A little while back, there was this submission to hacker news that didn't get much traction/attention, but which was about a research study that seemed to suggest that Vitamin D deficiency was a symptom of an underlying problem and just supplementing with Vitamin D was not sufficient as a fix and that the underlying cause of the deficiency the study said could be inflammation related to bacterial infection.

https://link.springer.com/article/10.1007%2Fs00011-014-0755-...