"Vitamin" D

I'm a listener of Steve Gibson and Leo Laporte's Security Now podcast, have been for years. And I trust Steve's reasoning and logic when it comes to Security stuff. So there's no reason to think that he couldn't apply that same logic and reason to any other subject he puts his mind to..
So here's some research he did on vitamin D. Its not actually a vitamin at all. If you stand by the strict definition of the term, a vitamin is something that your body can't make naturally and instead you have to get through nutritional sources.
I've been taking cod liver oil for years, and I am convinced its the reason why I don't get many flu's or colds. Living in northern climates we don't get enough vitamin D from the Sun. I've started taking supplements and I think everyone living anywhere thats not sunny and warm year round probably should too. Be aware though, vitamin D in large doses can be toxic so if you start taking it and you are not sure about how much to take, best to ask your doctor for a blood test to monitor your levels and they can tell you if you are deficient.

http://www.grc.com/health/Vitamin-D.htm