I routinely take 5,000 IU's of vitamin D3 in the summer alternating with 10,000 IU's beginning in September which increases my average daily dose to 7500 IU's per day in winter. I have been doing this for years.
With my last test (today) It's not enough.
Today's vitamin D test showed 58 ng/ml - down from 86 when I tested six months ago - and that is despite a lot of summer sun exposure (yep - diving).
I will be increasing my dose to a full 10,000 IU's per day for a few weeks in order to get my level back up into the 70's (ng/ml) - just in time for flu/covid season.
If I had not tested, I would have continued my protocol and likely experience a lowered D level without knowing it. Protection from flu (covid) is best at D levels greater than 55-60 ng/ml. Best levels for cancer prevention is 70 ng/ml.
So many people are deficient in D even if they supplement at the typical 400 - 2,000 IU's per day. As my own test shows, many people would need to take at least 4,000 - 6,000 IU's per day to stay in the therapeutic range.
(*note: I do take vitamin K2 both in supplement and via Natto. Vitamin K2 uses D for calcium reactions in the body - so I have no doubt maintaining my D level requires more supplement than most would need. Of course we all should be taking K2 as well)