D research at the Vitamin D Council website (http://www.vitamindcouncil.org/ http://www.vitamindcouncil.org/). That being said, I don't know if taking a vitamin D supplement would have any noticeable effect on your sense of well-being. I think it's probably a good idea to take one even if you don't notice a difference.