Every cholecalciferol molecule is the same. This is chemistry. Chemicals that are not the same cannot have the same chemical name. Every animal (without a genetic defect specific to the precursor chemicals) produces the same 7-dehydrocholesterol molecules, and every 7-dehydrocholesterol will, under UVB light, convert to cholecalciferol.

Industrial production of vitamin D3 irradiates the 7-dehydrocholesterol extracted from sheep's lanolin with UVB light.

There is some question as to whether ergocalciferol--a chemical mainly found in fungi that produce ergosterol and have been exposed to UVB light--is biologically equivalent to cholecalciferol in humans. It can alleviate vitamin D deficiency symptoms, but it is not known with certainty whether it can produce a sufficiency. As far as I am aware, the known cases of hypervitaminosis D have resulted from ergocalciferol supplementation, rather than from cholecalciferol.

If you are supplementing with vitamin D3, the chemical you are consuming is identical to that produced in your skin under UVB irradiation. There is no evidence whatsoever that it is destroyed in or poorly absorbed by the human digestive system. If you swallow 15000 IU of vitamin D3, that is the equivalent of standing shirtless in temperate midday sun for 15 minutes, after which time you will achieve no further benefit until some time has been spent absorbing the cholecalciferol and replenishing the 7-dehydrocholesterol in your skin.

It is poorly known, that if you have to preface a statement with "it is well known", what follows is less likely to be "known" than "unattributably rumored".

No it’s not required. This is a discussion board not a thesis paper review board. Do your own googling. If he or she is wrong then call bullshit.

IMO "Reputed and trustworthy citations from scientific sources required, please." is just a more elegant way of calling bullshit

It's an elegant way of covering noise with more noise.

It is required. Just claiming something is well-known to counter someone’s suggestion without providing any evidence isn’t very productive.

No other suggestion was offered in the post I am referring to, just a "sources pl0x" ask. Here is the simplest counter example I can think of.

"Gravity is real."

"Reputed and trustworthy citations from scientific sources required, please."

You can get less and less ridiculous from here ad nauseam, but the point is made.

"It is well known" is a weasel word, and hence requires more substantiation when asked. If one has to rely on Googling on these matters, one can find so many contradictory views. So I wanted to know what reputed sources the GP was relying on to use "It is well known" for that claim.

how about just search hacker news? https://www.npr.org/sections/health-shots/2018/11/10/6665455...

https://www.theguardian.com/science/2018/mar/09/is-vitamin-d...

And those studies saying otherwise have been backed by a lot of questionable money: https://www.nytimes.com/2018/08/18/business/vitamin-d-michae...

Oh by the way sunscreen is terrible, it doesn't have a proven link to reducing cancer and it destroys reefs so do yourself a favor and get more sun without all the slimy goop so you can feel better AND save the planet!