> "A whole body exposure to UVB radiation inducing the light pink color of the minimal erythema dose for 15–20 min is able to induce the production of up to 250 μg vitamin D (10,000 IU)"[1]
So when you expose someone to light until they get a sunburn, they produce up to 250 ug. 80 ng/ml serum is still equivalent to 400+ ug and that's in the blood alone.
The real issue is that over time that elevation caused calcium levels to rise. That can't happen with light exposure because you adapt to it.
> Surely naked homo sapiens got a bigger dose wandering around the highlands of Ethiopia 100,000 years ago.
Did they? They were black. If the "natural" level of vitamin D was significantly higher than modern levels in white people, current day black Africans would have higher levels than the RDA- they wear shorts and tshirts, so they should still have >50% of prehistoric levels. But in fact if anything people in Africa tend to have lower vitamin D levels than the rest of the world, and certainly much less than eg the US: https://www.thelancet.com/journals/langlo/article/PIIS2214-1...
On top of that, European-descended people have less efficient calcium utilization due to higher availability. They need more vitamin D to utilize the same amount of calcium, so looking at prehistoric levels is not a great indicator.
> What kind of perverse logic led to setting the RDA to 600 IU daily?
Well for one thing its certainly done with an assumption of daily sunlight exposure, since people don't usually figure that into their diet.
If you want the full logic, you can read the 1116 page book by the Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine. They chose the number: https://nap.nationalacademies.org/read/13050/chapter/1#xiii