Vitamin D Supplementing Gotchas

Is there a evolutionary adaptation to lower UV radiation at northern latitudes?  Gerdes has hypothesized that apo E4 allele carriers have less of a need for UV radiation derived vitamin D due to internal adaptations to preserving vitamin blood levels and maximizing intestinal absorption from dietary sources.
…
For those supplementing vitamin D exogenously, care and caution for adverse effects should be monitored. Sunlight derived vitamin D can be shut off -- we have enzymes and systems that control blood/cellular levels, however for supplementation just as taking a birth control or exogenous hormone medication, what goes in, stays in. Previously I listed contraindications for vitamin d supplementation ((a) hypomagnesemia -- get mag up before supplementation because vitamin D will lower serum Mag; (b) sarcoidosis or other condition associated with elevated 25OHD or 1,25OHD3). Now I would add those with E4 should like monitor closely and avoid supratherapeutic levels which probably need to be addressed on an individual basis. With E4 there may be suggestions that intracellular 1,25OHD3 may be higher and this would not necessarily be reflected in serum levels (just like Mag levels are not, intracellular $$$$$ tests are required to accurately assess). Supratherapeutic needs to be individually defined...