From glancing over, it does not really seem a confirmed fact, but a hypothesis.
"They chose to spend the worst part of the year trying to sleep through it inside of relatively safe caves, and to do that, they sacrificed nutrition and vitamin D from the sun."
Also it does not really matter for your vitamin D levels, if you are outside in winter in the north or in a cave. The sun is too low for the body to produce Vitamin D anyway (only the Inuit for example do not need sun, to produce Vitamin D btw. )
And that they lacked nutrition in winter can also just mean, that there simply was not much food in the winters, which is kind of expected, before humans developed storage and preservation technologies?
> only the Inuit for example do not need sun, to produce Vitamin D btw
As far as I can tell, they need vitamin D just like any other life form and can only synthesise it from the sun, just like all other lifeforms on Earth.
They might have adapted to reduced amounts of available vitamin D though[0], because they don't have much of it available to them in the first place and because there's really no other source than producing your own from the sun, or eating things that have produced it from the sun. So they have a vitamin D deficiency, but it's less severe than for most people.
Humans cannot synthesise vitamin D, but we can acquire it through diet from other sources, including particularly fresh fish, which are abundant in Inuit diets.
Note that numerous of the sources listed here are fortified (that is, have additional vitamin D added), however it does occur naturally in some sources as well:
> The major natural source of vitamin D is synthesis of cholecalciferol in the lower layers of the epidermis of the skin, through a photochemical reaction with Ultraviolet B (UV-B) radiation from sun exposure or UV-B lamps.[
Yeah, very poor wording on my part as I was addressing the false claim in GP that only sunlight-based synthesis would provide vit-D. Diet is an alternate source. More in my other follow-up response.
I said that they couldn't have another way to synthesise vitamin D than sunlight, it can only be synthesised by sunlight. So for the Inuit, vitamin D mostly comes from diet.
The claim I was responding to was "they [unlike other humans] don't need sun to produce vitamin D". They do, they just don't rely much on this source because it doesn't work in their environment.
It might seem like a cheat, but healthy humans can photosynthesize (metaphorically) vitamin D. It's one of the free lunches our bodies have, if you ignore sun damage.
What? I’ve never heard claim of this, we require sunlight (UV) as input but yes we do synthesize it. I mean the human body can’t spontaneously create anything, but it can take precursors and make various vitamins, proteins, and fats… https://en.wikipedia.org/wiki/Vitamin_D#Biosynthesis
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:
(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.)
"They chose to spend the worst part of the year trying to sleep through it inside of relatively safe caves, and to do that, they sacrificed nutrition and vitamin D from the sun."
Also it does not really matter for your vitamin D levels, if you are outside in winter in the north or in a cave. The sun is too low for the body to produce Vitamin D anyway (only the Inuit for example do not need sun, to produce Vitamin D btw. )
And that they lacked nutrition in winter can also just mean, that there simply was not much food in the winters, which is kind of expected, before humans developed storage and preservation technologies?