Hacker News Reader

As much as 90% of the US population may be deficient in vitamin D, so you have a good chance of it. That said, I agree that testing is good, either before or after supplementation.

https://www.grassrootshealth.net/document/grassrootshealth-p...

Ironically, supplementing vitamin D can cause a magnesium deficiency.

I don't disagree with anything you wrote except to say that in Canada & USA the medical system (insurance, etc.) presumes everyone is vitamin D deficient, and so usually testing is not covered unless there is some other pressing concern. So, most people don't get tested.