There can't be a recommended daily dose for everyone, every body is different. I quote:
"... in a correct analysis of the data used by the Institute of Medicine, it was found that 8895 IU/d was needed for 97.5% of individuals to achieve values ≥50 nmol/L."
That just means there are some people who need a very high dose. For comparison, 4000IU/d gets me well over 50nmol/L, this is also the "generally considered safe" upper limit. There was a case reported on HN where 5000IU/daily caused issues with calcium levels. You gotta get your bloodwork done.
50 nmol/l (20 ng/ml) is right in the middle of the 'insufficient' range. Did you mean ng/ml? The upper limit for most labs is between 175 and 200 nmol/l.
In my personal example, taking about 6000 UI (plus 300 mcg K2) for a year put me just under 200 nmol/l. I did an experiment and stopped taking it for 2 months (but kept taking K2). After testing again I had dropped to 70 nmol/l (top of the insufficient range) and had mild symptoms of muscle ache and fatigue. Needless to say, I've started taking it again (and will aim for 175 nmol/l).
In general I agree and these studies are what got me looking into it in the first place. But there's a ceiling too. Dr. Deva Boone has talked about it here on HN [1].
This. I was taking 5000IU as I thought it was mostly harmless. But I also noticed some of the symptoms mentioned in that thread such as crazy heart rhythm even when resting. I’ve dropped that supplement and am just accepting the more nominal amount from diet, multivitamins, and mild sunlight exposure. Since then I haven’t had the heart rhythm issues.
[1] https://www.sciencedaily.com/releases/2015/03/150317122458.h...
[2] https://pubmed.ncbi.nlm.nih.gov/28768407/ (two years later)