In the case of sunlight exposure to UVB an equilibrium is reached where the amount vitamin D produced in the skin equals the amount destroyed by UVB. This does not happen if you pop vitamin D pills. I've recently written an app that calculates the time required to obtain adequate vitamin D by lying prone in the sun, with most of your clothes off, and using MED the minimal erythema dose as a guide to safe sun exposure. At the height of summer it is a surprisingly short amount of time that is required to get adequate vitamin D even for people with dark skins.

When I look through some abstracts on NIH/NCBI, PNAS and so on I feel like there is some regulatory mechanism involved when production happens in the epidermis via UVB.

Maybe the same regulation mechanisms dont apply with oral intake, bringing Vitamin D directly into circulation without any further governance?

> "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

> 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.

Shorts and t-shirts? Em, a rather large amount of modern urban African spends most of their day indoors and wears long-sleeved shirt/pants/long dresses/long robe/hijabs/etc.

I'm not sure why one would expect them to have >50% of prehistoric levels.

The paper you linked to (or really, the metaanalysis it cited) showed low D levels were associated with urbanization, but the lowest concentrations observed in northern African countries and in South Africa with seasons. The highest levels were among those practicing traditional lifestyles.

It's an insincere argument

It's not reasonable to connect vitamin D to anything 100k years ago when vitamin D is so heavily influenced by diet and weather, both of which changed wildly. Lactose tolerance, megafauna, fishing, the last glacial maximum- just 20,000 years ago people were undergoing massive differentiation in vitamin D production and utilization.

Vitamin D production from skin UV exposure is self-regulated.

UV irradiation converts 7-DHC to pre-D3, but continued exposure to UV-B converts pre-D3 to lumisterol and tachysterol.

As pre-D3 levels fall, lumisterol and tachysterol can be converted back into pre-D3 or shed with dead skin.

Pre-D3 is slowly converted to D3 with heat exposure.