In the late 1800s and early 1900s, scientists realized that diseases such as:
- Scurvy
- Beriberi
- Pellagra
- Rickets
weren’t caused by infections, but by the absence of tiny dietary substances.
At the time, every newly discovered substance thought to be essential was simply given the next letter of the alphabet.
So we started with:
- Vitamin A
- Vitamin B
- Vitamin C
- Vitamin D
- Vitamin E
Scientists assumed many more vitamins would follow.
Instead, many of the compounds that were initially called vitamins turned out to be:
- mixtures of other vitamins,
- amino acids,
- fatty acids,
- or substances the body could manufacture itself.
Their letters were retired.
Vitamin B became an entire family
The biggest surprise was vitamin B.
Scientists originally thought vitamin B was one chemical.
Later they realized it contained many completely different compounds.
Today we know them as:
- B1 — thiamine
- B2 — riboflavin
- B3 — niacin
- B5 — pantothenic acid
- B6 — pyridoxine
- B7 — biotin
- B9 — folate
- B12 — cobalamin
Notice the missing numbers?
Those numbers were assigned to substances later shown notto be vitamins.
What happened to Vitamin F?
Vitamin F turned out not to be a vitamin.
It was actually:
- linoleic acid
- alpha-linolenic acid
These are essential fatty acids.
Because they’re fats—not vitamins—the name was abandoned.
What happened to Vitamin G?
Vitamin G eventually became:
Vitamin B2 (riboflavin).
Scientists realized it belonged with the B-complex rather than being its own vitamin.
Vitamin H
Vitamin H became:
Biotin (Vitamin B7).
The H came from the German word Haut (skin), because deficiency affected the skin.
Vitamin I
Never officially accepted.
Several compounds were proposed but none met the definition of a vitamin.
Vitamin J
Sometimes used experimentally for catechol or other compounds.
Never became an official vitamin.
Vitamin L
Originally thought to promote lactation in animals.
Later shown not to be an essential human vitamin.
Vitamin M
This became:
Folic acid (Vitamin B9).
“M” came from “monkey,” where it was first studied.
Vitamin N
Occasionally used for lipoic acid.
Today alpha-lipoic acid is considered an important metabolic cofactor but not a vitamin because humans can synthesize it.
Vitamin O
Never validated.
Vitamin P
One of the more interesting ones.
Vitamin P referred to:
bioflavonoids such as:
These compounds reduce capillary fragility and work alongside vitamin C.
They are beneficial but are not considered vitamins because deficiency doesn’t produce a well-defined disease.
Vitamin Q
Sometimes referred to:
Coenzyme Q (CoQ10).
Again, humans manufacture it.
Therefore it isn’t classified as a vitamin.
Vitamin R
Several compounds received this designation over time.
None remained vitamins.
Vitamin S
Never accepted.
Vitamin T
Sometimes referred to carnitine or other growth factors.
Since humans synthesize carnitine (assuming adequate vitamin C, iron, lysine, and methionine), it lost vitamin status.
This ties into your recent interest in vitamin C and carnitine—although carnitine is nutritionally important, it isn’t technically a vitamin because healthy people usually make enough of it.
Vitamin U
Very interesting.
Vitamin U is actually:
S-methylmethionine
found in cabbage juice.
It showed promise for stomach ulcers decades ago.
It is beneficial but not considered an essential vitamin.
Why did we jump to Vitamin K?
Vitamin K was discovered much later.
The researcher who identified it was Danish biochemist Henrik Dam, who noticed chickens fed a fat-free diet developed bleeding problems. He named the factor “Koagulationsvitamin” because it was required for blood coagulation. The “K” comes from the German and Scandinavian spelling of “coagulation,” not because it was the next unused letter of the alphabet.
Later we learned there are multiple forms:
- K1 (phylloquinone) from green plants
- K2 (menaquinones) produced by bacteria and found in fermented foods and animal products
So what is a vitamin?
Modern nutrition defines a vitamin as a compound that:
- is required in very small amounts,
- cannot be synthesized in sufficient quantities by humans,
- has a defined biochemical role,
- and whose deficiency causes a characteristic disease or syndrome.
Many substances once labeled as vitamins—such as CoQ10, carnitine, lipoic acid, bioflavonoids, and essential fatty acids—are undeniably important for health, but they don’t meet all of these criteria. Some are made by the body, while others are required in larger amounts or do not produce a specific deficiency disease.
An interesting perspective
The history of vitamin discovery also highlights how scientific understanding evolves. Early researchers correctly sensed that foods contained many essential factors beyond protein, fat, and carbohydrate. As analytical techniques improved, scientists separated these factors into distinct vitamins, amino acids, essential fatty acids, trace minerals, and numerous bioactive compounds. Today we recognize that health depends on far more than the classic 13 vitamins. Compounds such as omega-3 fatty acids, choline, lutein, carotenoids, polyphenols, CoQ10, and others play important physiological roles, even though they are not officially classified as vitamins.
