Followup questions:
2. Which letters are still unused?
3. Why aren’t the B vitamins split up? And why isn’t the most important one b1? Why isn’t there just a “B”?
4. Why didn’t the letter match the name of the chemical, e.g. Riboflavin is B, ascorbic acid is C, retinol/beta carotene is A, etc.
5. Why aren’t new vitamins being found?
6. Do countries that don’t use English still use the ABC designations?
Short answer.
In the 19th century, the general theory of medicine said that illnesses were always caused by a toxin of some sort entering the system. However, a series of experiments seemed to show that a deficiency of some nutrient might instead be the problem. You could test for these by seeing which set of foods had the mystery ingredient and trying to separate it into a solution that could be added to a control group’s diet. By doing this scientists found one substance that was fat-soluble and worked against a form of nightblindness and a second substance that worked against beriberi and was water-soluble. They didn’t yet know exactly what the substances were, so they coded them as vitamin A and vitamin B. Casimir Funk had done a series of experiments that showed that one of these had an amine group in it and so it gave them the general name of vitamines, or amine plus “life”. Later, when they found some didn’t have the amine group, they dropped the “e” ending to make it more general.
Another vitamin was found that worked against scurvy, obviously different from the first two, and this was called vitamin C. Cod liver oil was known to contain vitamin A, which was destroyed by oxygen. Yet even then what remained worked against rickets. Another vitamin must be involved, which was called vitamin D.
And so on. Some of the new vitamins named this way turned out in the end to be actual vitamins, like E and K. However, others were either false reports or proved to be rediscoveries of earlier vitamins or variants of them and so the names given fell out of use. See Wikipedia for a complete table.
The problem with identifying vitamins was that they were needed in such fantastically small quantities that they were extremely hard to isolate. However, this problem was solved over a period of years. As scientists refined their methods they discovered that the substance identified as vitamin B was just one of a series of chemically closely-related substances. They started putting them into subclasses of B[sub]1[/sub] and B[sub]2[/sub], etc. Some of the later-named letter vitamins were actually B variants. And some of the B variants were also named wrongly. Again, check that Wiki page. Today the B complex is given individual names to hold down the confusion.
Since each of the vitamins has a standard scientific name, that can be used in non-Roman alphabet countries just as they have become more common here.
Amazingly, there is not a good complete adult book on the history of vitamins. Isaac Asimov’s children’s book, How we found about VITAMINS, is a typically excellent primer. Walter Gratzer’s *Terrors of the Table: The Curious History of Nutrition *has several chapters on the history of vitamins and the horrible suffering that vitamin deficiencies caused.
So are they saving “vitamin Z” for something really special? You know, like the ultimate vitamin–the vitamin to end all vitamins?
So what happened to B[sub]10[/sub] and B[sub]11[/sub]?
Iirc, they got scared because B7 8 9?
<humor> http://www.youtube.com/watch?v=7CU-poRQPr0 (vitamin section starts a ways in) </humor>
It’s my understanding that vitamin K is named K because of the German Koagulation, since it is involved in blood clotting.
Nice work, Mapcase. Well, asssuming you didn’t make all that up. 
There’s already Vitamin X – makes you strong as an Ax or an Ox.
You small thinker you. From Gratzer’s book:
Really Not All That Bright, thanks. I was going to make a witty comeback, but any attempt at fakery would make me feel so outclassed by Silent George that I’m going to rest on my laurels.