How did they come to this?

Seems like a reasonable measure would be to look at a native person, from ones lineage, having a relatively average pre-industrial day.

If lineage doesn't matter, and all humans are the same, you could probably just go sample some tribe members that still lives mostly outdoors, outside of developed society.

And, regardless, I'll listen to my doctor.

Sunlight UVB radiation needed to make vitamin D is only available in Ireland during the months of April through October. This is due to our geographic location 52o - 55o North and means that Ireland experiences a ‘vitamin D winter’ that stretches from the end of October to late March.

Currently, 94 % of children, 94 % of teenagers and 90 % of adults on the island of Ireland have inadequate vitamin D intakes. Therefore, almost everybody in Ireland needs a supplement of vitamin D and 15 µg (600 IU) is recommended on a daily basis.

N.B. In the early 2000s, over 20 cases of rickets (a weakening of the bones caused by severe vitamin D deficiency) in infants and toddlers were reported in Ireland.

That study also claims 12-20ng/mL 25-hydroxyvitamin D is sufficient. My doctor says the post-COVID studies suggest 75ng/mL is closer to optimal, and <30ng/mL had been considered deficient for about a decade before COVID.

Personally I had no problem hitting 20ng/mL through sunlight alone and I'm pretty fair skinned but live somewhere sunny. But I haven't been able to get anywhere near 75ng/mL even when I've made an effort to increase vitamin D through supplements and exposure to the sun.

In Calgary, it's almost half the year that you basically cannot rely on the sun at all for vitamin D.

A pretty standard bottle of Vitamin D at 10k IU is 60 capsules/pills/whatever. That's a 2 month supply.

To hit the toxic eventually (77k IU) you would be going through a 2 month supply every ~7 days. To hit the acute toxicity in days or weeks (low end 600k), your going through that bottle every day.

However, every single time that someone mentions Vitamin D, people jump out of the woodwork about toxicity without ever actually bringing up just how much you have to take as if if you take the 400 IU capsule, going one IU over is going to kill you. I'm also pretty sure that a specific group of denialists shouldn't be counted as a reason to scream warnings from the mountaintops every time someone thinks about Vitamin D.