Of all the vitamins, D has the strangest backstory. It was named a vitamin because that's what the naming convention of the moment demanded but unlike true vitamins, your body doesn't need to get it from food at all, if it gets enough sunlight instead. Figuring that out took two rival camps of researchers, a chemist working with hog skin, and a professor who used $300 of his own money to patent a discovery he then gave away.
A Disease Misunderstood for Centuries
Rickets a childhood disease that leaves bones soft, bowed, and misshapen was described in detail as far back as the 2nd century C.E., by the physicians Galen of Pergamum and Soranus of Ephesus. English physician Francis Glisson gave a detailed clinical description in 1650, and by then the disease already had a grim nickname: "the English disease," for how common it had become in Britain's smoke-choked, sun-starved industrial cities. Children working indoors in factories, in cities blanketed by coal smoke that blocked what sunlight there was, developed the condition at alarming rates.
Doctors had stumbled onto a partial fix long before anyone understood why it worked. In 1824, physician D. Schütte became the first to specifically prescribe cod liver oil for rickets purely empirically, with no theory behind it at all, nearly a century before the concept of a "vitamin" existed.
Two Rival Cures, and Nobody Knew Why Either Worked
By the early 20th century, two completely separate camps had each found a real cure for rickets and neither could explain the other's results.
In 1918 and 1919, British researcher Edward Mellanby ran careful experiments on puppies, feeding them an oatmeal-heavy diet modeled on a poor Scottish diet. The puppies reliably developed rickets and adding cod liver oil to their food reliably cured it. Since the concept of "fat-soluble factor A" (later vitamin A) was already established, Mellanby reasonably, but incorrectly, concluded that the antirachitic substance in cod liver oil probably was vitamin A, or something distributed similarly to it.
At almost exactly the same time, in 1919, German physician Kurt Huldschinsky was achieving the same result through an entirely different method: he cured rickets in children in Vienna simply by exposing them to ultraviolet light from a mercury vapor lamp. No diet change involved at all. Two different researchers, two completely different interventions, and the same disease cured both ways with no explanation yet for how that was possible.
The Experiment That Split Vitamin A in Two
The resolution came from Elmer McCollum the same researcher whose Wisconsin lab had identified vitamin A less than a decade earlier. In 1922, McCollum ran a deliberately destructive experiment: he heated and aerated a batch of cod liver oil, a process already known to destroy vitamin A's activity. If Mellanby had been right that the antirachitic factor was vitamin A, the treated oil should have lost its ability to cure rickets.
It didn't. The oxidized oil no longer had vitamin A activity, but it still cured rickets in McCollum's test animals just as effectively as before. That meant cod liver oil contained two separate substances doing two separate jobs. Since letters B and C were already claimed by other vitamins, McCollum named this new one vitamin D. That same year, in a Vienna clinic treating malnourished children in the aftermath of World War I, researcher Harriette Chick and her colleagues showed that both cod liver oil and sunlight exposure could cure the rickets common among the city's children direct evidence that diet and sunlight weren't rival explanations after all, but two paths to the same nutrient.
The Trapper of the Sun
The practical breakthrough that made vitamin D a public health tool came from Harry Steenbock, a biochemist at the University of Wisconsin McCollum's old institution. Steenbock had been working with goats since 1916 and had noticed something curious: goats kept outdoors through summer maintained healthy calcium levels, while the same animals confined indoors over winter didn't. After reading about Huldschinsky's UV lamp results in Vienna, he began experimenting with ultraviolet light himself.
His method was pure trial and error. He irradiated the air his lab rats breathed nothing happened. He irradiated their cages still nothing. Then he irradiated their food directly, and it worked: the treated food alone cured rickets. Steenbock had discovered that UV light could impart antirachitic activity directly onto food, years before anyone understood the underlying chemistry.
He filed a patent on the food-irradiation process in 1924. When the university's Board of Regents declined to fund securing the patent, Steenbock paid for it himself, reportedly with around $300 of his own money and then, rather than profiting personally, turned the patent over to a newly formed foundation, the Wisconsin Alumni Research Foundation. Licensing fees from the process, used to fortify milk on a mass scale, eventually brought in roughly $14 million and funded decades of further research. Steenbock earned an enduring nickname for the achievement: "the trapper of the sun." Rickets went from a widespread public health crisis to a rare condition within a generation.
A Chemistry Puzzle With a Twist
The actual chemistry took another decade to fully sort out and it had a genuine twist. Researchers found that irradiating ergosterol, a compound from yeast, produced a highly potent antirachitic substance, purified independently by teams in London, the Netherlands, and Germany in 1931, and named vitamin D2 (calciferol). German chemist Adolf Windaus, who led the German team, was awarded the 1928 Nobel Prize in Chemistry for his broader work connecting sterol chemistry to vitamins the first Nobel Prize citation ever to mention vitamins by name.
But a nagging problem remained: ergosterol doesn't naturally occur in animal or human bodies at all. If sunlight on skin produced vitamin D, and ergosterol wasn't present in skin, something didn't add up. A 1934 finding by researcher Waddell that irradiated ergosterol behaved differently in chickens than in rats suggested a different, unidentified substance was responsible for the natural, animal version of the process. The puzzle wasn't resolved until 1937, years after Windaus had already received his Nobel Prize, when Windaus and colleague Bock isolated a compound called 7-dehydrocholesterol from hog skin later confirmed in human skin too. This compound, a relative of cholesterol actually found in skin, was the real natural precursor. Irradiated by UV light, it produced vitamin D3, and testing confirmed it was identical to the vitamin D naturally found in fish oils like halibut and bluefin tuna.
Why This Story Still Matters
Vitamin D's discovery is really two intertwined stories that took decades to recognize as one: a dietary deficiency disease with a food-based cure, and a completely separate sunlight-based cure, that turned out to describe the exact same missing compound reaching the body by two different routes. It's also the only vitamin discovery story that ends with a scientist turning down personal profit from his own patent to fund a research foundation instead a detail that's easy to leave out, but says something about how this particular piece of nutrition science reached the public at all.
Frequently Asked Questions
Who discovered vitamin D? No single person it emerged from parallel work by several researchers. Edward Mellanby linked diet to rickets in 1918, Kurt Huldschinsky linked sunlight to rickets in 1919, Elmer McCollum proved it was a distinct vitamin in 1922, and Adolf Windaus and colleagues worked out the underlying chemistry through the 1930s.
Why is vitamin D called "the sunshine vitamin"? Because, unusually among vitamins, your body can manufacture it on its own a compound in your skin converts to vitamin D3 when exposed to UV light, without needing any dietary source at all.
When was vitamin D officially named? 1922, when Elmer McCollum proved the antirachitic factor in cod liver oil was chemically distinct from vitamin A.
How did milk become fortified with vitamin D? Harry Steenbock discovered in the mid-1920s that irradiating food with UV light directly increased its vitamin D content, patented the process in 1924, and the licensing of that patent led directly to large-scale milk fortification.
Sources consulted: The Journal of Nutrition (Windaus historical review); PMC/National Institutes of Health (100-year anniversary review of vitamin D3 discovery); Academy of Nutrition Sciences (UK); UW–Madison News; EBSCO Research Starters; ScienceDirect historical reviews of vitamin D discovery.
