There are problems with taking high doses of Vitamin D3 that may be an allergic reaction or genetic, and controversy that the amount of non-supplement D3 consumed by people on average may be a lot higher than we think.

http://www.mgwater.com/vitamin_d.shtml http://www.livestrong.com/article/302715-risks-of-too-much-v...

(result of random refference googling)

Examine.com is a great website. In general, I would consider Vitamin D and Magnesium to be the most beneficial for a layperson without any specific health issues.

Is there an effective way to know which vitamins you're missing? Can full blood tests help tell you which vitamins you should be taking daily?

It would be so fantastic if we could just have a chip implanted to continuously measure this stuff, then download it and analyze on a daily, weekly, monthly and annual basis. There's so much cool science to be done here, especially if this is done on a global scale.

Yes. There are common panels available at Quest/Labcorp that will measure critical nutrients such as Vitamin D or Vitamin B12. In addition, there is a more comprehensive test called ION available from Metametrix.