Vitamin D isn’t technically a vitamin in the strict sense, because unlike the other vitamins the human body can produce it itself (by exposure to sunlight).
The body can also synthesize vitamin A from beta-carotene which is effectively two vitamin A molecules joined together (one rotated 180deg relative to the other).
Sure, many things are vitamins for one species but not another. (In fact, every vitamin must be able to be produced by at least one species – where else would it come from?)
Many animals can. There are a gene for it, humans don't have it. There is a lot of speculation as to why, but nothing really stands out (possibly just random chance - if you eat enough there is no advantage to keeping the gene and in turn no loss from losing it. However I'm unable to rule out other possibilities) https://pmc.ncbi.nlm.nih.gov/articles/PMC3145266/ is a really interesting survey of the issue across many different species.
From the article:
> Another argument supporting the suggestion that species which have lost their GLO gene were under no selective pressure to keep it, is that all species which have lost their GLO gene have very different diets but all of them have diets rich in vitamin C
What would a diet poor in vitamin C be considering that "everything else" makes it? I guess root vegetables? It feels like, if anything, this would imply a GLO gene decay more often than has happened, no?
That is probably a question for a nutritionist not me. My understanding is Grains, root vegetables, and meat are all low in vitamin C. Likely other things as well. But I'm not a nutritionist (I've read enough that I think I'm right here, but not enough to state it with confidence), so take the above with plenty of salt.
Most definitions of the word vitamin are not specific to humans. Wikipedia talks about "organisms", Britannica about "higher animal life", Webster about "most animals and some plants"
