[nn3]

Why? The younger doctor is likely just reading wikipedia too.

And the older ones are probably just telling you what was the conventional wisdom when they did their education 20 years ago.

[bird_monster]

You're asking "Why" as in "Why shouldn't we listen to untrained professionals' opinions on supplement dosages" ?

Also this person did not wikipedia what they posted, as wikipedia also lists tolerable upperbounds for adults as 4000 IU / day.