(n.) the common name for the vitamins phylloquinone, K1, and menaquinone, K2, used in the coagulation of blood
Have you ever wondered why the system for naming dietary vitamins starts off straightforwardly enough with vitamins A, B, C, D and E, and then jumps ahead six places to vitamin K? Well, you can thank German.
The very first inklings of the vitamin K group were discovered in 1929 by the Danish scientist Henrik Dam. He would go on to win the 1943 Nobel Prize for medicine (alongside the American biochemist Edward Doisy) for his work on vitamin K, which he initially introduced in a German-language medical journal as the Koagulationsvitamin. It was this name that stuck, and Dam’s discovery was given the name vitamin K accordingly, hence our alphabetical jump.
When vitamin K was named, however, the system wasn’t quite as floored as it is today. A number of different vitamins that were once included in the alphabetical sequence in the 1920s and 30s were later reclassified as either not vitamins at all, or else as subcategories (or more properly, vitamers) of other, pre-existing vitamins.
So in 1923, a new addition to the vitamin alphabet was discovered and given the name vitamin F. Six years later, it was found to be an essential fatty acid not a vitamin, and has been recorded as such ever since. Vitamin G was discovered even earlier, in 1920, but it was later found to be a vitamer and recategorized: it is better known as riboflavin today, and is listed as vitamin B2.
Vitamin K isn’t the only upset to the system, either. When it was discovered in 1931, biotin (vitamin B8) was given the placeholder name vitamin H because it aids in the growth of hair and nails (Haar and Haut in German). When flavonoids were discovered in 1938, they were given the name vitamin P because they were assumed only to come from fruits, vegetables, and other plant-based foods. Choline was once known as vitamin J, but actually isn’t a vitamin at all. And folate, or folic acid, is now classed as vitamin B9, but was once known as vitamin M because it was partly discovered through experiments conducted on monkeys.