The Nutrients That Don’t Fit

Look at a B-complex label and something is off: B1, B2, B3, B5, B6, B7, B9, B12. B4 is gone. So are B8, B10, and B11.

Choline raises the opposite question. It is an essential organic nutrient with its own dietary requirement, but it is not a vitamin. Humans can make some niacin, yet niacin is vitamin B3. Humans can make vitamin D in skin. Most mammals make their own vitamin C, while we cannot.

These are not separate quirks. The nutrients are biological. The names and numbering are historical. The modern vitamin list is what remained after more than a century of discoveries, mistaken identities, split compounds, renamed substances, and better definitions.

What is a vitamin?

A useful modern definition is an organic compound needed in relatively small amounts for normal physiology that the body cannot make in sufficient quantity and therefore must obtain, at least partly, from outside itself. That definition works surprisingly well, but not perfectly.

Vitamin D can be made in skin. Niacin can be synthesized from tryptophan. Intestinal bacteria can contribute some forms of vitamin K. Vitamin A and vitamin D participate in signaling rather than simply behaving like conventional enzyme helpers.

The category is still useful. The edges are simply less clean than the word “vitamin” makes them sound.

One distinction matters later in this article: many vitamins, especially B vitamins, participate in reactions as coenzymes or parts of coenzymes and can be used repeatedly. Other essential nutrients are incorporated into membranes, proteins, fats, or other body material and are required on a much larger scale. That distinction helps explain why nutrient requirements can differ by orders of magnitude. It is not a formal test that determines whether something is a vitamin.

Why do the B numbers have gaps?

Vitamin B was not originally eight vitamins. Early nutrition experiments identified a water-soluble dietary factor and called it vitamin B, in contrast with the fat-soluble factor called vitamin A. Researchers later discovered that the “B” material was a mixture containing several chemically unrelated substances.

As those factors were separated and investigated, numbers were attached to them. The process was not centrally managed, and different laboratories sometimes used the same number for different compounds.

Some candidates later proved not to be human vitamins. Others turned out to duplicate substances that already had another identity. Their numbers were not reassigned.

B8 may be the most revealing entry. Different researchers used the same number for different compounds. That is more informative than the missing number itself: the familiar B-vitamin sequence was never designed as a finished taxonomy.

The modern list simply contains the designations that survived: thiamin (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), vitamin B6, biotin (B7), folate (B9), and vitamin B12.

The letters were messy too

The alphabet has its own discarded names. Riboflavin appears in early literature as vitamin G before settling into B2. Biotin was studied as vitamin H before researchers determined that separately named growth and deficiency factors were the same compound.

Vitamin F is more interesting, because the underlying nutritional requirement did not disappear. The term was applied historically to what became recognized as the essential fatty acids. Linoleic acid and alpha-linolenic acid really are essential nutrients today, with requirements set by the National Academies. Nutrition simply classifies them as fatty acids rather than vitamins.

Vitamin P was used for flavonoid-related factors proposed to influence capillary function. Flavonoids remain biologically interesting, but no human vitamin requirement was established and the designation disappeared.

Vitamin K was not simply the next letter in sequence, and here the person responsible explained himself. In his 1943 Nobel lecture, Henrik Dam described the 1935 naming directly: the letter K “was the first one in the alphabet which had not, with more or less justification, been used to designate other vitamins, and it also happened to be the first letter in the word ‘koagulation’ according to the Scandinavian and German spelling.” Both things were true at once. The letter was available, and it fit.

Choline: essential, but not a vitamin

Choline received dietary reference values from the Institute of Medicine in 1998 and was classified as an essential nutrient rather than assigned a vitamin number. The body uses choline to make phosphatidylcholine and sphingomyelin, two phospholipids that are major components of cell membranes. It is also needed to produce acetylcholine, and it is a source of methyl groups used throughout metabolism.

The liver can synthesize some choline-containing phosphatidylcholine through the PEMT pathway. That synthesis is not sufficient to remove the dietary requirement, and common genetic variants in the PEMT gene change how much a given person needs.

Current Adequate Intakes are 550 mg per day for adult men and 425 mg per day for adult women. That amount looks strange beside many vitamins. Vitamin B12 is required in micrograms. Riboflavin is required in roughly a milligram. Choline is required in hundreds of milligrams.

Its jobs explain much of that difference. Many B vitamins participate in catalytic chemistry, where the vitamin or a derivative helps a reaction occur and can act repeatedly. Choline is also used as material: choline-containing phospholipids become part of cell membranes, and choline is consumed in making acetylcholine. Material built into body structures generally has to be supplied on a larger scale than a reusable catalytic helper.

That explains a difference in dose. It is not a formal rule separating vitamins from non-vitamins.

There does not need to be a demotion hidden in the terminology. “Essential nutrient” is the biologically important part. Calling choline a vitamin would not make it more essential.

Vitamin C is a vitamin because humans lost the ability to make it

Most mammals synthesize their own ascorbate. Humans, unlike most animals, cannot, which is why vitamin C is an essential dietary component for us.

The difference lies in the final step of the pathway that converts glucose-derived compounds into ascorbate. Humans lack a functional gulonolactone oxidase enzyme for that step. The gene is still present in the human genome, but as a nonfunctional pseudogene.

That makes vitamin C a clean demonstration that “vitamin” can depend on the species eating it. An animal that makes enough ascorbate internally has no equivalent dietary requirement. Humans need an external source because our own synthetic pathway no longer works.

Same molecule. Different nutritional requirement.

Vitamin D does not behave like an ordinary vitamin

Humans can also synthesize vitamin D3. Ultraviolet B radiation, at roughly 290 to 320 nanometers, converts 7-dehydrocholesterol in skin to previtamin D3, which becomes vitamin D3. The liver and kidney then convert it onward, with hydroxylation in the kidney producing the physiologically active form, 1,25-dihydroxyvitamin D, also known as calcitriol.

Calcitriol acts through the vitamin D receptor, a nuclear receptor found in many tissues, changing which genes are transcribed. In that sense the active vitamin D system behaves as an endocrine system rather than as a conventional enzyme helper. Chemically, vitamin D compounds are secosteroids.

The vitamin name reflects how the nutrient was found: through rickets, diet, cod-liver oil, sunlight, and the search for an antirachitic factor, well before the endocrine pathway was understood. Both endogenous production and dietary intake matter. The interesting point is that a compound the body can manufacture still became one of the canonical vitamins, because that production is conditional and can be insufficient.

Niacin can be made from an essential amino acid

Niacin makes the boundary even less tidy. The body can synthesize niacin compounds from tryptophan, one of the nine essential amino acids.

The contribution is large enough that requirements are expressed in niacin equivalents. The Food and Nutrition Board defines one niacin equivalent as 1 mg of niacin or 60 mg of tryptophan. The conversion is inefficient and variable enough that it does not eliminate the dietary niacin requirement.

What maize and pellagra show about availability

This connection mattered historically. Diets heavily dependent on maize were associated with pellagra where the rest of the diet supplied little niacin or tryptophan. Pellagra was common in the early twentieth century among people living in poverty in the southern United States and parts of Europe whose diets consisted mainly of corn.

Maize presents two problems at once. Much of the niacin naturally present in grain products is bound to polysaccharides and glycopeptides that leave it only about 30% bioavailable, and maize protein is relatively low in tryptophan.

Traditional Mesoamerican processing is the counterexample. Cooking maize in an alkaline solution, called nixtamalization, releases bound niacin and improves its availability. Where maize spread as a staple without that processing tradition, into populations whose diets were already limited, pellagra became far more common.

Diet composition, niacin availability, and processing all contributed. The disease was never simply a property of the grain.

Some vitamin-style numbers were never recognized vitamins

Not every missing B number belongs to the early scientific sorting process. B15 has been used for products marketed as pangamic acid. No recognized human vitamin requirement exists for it, and substances sold under the name have not had a consistent chemical identity.

B17 is a marketing name applied to laetrile and amygdalin products. It is not a recognized vitamin. Laetrile was promoted as a cancer treatment; the National Cancer Institute’s review of the evidence concludes it has shown little anticancer effect, and hydrogen cyanide released from amygdalin is the source of its toxicity.

Attaching a B-number to a compound does not establish vitamin status. That requires evidence of a genuine nutritional requirement, not a vitamin-shaped name.

Why nutrient amounts differ so much

The classifications become easier to read once the amount required is separated from the importance of the nutrient.

Those numbers do not rank importance. A tiny requirement can mean a compound is repeatedly reused. A large requirement can mean the nutrient becomes physical material, turns over rapidly, or does a job needed on a much larger scale.

That pattern explains why vitamin B12 is measured in micrograms while amino acids, fatty acids, calcium, choline, and water are needed in progressively larger quantities. It is an explanation of scale, not a classification rule.

So what does “essential” actually mean?

An essential nutrient is one that must ultimately be supplied from outside the body in enough quantity to support normal function, because endogenous production is absent or insufficient.

Vitamins are one subset of that larger problem. Essential amino acids are not vitamins. Essential fatty acids are not vitamins. Minerals cannot be vitamins, because they are elements rather than organic compounds. Choline has its own classification. Water is essential and sits outside all of them.

None of that makes one category more legitimate than another.

Why the gaps are worth noticing

The gaps in the vitamin list preserve some of the history of nutrition science.

Researchers found dietary factors before they knew their structures. They split mixtures apart. Different laboratories named the same compounds differently, and sometimes used the same number for different substances. Some candidates became established vitamins. Others became amino acids, fatty acids, metabolites, or discarded hypotheses.

The result is a classification system shaped by both biology and the order in which biology was discovered. That is why B4 can disappear while choline remains essential, why vitamin C depends on which species is eating it, why vitamin D became a hormone, and why niacin can partly come from an amino acid.

The questions that actually matter were never about whether a nutrient earned a letter or a number. They are whether the body needs it, whether the body can make enough, how much has to come from outside, what it does, and what happens when the requirement is not met.

Related: Choline · Vitamin B3 (Niacin) · Vitamin D

This article is educational and does not constitute medical advice. It describes how nutrients came to be classified, not how much of any nutrient a particular person needs. See the individual Nutrition Foundations pages for current reference intakes and nutrient-specific guidance.