Obvious omission from my prior comment:
in the absence of sunlight. I'd hoped that would be clear from context, it's an oversight on my part.
That is, either sunlight or dietary supplementation will provide the essential vitamin/hormone D (its biological function is ... interesting).
Which is to say, deprivation of either sunlight or dietary sources of themselves is not sufficient to induce vitamin D deficiency. There's also the fact that as an oil-based vitamin, can be, and is, stored for prolonged periods in body tissues. That's a sharp contrast to the water-soluable vitamin C of which excess is excreted in urine.
This was responding specifically to the prior claim "they need vitamin D just like any other life form and can only synthesise it from the sun" (emphasis added) by seszett here: https://news.ycombinator.com/item?id=40594479> The apparent claim that sunlight is necessary for acquisition of vitamin D is incorrect.
Periodic hypersupplementation of D can be effectively processed and retained by the body. By contrast, hyperdosing of vitamin C is a waste of resources as the excess is not retained and is flushed out. I've read previously that long-term injections of vitamin D have been effectively used, see:
"Lasting Moderate Increases in Serum 25-Hydroxvitamin D Levels and Shorter-Term Changes in Plasma Calcium " https://pubmed.ncbi.nlm.nih.gov/28492140/>
"Vitamin D Injections: Benefits, Side Effects, and Who Is a Candidate?
" https://healthnews.com/nutrition/vitamins-and-supplements/vi...>
(Neither of these is the specific item I'd had in mind, though both describe similar procedures and efficacy.)
(Overconsumption of various vitamins and minerals can be harmful in other ways as well, I'm not a doctor (not even of philology) and moreover I'm not your doctor.)