MD's that are Rheumatologists have their patients with RA take 50,000 IU/week or 9,000 IU/day.
The COVID19 MD consensus for people with dark skin, people indoors too much is 5,000 IU/day to be ready to battle Covid19 and win/live by not ending up in the Hospital and on a ventilator.
I have been in touch with MDs/virologists world wide on this discussing it since March.
If one is 20 YO, and a life gourd in the sun down south 2,000 IU is more than enough.
There is no one size fits all.