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From wikipedia

A vitamin is an organic compound and an essential nutrient that an organism requires in limited amounts.

There are many essential nutrients to an organism. Glucose for example. However, not all of them are classified as vitamins. Is there a clear, non-arbitrary definition of vitamin? If not, who decide what ought to be called a vitamin?

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  • $\begingroup$ I completely rewrote your question. I removed one of the two questions (which you might want to ask on a separate post). I think the question is good but you already got a close vote but it needed a lot of reformatting and needed is scope to be reduced. Feel free to roll back if you don't like my edit. $\endgroup$ – Remi.b Oct 14 '17 at 17:09
  • $\begingroup$ @Remi.b Hm, I posted an answer to the original two-part question just as you edited it. If we keep the question in this new form, feel free to edit the answer ... $\endgroup$ – Roland Oct 14 '17 at 17:16
  • $\begingroup$ Thanks for changing my question. I'm not native English people, so my english skill is not up to scratch. Before edited, I asked two questiom - 1. Why vitamin D is classfied even it can be naturally synthesized. $\endgroup$ – jong-hyeon-yeo Oct 14 '17 at 17:24
  • $\begingroup$ And Why vitamin A B C D E K are classified and named as 'Vitamine' $\endgroup$ – jong-hyeon-yeo Oct 14 '17 at 17:26
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    $\begingroup$ @jong-hyunYeo Vitamine is an abbreviation for vital amine. When vitamins were first being defined and categorized, they were termed as such because originally all vitamins contained an amine. Eventually though, they broadened the classification criteria of vitamins, and so the e was dropped, as it no longer applied. $\endgroup$ – Charles Oct 17 '17 at 6:13
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1) Vitamin D is a bit of an umbrella term that actually refers to a whole group of related molecules. One of these, vitamin D$_3$ (cholecalciferol) is formed spontaneously from ergosterol in the presence of UV radiation, which happens in the skin when exposed to sunlight. Cholecalciferol is then converted to calcitriol, the active form, in the liver. Here is some information on the metabolic pathway.

2) Actually, if you consider the definition carefully, only a few molecules are essential for human growth and nutrition, in the sense that they cannot be replaced with something else, and are required in small amounts. For example, the majority of amino acids can be synthesized by the body from other amino acids, and are therefore nonessential. The remaining amino acids are essential nutrients because there is no synthesis pathway -- but they are required in large amounts and not considered vitamins. This distinction between "large" and "small" might sound strange, but it is important since it tells you if the nutrient is consumed for energy/growth (like amino acids) or has some "supporting" role, such as antioxidants, cofactors for enzymes, or hormones (like the vitamins).

That said, it is sometimes a bit fuzzy if a nutrient is essential or not. Vitamin D is a borderline case: it should perhaps not be called a vitamin, since it can in fact be synthesized by most adults in adequate amounts. However, vitamin D deficiency can occur if one does not get enough exposure to sunlight, and this can cause the disease rickets. Some researchers have suggested that this is one factor behind evolution of lighter skin in northern human populations, since dark skin pigment reduces UV radiation needed for vitamin D synthesis.

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  • $\begingroup$ Thanks for your answer.I usally takes some supplements for my immune system and I wonder why some supplements are not classified as Vitamine - Especially Coenzyme Q10 and Omega-3-fatty acid. Your answer give me more clear view than my thoughts. $\endgroup$ – jong-hyeon-yeo Oct 14 '17 at 17:45
  • $\begingroup$ You're welcome! Btw, coenzyme Q10 is not an essential nutrient; omega-3 fatty acids are essential, but not considered vitamins since they are required in larger amounts. $\endgroup$ – Roland Oct 14 '17 at 18:10
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Please keep reading the Wikipedia article, it clarifies substantially in the next several lines, for example (emphasis mine):

An organic chemical compound (or related set of compounds) is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and it must be obtained through the diet

and also:

By convention the term vitamin does not include other essential nutrients, such as dietary minerals, essential fatty acids and essential amino acids

Glucose, as an example, is something that is both used in relatively large quantities by humans, and can be synthesized by humans (from carbohydrates and other energy sources).

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Wikipedia actually has a really good definition of an essential nutrient:

An essential nutrient is a nutrient required for normal physiological function that cannot be synthesized by the body, and thus must be obtained from a dietary source. Apart from water, which is universally required for the maintenance of homeostasis, essential nutrients are indispensable for the metabolic processes of cells, as well as the proper physiological functions of tissues and organs. In the case of humans, there are nine amino acids, two fatty acids, thirteen vitamins and fifteen minerals that are considered essential nutrients.

For vitamins, this appears to be a definition by exclusion: everything that is an essential nutrient but not a mineral, amino acid or fatty acid is considered a vitamin. Specifically:

Vitamins are organic molecules essential for an organism that are not classified as amino acids or fatty acids. They commonly function as enzymatic cofactors, metabolic regulators or antioxidants. Humans require thirteen vitamins in their diet, most of which are actually groups of related molecules (e.g. vitamin E includes tocopherols and tocotrienols).

I think the reason why food like glucose is excluded from this list is because no particular form of food is necessary in and of itself: as long as you're consuming carbohydrate or fatty acid, the human body can produce energy through carbohydrate catabolism or fatty acid catabolism. So it isn't one particular nutrient that's necessary, just "anything that can be used as an energy source".

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