Re #2: the figure gets thrown around a lot. But who is outside, naked, at noon for fifteen minutes? It's really hard to translate that to anything meaningful or actionable.
Here's a tool that can help calculate useful exposure depending on your location/time of year. Doesn't take clothing into account however..
The point is that being outside naked at noon every day for 15 minutes wouldn't hurt you. Therefore, a daily dose of 20,000 IU / day won't hurt you. Therefore, a 5000 IU supplement every day won't hurt you either.
One does not follow the other. Maybe taking 5000 IU orally has a vastly different effect to letting it acculumate through skin? That's purely speculation on my part though, just trying to illustrate that these two are not necessarily equivalent.
It is certainly a different mechanism. There are inactive isomers of pre-Vitamin D (tachysterol and lumisterol) that prevent over-production of Vitamin D from continued UV exposure.
I list these numbers to give a general idea of the levels of Vitamin D that people experience without oral supplementation.
My conclusion was that too many studies were being done to determine the correct or safe amount to supplement rather than testing regularly to determine a safe/optimal blood concentration.
Very excited for good prospective studies of Vitamin D.
That's a bold assumption to be making, especially regarding a fat soluble vitamin that builds up in the body.
A bag of carrots wont hurt you yet an equivalent dose of betacarotene (vit A) supplement definitely will and is strongly associated with increasing certain cancers. There's a reason you won't see those on the shelf apart from miniscule amounts in multivitamins.
is this just an extrapolation or does my body actually constantly produce the same amount of vitamin D if i sit in the sun even if I don't have a deficit?
Because usually phisiological processes are in some kind of equilibrium and regulated, so it's not straightforward to compare it to supplements.
7-dehydrocholesterol is photolyzed by UV to previtamin D. There is then spontaneous isomerization between this previtamin D, vitamin D and 2 inactive isomers. As vitamin D is used by the body, and the inactive isomers remain in the skin, there is an equilibrium - as vitamin D is used by the body it is drawn from the skin, leading to more conversion of inactive isomers to active vitamin D.
The article goes into some detail on this, my understanding is that Vitamin D is produced in inactive form with sunlight exposure and then this is more-slowly released as needed (over winter apparently, but nobody's quite sure apparently).
I'm not that pale but it's shocking how fast you can burn in some places. Denver, for example, has a mile less atmosphere protecting you and little UV-blocking humidity. While New Zealand famously has the ozone hole. I don't burn nearly as fast in San Francisco.
Here's a tool that can help calculate useful exposure depending on your location/time of year. Doesn't take clothing into account however..
https://fastrt.nilu.no/VitD-ez_quartMEDandMED_v2.html