There was a double-blind study which showed very significant improvements in healthy college students' self-reported cheerfulness when they took >=400 IU of vit

There was a double-blind study which showed very significant improvements in healthy college students' self-reported cheerfulness when they took >=400 IU of vitamin D in the winter. The results were surprisingly strong: http://www.springerlink.com/content/padt0xj3xk26llh3/ http://www.springerlink.com/content/padt0xj3xk26llh3/ So, I would say that taking vitamin D in the winters is a good idea.