The point is that being outside naked at noon every day for 15 minutes wouldn't hurt you. Therefore, a daily dose of 20,000 IU / day won't hurt you. Therefore, a 5000 IU supplement every day won't hurt you either.
I list these numbers to give a general idea of the levels of Vitamin D that people experience without oral supplementation.
My conclusion was that too many studies were being done to determine the correct or safe amount to supplement rather than testing regularly to determine a safe/optimal blood concentration.
Very excited for good prospective studies of Vitamin D.
A bag of carrots wont hurt you yet an equivalent dose of betacarotene (vit A) supplement definitely will and is strongly associated with increasing certain cancers. There's a reason you won't see those on the shelf apart from miniscule amounts in multivitamins.
Nothing in nutrition exists in a vacuum.
Because usually phisiological processes are in some kind of equilibrium and regulated, so it's not straightforward to compare it to supplements.