Why Are Some Nutrients Vitamins and Others Not? An Accident of Our Own Biology

A vitamin is an organic compound required in small quantities for normal function that the organism cannot synthesise in sufficient amounts and must obtain from the diet. Every part of that matters. Organic excludes minerals, which are also essential and are not vitamins. Small quantities distinguishes them from the fats, proteins and carbohydrates that supply energy and bulk. And the requirement that the body cannot make it is what makes the category species-specific rather than a property of the compound, so the same substance is a vitamin for one animal and not for another depending on whether that animal retains the machinery to produce it.

Several examples show the category shifting between animals:

• •Most mammals make their own vitamin C and do not need it in the diet
• •Humans, other primates, guinea pigs and some bats lost that ability through mutations in the same gene
• •The broken gene is still present in the human genome, recognisable and non-functional
• •Cats cannot make taurine and must obtain it, while dogs can, which is why cat food differs
• •Some vitamins are produced by gut bacteria in quantities that vary between species
• •Vitamin D is made in skin exposed to sunlight, so whether it is dietary depends on circumstances

The concept arrived through deficiency diseases rather than through chemistry. Scurvy among sailors was shown to be prevented by citrus in a comparative trial conducted by James Lind in 1747, which is among the earliest controlled clinical trials, though the finding was applied inconsistently for decades. Beriberi among populations eating polished rice was traced to something present in the discarded husk, with experiments on chickens in the 1890s demonstrating it. The general idea that trace dietary factors were essential was formulated in the early twentieth century and the word coined in 1912, initially on the mistaken assumption that all such factors were amines. Isolation and synthesis of each compound followed over subsequent decades, and the alphabetical naming records the order of discovery rather than any chemical relationship.

The numbers printed on packaging come from a specific process that is less precise than it looks. Committees establish an estimated average requirement from studies of how much is needed to prevent deficiency or to maintain a marker in a normal range, then set a recommended intake above that, at a level covering nearly everybody in the population, which means most individuals need less than the recommended figure. Separate upper limits mark where harm begins. The underlying evidence is frequently thin, resting on small studies and on assumptions about which marker matters, and figures differ between countries for the same nutrient, which is a straightforward indication of how much judgement is involved. Values are revised as evidence accumulates, sometimes substantially, and vitamin D has been revised in both directions.

Correcting a deficiency works, and supplementing beyond sufficiency mostly does not, which is the consistent finding across a great deal of research. Populations with documented deficiency benefit clearly, which is why folic acid before and during pregnancy, vitamin D in low-sunlight populations and vitamin A where deficiency is endemic are all supported. Trials of supplementation in populations that are not deficient have repeatedly found no benefit and occasionally harm, with high-dose antioxidant trials producing increased mortality in some groups and beta carotene increasing lung cancer risk among smokers, which was unexpected and stopped the trials early. The fat-soluble vitamins accumulate and can be toxic in excess, unlike the water-soluble ones, which are largely excreted.

A vitamin is defined by the organism being unable to make it, so the category depends on the species rather than on the compound, and most mammals make their own vitamin C while humans carry the broken gene. The concept arrived through deficiency diseases, with citrus and scurvy demonstrated in 1747. Correcting deficiency works and supplementing beyond sufficiency mostly does not.