[theendisney]

I like vitamine D. The salmon is full of mercury, the hemp is criminalized, there is either not enough sun, to much, we dont have time or we have reduced capacity to produce. 50% has a deficiency and many can probably use more than the bare minimum.

UVB shouldnt just be legal, it should be mandatory.

[malcolmgreaves]

About 5 minutes of sunlight exposure is the maximum amount you can absorb and use to make vitimin D. Everything after that point is harmful radiation.

[amavect]

Mostly right, but only for light skin (and depends on the month and the latitude) (Table 1, Table 3). Dark skin usually needs 10-15 minutes (Table 2), and even 20 minutes for the darkest skin (Table 4). https://doi.org/10.3390/nu16101489

[kaikai]

I read the summary thoroughly and scanned the rest, and I don’t think the paper supports the grandparent comment.

The paper says you can produce enough vitamin d to maintain healthy levels from a specific amount of sunlight per day, depending on latitude and skin color.

The original comment suggests that there’s some (very short!) limit beyond which the body is unable to produce more vitamin d, which is very different. I’d be very curious to see sources for that.

[amavect]

Ah, I didn't read their comment too strictly.

UVB synthesizes cholecalciferol (vitamin D3) in the skin, which the liver converts into calcifediol (what blood tests usually measure), which the kidneys convert to calcitriol (the active hormone). Wiki claims the kidneys have a negative feedback loop, converting excess calcifediol into inactive 24,25-dihydroxycholecalciferol. I wish I had better sources (for my vitamin D pdf folder).

https://en.wikipedia.org/wiki/Vitamin_D#Excess

But just knowing that, I don't immediately see anything limiting cholecalciferol or calcifediol amount and storage.

[plorg]

The same wiki article says there is a limit to the capacity of synthesis by UVB due to the quantity of reagent 7-dehydrocholesterol produced in the skin, but I don't know the math on what amount of exposure would be required to hit that limit - presumably it (or something like it) is covered in the article above.

[amavect]

Some napkin math, then. About 25–50 μg 7-DHC / cm2 skin (Wiki). About 1.5 m^2 human skin area (google AI summary). About 35% skin exposed (R. Kift, linked earlier). 1.5 * 100 * 100 * 25 * .35 = 131250 μg. Need 50-250 μg of colecalciferol (2000-10000 IU). Anyone would likely get sunburn before running out of 7-DHC (excepting a low 7-DHC condition, or me getting the wrong numbers).

https://en.wikipedia.org/wiki/7-Dehydrocholesterol

This study says "Findings include that small UV doses on a regular basis are more efficient for vitamin D synthesis than larger sub-erythemal doses", using a logarithmic model for blood calcifediol as a function of exposure.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8558903/

But it doesn't address colecalciferol production and storage. Fat stores colecalciferol, and I don't know of any way to measure that directly. I would guess that further UVB exposure linearly produces colecalciferol (with linear DNA damage, minus DNA repair with time), but the liver and kidneys logarithmically produce calcifediol and calcitriol. Just a guess. Still more questions :)

[pstuart]

It's not just vitamin D, it's other things too (e.g., NO -- https://pmc.ncbi.nlm.nih.gov/articles/PMC10539323/).

But yes, one can have too much of a good thing in this regard.

[solid_fuel]

Don't bother quoting facts to these guys. We're being governed by people who make decisions based on their feelings, not material reality.