Vitamin D Isn't Really a Vitamin: The Discovery Story, the Science, and the Facts Worth Knowing

Here's a fact that surprises most people: vitamin D isn't really a vitamin. By the strict definition, a vitamin is a compound the body cannot make on its own and must get entirely from food. Vitamin D breaks that rule. Your skin can manufacture it from nothing more than sunlight, which is why nutrition scientists classify it as a prohormone, a precursor your body converts into an active hormone through its own internal chemistry. It's the only nutrient in this category that works quite this way, and that single fact explains almost everything else worth knowing about it, from how it was discovered to why its effects reach so far beyond bone health.

What Vitamin D Actually Is

A true vitamin is biologically active the moment you consume it. Vitamin D is not. As the National Institutes of Health's Office of Dietary Supplements explains, dietary and skin-synthesized vitamin D is biologically inert and has to undergo two separate hydroxylation steps inside the body before it becomes usable. That two-step activation process, first in the liver and then in the kidneys, is exactly what defines a prohormone rather than a vitamin in the traditional sense. Every other essential vitamin, C, the B complex, A, E, K, has to come from outside the body in a form the body can use more or less as-is. Vitamin D is the outlier: give it nothing but UVB sunlight on bare skin, and the body will build it from scratch.

The Discovery Story: From Rickets Epidemics to a Named Vitamin

The story of vitamin D doesn't start in a laboratory. It starts with a disease that reshaped the skylines of industrializing Europe. As cities across 18th and 19th century Britain, Germany, and the Netherlands filled with factories, coal smoke, and crowded tenement housing, a bone-softening childhood disease called rickets became almost routine in industrial neighborhoods. Children's legs bowed, their chests deformed, their growth stalled. Contemporary physicians didn't yet understand why, but two things about it were consistent: it clustered in smog-choked cities where sunlight barely reached the ground, and it was far less common in the countryside.

Long before anyone understood the mechanism, fishing communities across Northern Europe already had a folk remedy: cod liver oil. By the 18th and 19th centuries, physicians in England and Scandinavia were recommending spoonfuls of the foul-tasting oil to rachitic children, and it worked, often dramatically. Nobody at the time could say why an oily fish byproduct would fix a bone disease. The mechanism sat unexplained for well over a century, a folk remedy that outran the science behind it.

That started to change with the British biochemist Edward Mellanby. Beginning around 1918 and continuing through the early 1920s, Mellanby ran a now-famous series of experiments on dogs, raising puppies indoors on a rickets-inducing diet of oatmeal and then reversing the disease by feeding them cod liver oil. It was the first solid experimental proof that rickets was a nutritional deficiency disease rather than an infection, and that whatever was in cod liver oil was the missing piece. What Mellanby didn't establish was that this factor was something new. He initially believed the antirachitic effect might be the work of vitamin A, which cod liver oil also happens to contain in abundance.

That confusion was resolved by the American biochemist Elmer McCollum, who had already been central to identifying vitamin A a few years earlier. In 1922, McCollum's team heated and aerated a sample of cod liver oil, a process that destroyed its vitamin A activity, and tested it again on rachitic rats. The treated oil had lost its ability to protect against a vitamin A deficiency, but it still cured rickets. That was the proof: whatever was preventing and healing rickets was a distinct nutrient, separate from vitamin A entirely. McCollum named it vitamin D, simply the next letter in line after A, B, and C had already been claimed by earlier discoveries. It became the fourth vitamin identified by science, and the name has stuck ever since.

Knowing that a nutrient existed and knowing what it actually was chemically were two different problems, and the second one took another six years to solve. That fell to the German chemist Adolf Windaus, who spent much of the 1920s working out the structures of sterols, the broad family of fat-soluble compounds that includes cholesterol and, as it turned out, vitamin D itself. Windaus and his collaborators identified the specific compound in skin, 7-dehydrocholesterol, that sunlight converts into vitamin D3, and mapped its full chemical structure. For that body of work, Windaus received the 1928 Nobel Prize in Chemistry, awarded, in the Nobel committee's own words, for research into the constitution of the sterols and their connection with the vitamins. Within roughly a decade, vitamin D had gone from an unexplained folk remedy to a named nutrient to a fully mapped molecule, one of the faster turnarounds in the early history of nutritional science.

D2 and D3: Two Discoveries, One Family

One detail that often gets lost in the discovery story is that researchers actually identified two distinct forms of the antirachitic factor, not one. Vitamin D2, or ergocalciferol, was isolated from irradiated plant and fungal sterols, specifically ergosterol, in the years following McCollum's initial identification. Vitamin D3, or cholecalciferol, is the form Windaus traced back to 7-dehydrocholesterol in animal and human skin, the compound sunlight actually converts on your body. Structurally, the two molecules are extremely close relatives. Both belong to the secosteroid family, and both differ from ordinary steroids by a single broken ring in their chemical backbone, a "seco" structure that's the source of the name. The difference that matters is a small variation in the side chain, present in D2 because of its plant and fungal origin, absent in D3 because of its animal origin.

That small structural difference is also where the practical distinction between the two forms comes from, but that's a separate conversation from their discovery. We've covered which form is generally considered more effective at raising and maintaining blood levels, and why, in our piece on common vitamin D myths in the Gulf. Here, the more interesting fact is historical: two research paths, one starting with irradiated plant sterols and the other with human skin chemistry, converged on two closely related molecules that both ended up carrying the same letter.

How Your Body Actually Makes and Uses Vitamin D

The process that made Mellanby's dogs recover, and that still keeps you supplied with vitamin D today, starts in the skin. When UVB radiation from sunlight hits skin cells, it converts a cholesterol-derived compound called 7-dehydrocholesterol, the same molecule Windaus identified, into previtamin D3, which then settles into its final form as vitamin D3 over the following hours. This is genuinely unique among nutrients: it's the only one your body can build essentially from scratch, given nothing but sun exposure.

But raw vitamin D3, whether made in the skin or absorbed from food or a supplement, isn't yet active. It travels to the liver, where an enzyme converts it into 25-hydroxyvitamin D, sometimes written as 25(OH)D or calcidiol. This is the form doctors actually measure in a blood test, because it reflects your overall vitamin D status more reliably than any other marker. From there, as your body needs it, the kidneys perform a second conversion, turning 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D, better known as calcitriol, the fully active hormone form. Only at this final stage does vitamin D actually do its work at the cellular level.

And that work turns out to be remarkably widespread. Vitamin D receptors, the docking sites calcitriol needs to act on a cell, exist in nearly every tissue type in the human body, not just bone and gut, where the classic calcium-absorption story plays out. They're found in immune cells, muscle tissue, the nervous system, the cardiovascular system, and more. That's a large part of why the science on vitamin D reaches so far past bone health, and it's also why we've dedicated an entire piece to how it operates specifically in bone metabolism in the Gulf, since that mechanism alone deserves its own explanation. The prohormone framing from the very start of this piece is really the same idea in different words: this isn't a single-purpose nutrient with one job to do. It's a signaling molecule with receptors wired into most of the systems that keep the body running.

What Daily Essentials Provides, and Why That Number Is Safe

Vitameenat's Daily Essentials box delivers vitamin D from two separate places within the daily sachet, and it's worth being plain about the total. The dedicated vitamin D capsule provides 20mcg (200% of the Reference Intake), and the MultiVitamin component contributes a further 10mcg (100% RI). Added together, that's 30mcg of vitamin D per day, or 1,200 IU, working out to 300% of the standard Reference Intake.

That number is worth pausing on rather than glossing over. Three hundred percent sounds high at first glance, but it sits well within the range that regulatory bodies consider safe for daily use. The NIH Office of Dietary Supplements lists the tolerable upper intake level for adult vitamin D at 100mcg, or 4,000 IU, per day, a ceiling more than three times higher than what Daily Essentials provides in total. The reason vitamin D appears in two components rather than one comes down to formulation: because the sachet's five components are physically separate capsules and tablets rather than a single combined pill, each one can be dosed independently, and in this case, two smaller contributions land at a total that's generous without approaching any safety threshold. If you're comparing labels or stacking supplements yourself, this is exactly the kind of detail worth checking on any multi-component regimen, and for anyone weighing dosing questions in more depth, our guide to vitamin D3 in the UAE covers how much you actually need and when.

You'll also see vitamin D paired with vitamin K2 fairly often in supplement conversations. That's a related but separate topic worth reading on its own terms, including exactly what Daily Essentials does and doesn't include on the K2 side, and we've covered it in full in our dedicated vitamin D and K2 guide.

Quick Facts: Vitamin D at a Glance

Frequently Asked Questions

Why is vitamin D called a prohormone instead of a vitamin?

Because it doesn't meet the strict definition of a vitamin. A true vitamin must be obtained from the diet because the body cannot make it, and it's biologically active as consumed. Vitamin D fails both tests: the body can synthesize it from sunlight, and it has to be converted through two hydroxylation steps in the liver and kidneys before it becomes the active hormone, calcitriol.

Who actually discovered vitamin D?

No single person discovered it in one moment. Edward Mellanby's dog experiments in the late 1910s and early 1920s first proved rickets was caused by a missing dietary factor. Elmer McCollum identified that factor as distinct from vitamin A and named it vitamin D in 1922. Adolf Windaus then determined its precise chemical structure, work recognized with the 1928 Nobel Prize in Chemistry.

Why is it called vitamin D specifically?

Simply because it was the fourth vitamin identified by science, following the earlier discoveries of vitamins A, B, and C. The naming follows the order of discovery, not any property of the nutrient itself.

Is vitamin D3 better than D2?

They come from different sources, D2 from plant and fungal sterols, D3 from the same pathway your skin uses, and D3 is generally considered more effective at raising and sustaining blood levels. We cover that practical comparison in more detail in our vitamin D myths guide.

Does everyone have the same vitamin D receptors?

Everyone has vitamin D receptors distributed across nearly every tissue type, which is why researchers have studied its role in immune function, muscle performance, and cardiovascular health, not just bone. Individual response can still vary based on genetics, skin tone, and overall health status.

The Takeaway

Vitamin D earned its name almost by accident, it was simply the fourth letter available when McCollum needed one in 1922, but the science behind that name is genuinely unusual. It took a bone disease reshaping the skylines of industrial Europe, a folk remedy nobody could explain, a set of dog experiments, and a Nobel Prize-winning chemist to fully understand a compound your own skin can build from sunlight alone. That prohormone status, and the fact that its receptors reach almost every system in the body, is exactly why vitamin D keeps showing up in research far beyond bone health. If you're weighing how much you actually need day to day, our guide to vitamin D3 in the UAE is the more practical next read, and if bone health specifically is what brought you here, this piece walks through that mechanism in depth. For the bigger picture on why deficiency is so common despite the region's sunshine, start with the sunshine paradox.

Reviewed by Fatima Kassem - pharmacist and founder of Vitameenat, MSc in Pharmaceutical Sciences from the University of Copenhagen, with early career experience at Novo Nordisk.