[kovach]

Here is an old clinical practice guideline article (2011) from The Journal of Clinical Endocrinology & Metabolism talking about Vitamin D deficiency and recommendations for correcting it:

https://academic.oup.com/jcem/article/96/7/1911/2833671

You can see that prevalence of deficiency is very high, as this other peer-reviewed study confirms:

https://pubmed.ncbi.nlm.nih.gov/21310306/

In the first article, after correcting the deficiency, the maintenance dose that they give is: 600-1000 IU/d for children; 1500–2000 IU/d for adults; 3000–6000 IU/d for obese. As you can see that's much higher than the current RDA of 400 IU/d.

And to correct the deficiency in the first place, the dose needs to be 2-3 times higher still for 8 weeks.

So around half the people in the US need to be taking a very large dose for 8 weeks, and then a maintenance dose 4-15 times higher than the RDA.

This is all from established studies done long ago that no one has disputed. But their recommendations have yet to be implemented in a lot of places, which might be why some countries fare better than others.