This varies a lot between different vitamins. They are not all the same. It sounds like you are talking about vitamin D. Humans are not the only ones who make vitamin D, almost all animals do. It is particularly plentiful in fish liver, sheep skin and in some yeast. We can extract the enzymes responsible of synthesizing vitamin D in these places and then feed them the starting compounds and shine bright UV lights at them and they will produce lots of vitamin D.
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