The cold weather has got me thinking about food sources of vitamin D.
I know that many kinds of fish are a good source of vitamin D for us humans. I was wondering how the fish get the vitamin D in the first place. Also, why are there other kinds of animals which are not a good source of vitamin D?
Qualified guess based on what wikipedia says about the evolution of vitamin D, phytoplankton make vitamin D, zooplankton eat phytoplankton, little fishes eat zooplankton, big fishes eat little fishes, we eat big fishes. Everyone gets vitamin D.
Either they synthesize it within their own bodies, out of chemically simpler substances in their diet or they get it by eating a lot of something else that synthesizes a lot of it in its body).
All types of animals (and plants) can synthesize most of the types of complex molecules that they need to live for themselves, within the cells of their body. However, if a species of animal happens to get a lot of such a substance in its normal diet, it may lose the ability to make it for itself (because there is no evolutionary advantage in being able to make stuff that you have plenty of anyway). When this happens, that substance becomes an essential nutrient or vitamin, for that species, and perhaps for species descended from it. Thus different substances are vitamins for different species. For example, our monkey ancestors got a lot of ascorbic acid in their diet, because monkeys mostly eat fruit and there is a lot of ascorbic acid in most fruit (synthesized within the plant). Thus the monkeys lost the ability to make ascorbic acid, and it became a vitamin for them and for us, their descendants. For monkeys, apes and humans, ascorbic acid is a vitamin (vitamin C). For most other animals, it is not a vitamin, not because their bodies do not need it (they do) but because they can make their own. (For other types of animal, other substances are probably vitamins that are not vitamins for us because their bodies cannot synthesize that substance, but ours can.)
If naita’s post, above is correct (and I have not reason to think it is not) the ability to synthesize vitamin D without the help of sunlight was lost long ago by our distant fish ancestors, because they were getting plenty of it from their diet of plankton. At that point it became a vitamin for them, and remains one for their descendants, including us.
Fish is a better source of EPA and DHA, but you don’t ask how they acquired that. They acquire that by eating tiny plants that contain those fatty acids.
I don’t believe that the definition of a vitamin requires that it be obtained from food. Vitamin D is generally recognized as a vitamin even though we can synthesize it, although some now classify it as a hormone/vitamin.
You can find different definitions from other sources that require the substance be found in food, but as noted above, vitamin D3 can be synthesized.
That is most certainly not an adequate definition of vitamin. All sorts of substances are essential for normal cell function, growth, and development, but they are not vitamins. In most cases this is either because you will inevitably get enough of of them from virtually any likely and otherwise adequate diet, or because your cells can synthesize them from substances common in any otherwise adequate diet. Generally speaking, to count as a vitamin something does have to be required in the diet.
Vitamin D is a bit of an odd case in that we can synthesize its bioactive form from a precursor molecule if we get enough sunlight. However, that precursor molecule is itself not something that can necessarily be found in any otherwise adequate diet. Thus you really do need to eat the right things to get enough vitamin D, even if you get plenty of sunlight. If you eat enough of the bioactive form, you can do without the sunlight, but I believe it is not as common in most foods as is the precursor.
D is also an oddity among vitamins in that it acts rather like a hormone, whereas most things we call vitamins are, or are precursors of, coenzymes.
Anyway, I doubt whether vitamin is really a natural-kind term, so probably it is impossible to provide a rigorous, scientifically-meaningful definition of it that matches the way the word is actually used. There are lots of essential nutrients that you need only in fairly small quantities, cannot necessarily get from an otherwise adequate diet, but that are not normally counted as vitamins: essential amino acids, essential fatty acids, certain trace minerals, etc. Which molecules we actually call vitamins has more to do with history than with biochemistry.
No, if my post is correct, there is no way to synthesize vitamin D without the help of sunlight.
To nitpick your nitpick, I very much doubt whether that is actually true. I would bet you could do it in a lab. It may be the case that no living organism in fact does synthesize it without using sunlight.
This was my guess as well.
What about other animals. Beef is not such a great way to get vitamin D although cows need it and make it from their own exposure to the sun. Perhaps I am just eating the wrong part of the cow.
Yeah, I thought of that inaccuracy after posting but left it as an exercise for the reader… 
Re: Above discussions of which nutrients should be called “vitamins” and why: Why, in fact, are any of these chemicals called by the vague and mystical names “Vitamin A, B, C, etc.”? It seems that various foods were found, over the ages, to provide mysterious protections from various diseases long before any modern knowledge of the detailed biochemistry. Thus, foods were known to provide mysterious substances that became known simply as “Vital amine A”, etc., later “Vitamin”.
Famously, the empirical discovery that citrus prevented (and even cured) scurvy led to the postulation of some mysterious substance, called “Vitamin C” before anything further was known about it.
Wikipedia article
Another article, The History of Vitamins
If the chemical details had been known earlier, they would never have been called simply “Vitamin X” in the first place. This is true of all the later-discovered micronutrients which are known by chemical names, and were never called “Vitamin” P, Q, R, S, T, …
That is a very misleading account. Vitamins were not “discovered over the ages” but in the early 20th century. Although it was known that certain foods would prevent or even cure certain diseases before this, most notably it was known that citrus fruits and certain other vegetables would prevent or cure scurvy in the 18th century, it was a 20th century discovery, principally due to Frederick Gowland Hopkins, that these effects were due to specific chemical substances, several of which he was the first to isolate, in those foods. Hopkins also introduced the alphabetical naming system for these substances (at a time when their actual chemical structure was still unknown), although he was not responsible for the misleading word “vitamin”. Hopkins was also responsible for the discovery of the essential amino acids (or, rather, he was responsible for the discovery that certain particular amino acids are essential in the human diet), and, to a considerable extent, for creating the modern scientific discipline of biochemistry.
What I wrote is largely my own interpretation of what I think I know about it. But wait a minute, I think you’re partially agreeing with my hypothesis. Namely, that the existence of helpful substances in food was known or surmised well before the actual biochemistry was known, and were thus simply called “Vitamins” before anybody knew what else to call them. You’re right, of course, that the term, derived from “vital amine” turned out to be a misnomer. The snippet of quote I included made at least that much clear.
The point is they weren’t called Vitamins before it was shown that one of them was an amine. References to suspected or known chemicals rather than “Citrus fruits prevent scurvy” were not happening "well before the actual biochemistry was known*. And they weren’t knowingly misnamed for very long:
Just eight years between coining the word and changing it because it was wrong.
*Your definition of “well before” may vary from mine.
No, I am saying that (a few) helpful foods were known before the biochemistry was known. That is quite different from the notion of helpful substances in a food. Although it may seem obvious to us looking back (as some of the most significant scientific advances often do), in fact the move from thinking about foods to thinking about particular chemical substances in food was a major and hard-won conceptual advance.
In any case, relevant foods that were known about before the early 20th century were very few in number: citrus etc. for scurvy, brown rice for beriberi (and that was only discovered in the late 19th century). I am not sure that there are any other relevant examples. The fact is that most serious vitamin deficiency diseases (as opposed to minor ill health caused by not getting quite enough of some vitamin) are actually pretty rare, especially before industrialization, so folk medicine never really encountered many of them, let alone developed any ideas about preventing them. Hopkins discovered the vitamins he discovered in milk; he was not looking at milk because there was a folk tradition about it curing particular diseases.
The term “vitamin” arose after this discovery, at a point when it was briefly thought that all the relevant substances might, chemically be amines (because some of the first few whose chemical structure was elucidated were in fact amines). It does not come from the folk medicine tradition. It comes from biochemistry.
Incidentally, a historical misconception based upon having read a Wikipedia article (and another web page apparently plagiarized from Wikipedia), does not amount to a “hypothesis”.