I take vitamin D3, the pharmacist recommended I take 2 1000 iu/day (2000 total) cuz of my crohn's and living in Canada where we have such long winters and therefore for many, not enough absorption through sun exposure...vitamin D is important for us CDers cuz it helps our bones, that's actually the main thing about
vitamin D, lack of vitamin D causes rickets...vitamin D helps the immune system and with those that have immune diseases...here's a good link.
http://www.mayoclinic.com/health/vitamin-d/NS_patient-vitamind
And (VERY IMPORTANT), here's a link about the dangers of taking too much vitamin D...
http://www.mayoclinic.com/health/vitamin-d-toxicity/AN02008
:)