Let's end this one with some certainty....there are so many conflicting POVs:
Is supplemental vitamin D somehow different than sunshine in that it uses up magnesium and retinol to be converted?
Do you need more supplemental retinol (vit A) when taking Vit D?
Ive seen the WAPF and Chris Masterjohn POVs, as well as John Cannell. These disagree. I've experimented personally for ~6 months now and can't come to a conclusion. I've seen other posts where people raise the issue but it's never really settled to the point where someone described the WHY for the correct point of view. I think its simple, does supplemental D make us need retinol (and why)?
I think it could make sense because the only source of vit D in nature (eggs, fish) is also coupled with vit A.
However, the other point of view would be that we need 5-10 k IU of D per day...and it is not safe (and actually antagonistic) to take higher amounts of retinol to offset that much vit D.
On top of all of this, I personally dont seem to do well taking isolated vitamin D, but am also not clear what combo of co-factors make it effective and tolerable.