The letter-naming system for vitamins dates to the early 1900s, when scientists knew certain substances in food were essential for survival but had no idea what those substances actually were chemically. Letters served as convenient placeholders: “A” for the first one identified, “B” for the second, and so on. The convention stuck even after chemists figured out the molecular structures, partly because names like “ascorbic acid” and “retinol” are harder to remember than “C” and “A,” and partly because the alphabet system had already embedded itself in medicine, food labeling, and public consciousness.
Casimir Funk and the Word “Vitamin”
Before the letters came the word itself. In 1911, the Polish-American biochemist Casimir Funk proposed that several mysterious dietary factors shared a common chemical trait: they were amines, a class of nitrogen-containing compounds. He combined the Latin word “vita” (life) with “amine” to coin “vitamine,” and the term was adopted by the broader scientific community by 1912.1PubMed. Casimir Funk: his discovery of the vitamins and their deficiency disorders The reasoning was elegant but slightly wrong: not all vitamins turned out to be amines. The British biochemist Jack Drummond later suggested dropping the final “e” to produce “vitamin,” freeing the word from implying a specific chemical structure. That small edit let the term survive its own inaccuracy, and we have used it ever since.
How the First Letters Got Assigned
The alphabet entered the picture through a pair of researchers at the University of Wisconsin. In 1913, Elmer McCollum and Marguerite Davis identified a fat-soluble factor in butter and egg yolk that was essential for growth in laboratory rats. They called it “fat-soluble factor A.” Shortly afterward, they recognized a separate water-soluble factor in rice polishings that prevented beriberi, and labeled it “water-soluble factor B.” The naming logic was practical and sequential: A came first, B came second. There was no grand taxonomic plan.
The convention spread quickly. When the British navy’s long battle with scurvy was finally traced to a specific dietary compound in citrus fruits, researchers assigned it the next available letter, C, in the early 1920s. Vitamin D followed when a factor that prevented rickets was isolated around the same time. Each new discovery simply took the next letter in line, which gave the system an air of orderliness it did not really have. The letters implied a clean sequence of discoveries, but the reality was messier: multiple labs in different countries were chasing overlapping leads, and the letter each factor received often depended on who published first and which naming convention their journal preferred.
Why B Vitamins Have Numbers
The original “water-soluble factor B” turned out not to be one substance at all but a tangled mix of chemically unrelated compounds that happened to dissolve in water and show up together in similar foods. As researchers peeled apart this mixture through the 1920s and 1930s, each newly isolated component received a subscript number: B₁ (thiamine, the anti-beriberi factor), B₂ (riboflavin), and eventually B₃, B₅, B₆, B₇, B₉, and B₁₂. The numbering itself has gaps because some candidates initially given B-subscript designations were later found not to meet the criteria for a vitamin, either because the body could manufacture them on its own or because they turned out to be the same compound as one already identified.
This splitting of vitamin B into a whole subfamily is a big reason the alphabet looks so uneven today. There is no “vitamin B complex” molecule. The phrase “B complex” just means the full group of eight chemically distinct nutrients that once shared a single letter. Think of it as a filing error that was patched with subscripts rather than corrected from scratch.
The Missing Letters
If you scan the alphabet from A to K, you notice conspicuous absences. There is no commonly recognized vitamin F, G, H, I, or J in everyday nutrition. Research has found that all 26 letters of the alphabet have been used at one time or another to label putative vitamin compounds.2Oxford Academic. The missing vitamin alphabet Most of those designations did not survive peer review or turned out to be duplicates. “Vitamin F,” for instance, was an early name for essential fatty acids like linoleic and linolenic acid. When scientists realized these were fats rather than the trace organic compounds the vitamin concept was meant to capture, the letter was quietly retired. “Vitamin G” was an early synonym for riboflavin before it was folded back into the B group as B₂. “Vitamin H” was biotin, which eventually settled into its current identity as B₇.
The pattern repeated dozens of times. A researcher would isolate something from food, declare it a new vitamin, assign it a letter, and then further work would reveal that the substance was either already known under a different name, was not essential in the diet, or was not a single compound at all. The surviving letters (A, the eight B vitamins, C, D, E, and K) are the ones that passed every test: the substance is organic, the body cannot produce enough of it on its own, and a deficiency causes a specific disease or dysfunction.
Vitamin P and the Flavonoid Puzzle
One of the more interesting abandoned letters is vitamin P. In the 1930s, researchers noticed that pure synthetic vitamin C was less effective at treating scurvy than whole citrus extracts containing the same amount of ascorbic acid. Something else in the plant material seemed to be helping. Albert Szent-Györgyi, who won a Nobel Prize for his vitamin C work, proposed that a group of plant compounds affecting blood vessel permeability deserved their own vitamin designation: P, for “permeability.”3PubMed Central. Do Certain Flavonoid IMPS Have a Vital Function?
The vitamin P story illustrates why many letter designations failed. Attempts to isolate a single compound responsible for the permeability effect gave confusing and sometimes irreproducible results. Over the decades, the candidates narrowed to a large family of plant chemicals called flavonoids, but no single flavonoid could be pinpointed as “the” vitamin P. The inability to define one compound with one specific biological role bred skepticism, and the designation was officially dropped. More recent analysis suggests the original observation was not wrong, just premature: several flavonoids likely play overlapping roles that a single-compound framework could not capture.3PubMed Central. Do Certain Flavonoid IMPS Have a Vital Function? The vitamin P saga shows that the letter system was never just about discovery. It was about whether the scientific community could agree that a substance met a strict set of criteria, and agreement was often hard to reach.
Why Letters Stuck When Chemical Names Exist
Every letter-named vitamin now has a proper chemical name, and in research settings those names are standard. Retinol is vitamin A. Cholecalciferol is vitamin D₃. Alpha-tocopherol is vitamin E. So why does the public still talk in letters? Partly because the chemical names vary depending on which form of the vitamin you mean (vitamin D alone encompasses D₂ and D₃, each with its own chemistry), and partly because the letters function as brands at this point. Food labels, dietary guidelines, and supplement bottles all use the letter shorthand, and there is no regulatory or scientific push to change that.
Professional bodies have tried to standardize nomenclature where it matters most. The International Union of Nutritional Sciences and the American Institute of Nutrition endorsed formal naming rules for vitamin E as far back as the early 1970s, specifying which chemical forms count under that letter.4Oxford Academic. IUNS-AIN Nomenclature for Vitamin E Similar standardization efforts exist for other vitamins. But these naming rules are aimed at researchers and regulators, not the general public. For everyday communication, the letters remain far more practical than asking someone to remember “pyridoxine” instead of “B₆.”
The Definition of “Vitamin” Is Less Obvious Than It Sounds
One reason the naming story keeps generating confusion is that the word “vitamin” itself is harder to define than people assume. The working concept has evolved over the past century from “a substance whose absence causes a specific deficiency disease” to something broader, encompassing compounds that support long-term health in ways that go beyond preventing scurvy or rickets. Recent work on updating the definition acknowledges that existing definitions often fail to clearly separate vitamins from other bioactive compounds in food, and that the nutritional requirements for these substances vary under different conditions.5PubMed Central. What Is a Vitamin? Towards a Contemporary Definition.
This fuzziness matters because it determines which substances get (or keep) a letter. If the criteria for “vitamin” shift, the roster of recognized vitamins could shift too. Some researchers have argued that certain compounds currently considered merely beneficial, such as specific carotenoids or polyphenols, should be reclassified as conditionally essential nutrients. If that ever happens, the alphabet might need to expand again. For now, the list of 13 recognized vitamins (A, the eight Bs, C, D, E, and K) has been stable for decades, but the boundary is not as firm as a textbook entry might suggest.
Whether Something Counts as a Vitamin Depends on the Animal
A quirk of the vitamin concept that rarely makes it into popular discussion: what qualifies as a vitamin is species-specific. The classic example is vitamin C. Humans need it in their diet because we lack the enzyme that catalyzes the final step in producing ascorbic acid from glucose.6PubMed Central. Vitamin C: update on physiology and pharmacology Most other mammals, including rats and dogs, make their own vitamin C internally and have no dietary requirement for it. For them, ascorbic acid is just a metabolite, not a vitamin.
The loss of this ability in humans, other primates, and guinea pigs traces to a shared genetic mutation. A 1957 study in Nature confirmed that humans and monkeys, like guinea pigs, cannot convert the precursor compound L-gulonolactone into L-ascorbic acid, a step that rats perform easily using liver enzymes.7Nature. Missing Step in Man, Monkey and Guinea Pig required for the Biosynthesis of L-Ascorbic Acid The gene for that enzyme still exists in the human genome, but it is broken: a pseudogene riddled with mutations that render it nonfunctional. One hypothesis for why this mutation was tolerated, rather than eliminated by natural selection, is that our red blood cells evolved an efficient recycling mechanism for vitamin C that reduced the daily amount needed by a large factor, making dietary intake sufficient.8Evolution, Medicine, and Public Health. Glut-1 explains the evolutionary advantage of the loss of endogenous vitamin C-synthesis: The electron transfer hypothesis
The practical upshot is that a substance’s vitamin status is not an intrinsic property of its chemistry. It is a statement about the relationship between a specific organism and that compound. Vitamin D behaves similarly: humans can synthesize it through sun exposure, which means it technically does not meet the classic definition of a vitamin (a compound the body cannot make). It holds its letter designation because many people get too little sunlight for adequate production, making dietary intake conditionally necessary. This species-dependent, context-dependent framing is exactly what makes the concept so slippery and why the letter system, for all its messiness, provides a useful shorthand that sidesteps the definitional debates.
Why Vitamin K Jumps Ahead in the Alphabet
One letter that puzzles people is K. If vitamins were named sequentially and we only commonly use letters up to E before jumping to K, what happened to F through J? As discussed, those letters were assigned to compounds that later lost their vitamin status. Vitamin K’s letter does not come from the English alphabet sequence at all. It comes from the German word “Koagulation.” The Danish researcher Henrik Dam discovered in 1929 that a fat-soluble factor in green vegetables was essential for blood clotting. When he published his findings in a German-language journal, the substance was designated “Koagulationsvitamin,” shortened to vitamin K. The letter happened to be far enough along in the English alphabet that it did not collide with anything already in use, so it stuck without confusion.
Vitamin K is the clearest example of how the naming system was never as systematic as it appears. Some letters were sequential (A, B, C, D, E), some were reclassified or dropped (F through J), and at least one was borrowed from a foreign word. The alphabet was a convenient scaffold, not a master plan, and its surviving form reflects a century of ad hoc decisions made by researchers who were far more interested in curing diseases than in building a tidy naming convention.
Supplements, Labels, and the Alphabet on the Shelf
Walk down the supplement aisle of any pharmacy and you will see the letter system doing its heaviest practical work. Bottles are labeled with bold single letters or letter-number combinations, and most consumers navigate their choices entirely through that shorthand. This creates a few misunderstandings worth knowing about. When a bottle says “Vitamin E,” it could contain any of several related compounds (tocopherols and tocotrienols) with different biological potencies. The letter hides real chemical variation that can matter for dosing. Similarly, “Vitamin D” on a label might be D₂ (ergocalciferol, plant-derived) or D₃ (cholecalciferol, animal-derived), and the two are not equally effective at raising blood levels of the active form.
The B vitamins cause their own confusion because supplement makers often sell “B-complex” formulas containing all eight, even though deficiency in any single B vitamin is a distinct clinical problem with its own symptoms and dietary causes. The letter-based marketing encourages people to think of “the B vitamins” as a unified group, which they are not, biochemically speaking. Each has a different function, and lumping them together under one letter is a historical accident, not a reflection of how your body uses them.
None of this means the letter system is broken. It does what it was designed to do over a century ago: give non-specialists a quick way to talk about essential nutrients without needing a chemistry degree. The trade-off is that the simplicity that makes the system useful also makes it imprecise. Understanding that the letters are placeholders from a time when the chemistry was unknown, not a carefully engineered taxonomy, goes a long way toward navigating the alphabet without being misled by it.