A couple of years ago I was sick a lot, but my symptoms were all over the place. My doc tested and tested and tested and tested. Finally, he tested for Vitaman D and I was dangerously low. As soon as I started taking over the counter supplements I felt great!
I was astounded at how easy it was to become D deficcient and how much it effected me.
I’m trying to do that now. I live in a dungeon, literally. No sunlight.