I thought this was going to be the easiest thing to do. Well even on PUBMED which only has peer reviewed published medical research there are over 85,000 articles on vitamin D, 22,000 articles on vitamin D deficiency, and 828 articles on vitamin D and covid. After 2 hours of reading, I think I found one that encapsulates a lot and gives some basic advice which is what my goal is. Its a review of the preliminary research as it applies to Covid which is top of mind for now. I can put up other summaries on other diseases in the future. But let me start with the conclusion first, and I’ll put the whole article here ( at the risk of getting sued) for those who like to read it for themselves.

Actually my summary of the conclusion so you dont even have to read it. It recommends taking 2000 IU of vitamin D a day. Its a summary of 141 articles and 47 studies.

“CONCLUSION
The 141 articles (Table A1) presenting primarily biological
plausibility evidence overwhelmingly support the assertions that
vitamin D sufficiency increases resistance to viral infections and
helps prevent every symptom of severe COVID-19 that results in
fatalities. They show that vitamin D deficiency can also explain
every major risk factor, including the mystery of why children
seem relatively protected and why males, the elderly, and people
with naturally melanin-rich skin are especially vulnerable.
The 47 studies (Table A2) summarized here demonstrate that
vitamin D deficiency explains the geographical differences in
COVID-19 case and fatality rates. They provide overwhelming
correlational evidence for the hypothesis, and causal evidence
as well. COVID-19 mortality was predicted by vitamin D in 16
studies (4, 14, 24, 27, 63, 122, 127, 146, 150, 182, 197, 206, 209–
211, 219) and vitamin D levels or sunlight predicted contracting
COVID-19 in 17 (34, 122, 124, 150, 197, 198, 200, 201, 204,
205, 207–209, 211, 216, 221, 222). Both causal modeling studies
and eight chart reviews demonstrated that lower 25(OH)D was
linearly associated with more severe COVID-19 outcomes (3–5,
63, 157, 211, 215, 217, 219, 221).
None of the four objections to recommending universal
vitamin D supplements are supported by the evidence. The
exhaustive literature search found no vitamin D proponent who
suggested that COVID-19 could be completely eliminated with
supplementation. Rather than overstating the case, they present
compelling evidence that vitamin D deficiency is one factor
which increases risk for COVID-19 infection and progression.
Although overdoses are theoretically possible, they are highly
improbable. The recommended dose by consensus, 2,000 IU/day
for adults, is 1/20th the amount that must be taken for many
months to risk toxicity (28, 88, 91, 92). The evidence strongly
suggests that vitamin D deficiency is an easily modifiable risk
factor and correcting it is potentially life-saving. Suppressing this

evidence out of fear that the public might believe supplements
will make them “immune” to COVID-19 is not only elitist,
but it is inconsistent with existing public policy approaches.
Many mitigation strategies are publicized. None are seen as
conferring immunity.
This succinct but comprehensive review of the evidence found
that despite almost complete absence of official government
guidelines favoring vitamin D supplements to potentially
decrease COVID-19 risk and severity, support among clinicians
and other researchers for correcting and preventing vitamin D
deficiency with modest daily vitamin D supplementation during
the COVID-19 pandemic is very strong, worldwide. The evidence
supports recommending 2,000 IU (50 mcg) vitamin D daily for
at-risk teens and adults, which is well within safe limits andmight
dramatically reduce COVID-19 fatalities.”

And here is a link to the whole article https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7513835/