What is vitamin D? While the question would seem to call for a simple answer, the query hasn’t always prompted as straight forward a response as one might expect. Researchers often squabble over whether to classify vitamin D as a “vitamin,” a “hormone,” a “pro-hormone,” a “pre-hormone,” an essential “nutrient” and more. This might lead you to ask, then why do we call vitamin D, vitamin D?
The field of nutrition took off in the nineteenth century by German chemists. They believed that an adequate diet solely consisted of proteins, carbohydrates, fats and minerals; unaware of nutrients we now call vitamins.
Scientists began questioning this, as they noticed certain diseases, like scurvy and beriberi, could be prevented by certain foods. However, when these certain foods were “purified,” they somehow lost their ability to treat the diseases. A biochemist named Casimir Funk coined the term “vital animes,” theorizing that there were vital compounds called animes in food, essential to the diet of animals. This term paved the way for the now current descriptive word, “vitamins.”
Around 1916, Professor Elmer McCullum of the University of Wisconsin-Madison discovered that some type of fat-soluble substance in butter prevented xerophthalmia (dry eyes). He later called this factor “vitamin A.”
In later animal experiments, he used this butter and compared it to cod liver oil. He noticed that in animals that took cod liver oil, they developed neuritis, while the animals taking the butter did not. This was attributable to a set of vitamins they penned “vitamin B,” which was found in the butter but not in the cod liver oil.
Meanwhile, physicians elsewhere discovered a nutrient in food that staved off scurvy in guinea pig experiments. They followed suit and called this factor “vitamin C.”
Professor McCollum transferred to John Hopkins, continuing his experiments on fat-soluble nutrients. Cod liver oil was long known to prevent and cure rickets, and since McCollum and colleagues discovered vitamin A years earlier, they thought that vitamin A might also be responsible for curing rickets. McCollum wanted to know for sure, was it vitamin A or some other nutrient?
In another series of experiments, McCollum aerated and heated cod liver oil to destroy the vitamin A content. He knew he had rid the cod liver oil of the vitamin A because the altered cod liver oil no longer prevented xerophthalmia. When he used this cod liver oil in rickets experiments, it still retained its ability to cure rickets. Thus, he reasoned there must be another nutrient in cod liver oil that prevents and cures rickets.
And so McCollum named this new nutrient, “vitamin D.”
Around this same time, a different group of researchers carried out a set of experiments and independently found that sunlight or artificial UV light could prevent and cure rickets, too. While some scientists observed this connection as far back as a hundred years earlier, these experiments were the first to verify that sunlight indeed plays a role in rickets.
To the bewilderment of scientists at the time, somehow cod liver oil equaled sunshine, requiring much further investigation.
In the 1920s, Professor Harry Steenbock of the University of Wisconsin-Madison carried out a series of experiments, irradiating goats and their food and noticing again that UV light prevented rickets. Furthermore, he found that UV light activated an in-active substance into something vitamin D-like in foods. Thus, he established that UV light could both irradiate foods and animals and induce vitamin D activity.
In the 1930s, researchers were able to pinpoint the structure of vitamin D, and they were also able to identify the structure of its precursor in the skin, 7-dehydrocholesterol, which converts into vitamin D once exposed to UV light.
These findings raised the issue that maybe vitamin D is not a vitamin. A vitamin is called a vitamin when it cannot be synthesized in sufficient quantities by an organism. In these series of experiments, it was shown that with sunlight, rickets could be eradicated just as well with sun exposure as with cod liver oil. Thus suggesting, given adequate sun exposure, maybe the vitamin D from food isn’t necessary, and so maybe vitamin D isn’t even a “vitamin” in the true sense of the word.
This is something we speculate over now, but it is unlikely they speculated over what to call vitamin D then. The field of nutrition, still in its infancy, still didn’t know enough about vitamin D and still sought to know more about nutrition. The term “vitamin D” worked just fine.
About forty years later with advanced technology in place, researchers wanted to know what vitamin D converted into once it entered into the animal body. Through radiolabeling techniques, they discovered that vitamin D turns into 25(OH)D and then into 1,25(OH)₂D. We commonly refer to 25(OH)D as your “vitamin D level” and “1,25(OH)₂D” as your “activated vitamin D,” the latter usually unimportant in a clinical setting.
This activated vitamin D, they realized, was a hormone, secreted by the kidney into the blood stream, part of a feedback loop with parathyroid hormone and calcium, telling the body how to maintain the right amount of calcium in the blood. This is the basis of why people sometimes refer to vitamin D as a hormone.
But this isn’t quite right either, as vitamin D is not a hormone itself, while its activated form is. Thus some choose to call vitamin D a “pro-hormone” or a “pre-hormone,” meaning that it helps, assists, is a precursor, and is necessary to the production of a hormone.
While this might be a bit more accurate of a description, I can’t envision a world in which we call vitamin D, “prohormone D” or some other name insinuating hormonal activity; mainly because it’s called “vitamin D” and always has been, regardless of the inaccuracy of the word “vitamin.”
And I’m not sure it matters so much. I’ve read my fair share of internet debates, some dim-witted:
“I’m not going to take a vitamin D supplement because it’s a hormone, and I don’t mess with hormones. I’ll let my body do the regulating.”
And some founded and hopeful:
“Doctors would take vitamin D more seriously if they knew it was hormonal.”
What’s important is that we improve the world’s vitamin D status. It’s not known if changing the name of vitamin D would improve this. My guess is that it wouldn’t.
In all this speculation, I think of biotin, a B-vitamin, and something no one takes because the body synthesizes it in the gut. Now let’s pretend for a moment that the research behind biotin is equally compelling to vitamin D’s: many people are deficient in it and treating this deficiency may prevent and treat many diseases. One solution to this issue would be to convince the public to begin supplementing with biotin.
As a guy who’s been out on the ground trying to explain that being aware of vitamin D is important, I can definitely say the word “vitamin” is on my side. It seems to lure people in, even if they know nothing of vitamin D, and how the body can synthesize it when out in the sun. They hear the word “vitamin,” and they assume that it must be something they need.
With biotin, I’m not sure I’m given this benefit. If biotin deficiency were a serious issue, I think there would be bigger barriers in its promotion, as fastidious as that might sound.
In conclusion, it’s interesting to know why vitamin D is called vitamin D, and why some argue that we shouldn’t call it a vitamin, rather a pro-hormone or something of the like. But from a public health standpoint, it’s probably unimportant getting this right and futile to get bogged down by specifics. In fact, the word vitamin might even play to its favor. For all we know, vitamin D could have been ill-graced with a name like biotin.
And without digressing further, what’s important is that we improve vitamin D status on the population level, and with that in mind, calling vitamin D — vitamin D — does just fine.