The argument is generally: Vitamin D has a lot of complex interactions in the body and is fat-soluble. That makes it very easy to overdose, and the recommended

The argument is generally: Vitamin D has a lot of complex interactions in the body and is fat-soluble. That makes it very easy to overdose, and the recommended doses are substantially higher than normal dietary intake. Most studies on vitamin D don't really differentiate between vitamin D from supplementation and vitamin D from going outside, meaning that the protective effect might be from the activity not the vitamin. If everyone takes 5K IU/day, then you'd expect issues from about half of people (if baseline of 50% of people are low is correct). Those issues are nontrivial. I personally think that most people (especially those who spend a lot of time indoors) should probably supplement, and most of the reaction is overstated.