Scientists aren’t sure exactly why vitamin D developed, but one theory is that it functioned as a kind of early sunscreen. It also helped with another stumbling block on the evolutionary road out of the water and onto land: calcium. Going from the calcium-rich environment of the sea onto dry land presented certain difficulties, namely getting enough of it, and it just so happens that the production of vitamin D in the body changes the ability of calcium to get into each cell. This likely made more of the calcium already present in the body usable.
Much like the noble phytoplankton, when rays from the sun strike your body, you (along with amphibians, reptiles, all bird species and most mammals) "photosynthesize" vitamin D to allow the body to metabolize calcium. It isn't actually the same process by which plants create food from sunlight, but it is literally a form of photosynthesis — the use of light to synthesize a chemical.
So necessary is this process to life on this planet that some have theorized the dinosaurs died out in part because — when the debris from an infamous asteroid blocked out the sun — the creatures couldn't produce enough vitamin D to carry on.
After all, it was tiny, nocturnal rodents that survived the dinosaurs — and then the apocalypse — to give rise to the age of mammals, and such organisms would have already sorted out how to acquire enough vitamin D without abundant sunlight.
So what is vitamin D, anyway?
First, a word on vitamins: The story of vitamins begins with a bunch of scientists trying to understand what we now know as deficiency syndromes, like scurvy and beriberi. Their work differentiating these types of diseases from those of an infectious nature led to our modern understanding of vitamins as organic compounds (that’s organic as in carbon-containing, not as in “the more expensive bananas.”) Vitamins are compounds that an organism requires for survival and cannot make enough of itself.
Today, there are 13 vitamins (that we know of) that keep the ol’ human meat sack in running order — vitamins that must be consumed, because the body cannot make enough to meet meat sack demands. But vitamin D is a weird one: Your body can make just plenty, provided some sunshine and perhaps a bit of nudity.
We can make all the vitamin D we need by flashing bare skin at a carcinogenic ball of plasma nearly 93 million miles away, but we can also get some of it from our food. And if you’ve ever wondered how a blast of midday UVB rays to the epidermis can get your body to synthesize the same compounds food chemists are out there fortifying milk with, then today is your lucky day.
The modern understanding of vitamin D begins with a bunch of researchers torturing puppies in the name of science. Yes, really. Aren't you glad you started down this sunny rabbit hole?
Rickets, a disease that causes bones to soften, became especially prevalent as the industrial revolution sent child workers indoors and factory pollution began to blot out the sun. When a rash of urban children developed skeletal deformities, researchers stepped up to find the cause and figure out a solution.
Enter the puppies.
In 1919, a scientist named Edward Mellanby successfully induced rickets in puppies by feeding them only bread and low-fat milk, noting that the resulting bone imaging and physical appearance of the dogs mimicked that of children suffering from rickets. Supplementing the diets of the wee puppers with yeast and orange juice (for B vitamins and scurvy-prevention, respectively) did nothing to stave off the bone disease; however, supplementation with both butterfat and cod liver oil did the trick. And with that, rickets was officially outed as a disease of deficiency. And where there's a deficiency, so, too, must there be a vitamin to be deficient in.
Today we know that rickets can be caused by a lack of or malfunction in the metabolism of phosphorus or calcium, and that the syndrome primarily results from vitamin D deficiency. During the early 20th century, it was discovered that certain foods — such as cod liver oil and whole milk — could heal rickets, but then research threw science a little curveball: It seemed as if going outside could also cure the disease.
In a most plant-like fashion, the bodies of these factory-working tykes could use sunlight to make an essential vitamin.
The term vitamin D refers to a group of fat-soluble steroids with a special “broken ring” formation. The shape of the compound is notable because it helps the body absorb critical nutrients such as calcium, iron, magnesium, phosphorus and zinc through the gut wall. This is all well and good, and explains in part why we add vitamin D to calcium-rich milk, but what does any of this have to do with sunlight?
As mentioned, there are two avenues to acquiring vitamin D: food and sun. And while yes, fatty fish flesh is a decent source of vitamin D compared to other foods — few of which naturally contain the compound — the bulk of vitamin D is produced in the skin thanks to a process known as photosynthesis, photodissociation, photolysis and — my personal favorite — photodecomposition.
Here’s how it works:
Human skin contains a compound that functions as a precursor to vitamin D, called 7-dehydrocholesterol; the compound is also a precursor to cholesterol. (Cholesterol gets a bad rap for giving bankers heart attacks, but is, in fact, completely necessary for the production of steroid hormones like androgens and estrogens, among other things.) When light from our neighborhood star lands on the skin, a specific portion of the light is able to transform 7-dehydrocholesterol into vitamin D3 by breaking a bond in the precursor molecule. The light that can do this is ultraviolet, meaning the length of the waves in which it travels are too short for the human eye to see. Specifically, it’s the UVB portion of the spectrum, which moves at a faster wavelength than its slower cousin, UVA rays. Simply put, sunlight breaks a bond in a molecule in your skin and then your body uses the new, sun-altered compound to manage its calcium needs.
And that’s how your body uses the photons streaming out of a 6-billion-year-old star to make its own vitamin D.
It doesn’t take much time to make enough vitamin D, if you time your light exposure correctly and you live in the right place. For a pale person – they make vitamin D the fastest — about 10 minutes in the midday summer sun with their arms and legs exposed is enough to make 50 times the vitamin D you need in a day. (But don’t worry, it's impossible to O.D. on vitamin D in the sun.)
However, even though it's astronomically easy for most people meet their vitamin D needs on a nice, June day, vitamin D deficiency is rampant in modern life. This is a significant problem, as those with the lowest levels of the vitamin may have more than twice the risk of death from heart disease, compared to those with the highest levels. And while it doesn’t take much summer light to make lots of the vitamin, those north of Atlanta are virtually doomed to not synthesize enough of the vitamin during the wintertime. This is because the angle of the light hitting the earth makes it so UVB rays cannot penetrate the atmosphere; this can be mitigated with supplementation, but has always struck me as curious, given that people have been living in darker, colder areas for millennia without the synthetic bolstering of vitamin D levels.
Then I remember that cultures in the darkest, coldest parts of the world often eat a lot of fatty fish — a great source of vitamin D.
Researchers estimate that many Americans are vitamin D-deficient, which can lead to heart trouble, cognitive impairments in older adults, severe asthma in children, increased risk of cancer, and, of course, rickets. One 2010 study found 41 percent of Americans to be vitamin D deficient, a staggering number with wide-reaching implications for an already-burdened health-care system.
All the more reason to get outside while the getting is good to make like a plant and photosynthesize.
Leigh Cowart is a freelance journalist covering science, sex and sports. She is fully vaccinated for rabies.