system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there so many different "B-vitamins"
gpt
When vitamins and their functions were first being researched in the early 20th century an American scientist named Elmer McCollum isolated what we call vitamin A from butter fat and called it "factor A." He called another important nutrient that had previously been isolated from rice polishings by Casimir Funk, a Polish scientist, "factor B." Scientists continued to name vitamins in alphabetical order after that. (Edit: The German scientists who discovered vitamin K named it K for "Koagulation," the German word for coagulation, since it helped blood coagulate. They skipped a letter or two in doing so, but it generally stuck to convention).

However, scientists discovered that "factor B" was actually a bunch of smaller compounds that were often found concentrated together in the same foods. These smaller compounds are the separate B vitamins (thiamin (B1), riboflavin (B2), etc.) and together they make up what we now call a vitamin B complex (the same as the original "factor B"). We also now know that our bodies use the different B vitamins for distinct processes and that not all of the vitamins that have been classified as B vitamins are actually vitamins (this is the reason why there are only 8 B vitamins but the list appears to be longer - Bs 4, 8, 10, and 11 are no longer considered vitamins). This is the same thing that happened to vitamins F through J; they are no longer considered vitamins, so we skip from vitamin E to vitamin K.

Edit: the second paragraph for accuracy.