What If Vitamin D Was Never a Vitamin at All?

Here is the definition of a vitamin: an organic compound your body requires but cannot manufacture, which therefore has to come from food.

Vitamin D fails that test completely. Your skin makes it. Given sunlight and cholesterol, you produce it yourself — which means by the only definition that matters, it isn't a vitamin and never was.

What it actually is: a secosteroid hormone. "Seco" means cut — it's a steroid molecule with one of its carbon rings broken open. Structurally, it belongs to the same family as cortisol, oestrogen and testosterone. It behaves like them too, travelling through the bloodstream, binding to a dedicated receptor, and instructing cells to change what they're doing.

So why is it called a vitamin? The answer is a genuinely good story, and it explains a great deal about why we still think about this the wrong way.

How It Got the Wrong Name — and Kept It

To understand the mistake, you have to start with the disease.

In the industrial cities of the nineteenth and early twentieth centuries, rickets was everywhere — soft, bowed bones in children, at rates approaching the majority of the child population in places like Glasgow, Manchester and the tenements of New York. It was so associated with Britain's factory towns that it was known in Europe as the English disease.

Nobody knew the cause. In hindsight it is almost unbearably obvious: coal smoke thick enough to block the sun, narrow streets that never saw daylight, and children working indoors. Rickets was the first mass disease of industrial indoor living. It was a sunlight deficiency.

1919: two cures that made no sense together

In the same year, two entirely separate discoveries landed.

In London, Edward Mellanby reared dogs indoors until they developed rickets, then cured them with cod liver oil. A dietary cause, a dietary cure — case closed. He assumed the active ingredient was vitamin A, already known to be in the oil.

In Berlin, Kurt Huldschinsky cured rachitic children using a mercury-vapour ultraviolet lamp. No food involved at all. Two years later, Hess and Unger in New York did the same thing with nothing but rooftop sunshine.

So there were now two cures for one disease — one you swallowed, one you stood under — and no plausible way to connect them.

1922: the letter is assigned

At Johns Hopkins, Elmer McCollum ran the experiment that settled the food half. He bubbled oxygen through cod liver oil to destroy its vitamin A, then fed it to rachitic animals. It still cured them. Whatever was doing the work was a distinct, unnamed factor.

Vitamins A, B and C had already been claimed. This was the fourth. It was named vitamin D for no better reason than that D came next.

The label was applied in the same decade the ultraviolet cure was being demonstrated on the other side of the Atlantic — and it took several more years for anyone to establish that the substance in the fish oil and the substance made by UV light in skin were the same molecule. Adolf Windaus later worked out the chemistry, showing it was a modified sterol built from cholesterol — a secosteroid — and won a Nobel Prize along the way.

By then the name was in the textbooks, on the labels, and in the fortified milk that Harry Steenbock's UV-irradiation patent had put in every American kitchen. It was, by any reasonable measure, too late.

The full hormone picture didn't arrive until the 1960s and 70s, when researchers mapped the liver and kidney conversions and identified the receptor it binds to. That's when it became undeniable that this was an endocrine system, not a nutrient.

But scientific names are sticky in a way scientific understanding isn't. Nobody was going to relabel every bottle, textbook and food package on earth over a technicality. The name records the experiment that found it, not the molecule itself — and a hundred years later it's still quietly shaping how we think about the problem.

Which is not a trivia point. Call it a vitamin and the solution sounds like a purchase. Call it a hormone and the solution sounds like sunlight — which is what it has always actually been.

The Three-Stage Conversion Nobody Explains

What arrives on your skin is not what your cells use. There's a relay involving three organs, and it's worth understanding because it explains why blood tests measure one thing while your body runs on another.

And once it binds that receptor, the scale of what it does is genuinely startling.

The vitamin D receptor isn't confined to bone and gut, which is where the textbooks used to leave it. It has been found in almost every tissue type in the human body — immune cells, muscle, brain, breast, colon, vascular tissue. When calcitriol binds it, the complex moves into the cell nucleus and acts as a transcription factor, influencing the expression of over a thousand genes, by some estimates several percent of the entire human genome.

That's not how a nutrient behaves. That's how a hormone behaves.

The Most Natural Source Is Also the Free One

Sunlight remains, by an enormous margin, the most efficient way to raise vitamin D. Brief exposure to summer sun can generate several thousand international units — far more than any realistic amount of food delivers.

But there are four things about sun exposure that almost nobody gets right.

Glass blocks it entirely

This is the one that changes people's behaviour immediately. Standard window glass blocks essentially all UVB while letting UVA through. A sunny office, a conservatory, a long drive — none of it produces any vitamin D at all. You get the ageing wavelength and none of the benefit. If you're indoors, you're getting nothing, however bright it looks.

Midday is when it works

Counterintuitively, gentle morning and late-afternoon sun is nearly useless for this purpose. UVB only reaches the ground in usable quantity when the sun is high — roughly between 10am and 3pm. A shorter exposure at noon does more than an hour at five o'clock.

It plateaus, and then it only harms

This is the most important thing in the article. Your skin has a ceiling. Once it has produced its maximum, further UV exposure creates no additional vitamin D — the body degrades the excess to prevent toxicity — while UV damage keeps accumulating.

There is no version of this where burning helps. Roughly ten to thirty minutes on arms and legs for fair skin in summer, several times a week, gets most people there. Longer buys you nothing but risk.

Your skin tone changes the maths considerably

Melanin is natural sun protection, and it's effective. Deeply pigmented skin may need three to six times the exposure to produce the same amount — which is a substantial and under-discussed reason vitamin D deficiency is far more common among people with darker skin living at higher latitudes.

Sunscreen and Vitamin D: The Contradiction That Isn't One

I need to deal with this properly, because I've argued hard for daily SPF elsewhere on this site, and on the face of it the two positions don't sit together.

The tension is real and it's chemical. The exact same UVB wavelengths that make vitamin D are the ones that burn you and drive skin cancer risk. There is no separating them. Sunscreen is explicitly designed to filter those rays out, so in a laboratory — correct application, full coverage, thick layer — high-SPF sunscreen does substantially reduce synthesis.

And yet regular sunscreen users don't turn up deficient in the research. Four reasons, and the last one genuinely surprised me.

SPF 30 leaves 3% through — and 3% is plenty

SPF is a filter, not a wall. SPF 30 blocks roughly 97% of UVB; SPF 50 blocks about 98%. That remaining fraction is small in percentage terms and substantial in absolute terms, because midday summer sunlight is extraordinarily strong. Three percent of a very large number is more than enough to trigger synthesis.

Nobody applies it properly

The SPF on the bottle is measured at 2mg per square centimetre of skin. Most people apply somewhere between a quarter and a half of that, miss patches entirely, and rarely reapply on schedule. You are almost certainly getting materially less protection than the label promises — which is a problem for burn prevention and, incidentally, the reason your vitamin D keeps working.

The threshold is low, and then it stops

You need far less than people imagine. Roughly five to fifteen minutes on face, arms and legs, two or three times a week maintains levels for most fair-skinned people in summer. After that your skin hits its ceiling and begins breaking down the excess to prevent toxicity — so prolonged exposure adds no vitamin D whatsoever while continuing to accumulate damage. The useful window is genuinely brief.

Broad-spectrum sunscreen may actually protect your vitamin D

This is the part that reframes the whole argument. UVB makes vitamin D — but UVA degrades it, breaking down vitamin D and related compounds in the skin.

A broad-spectrum sunscreen filters UVA effectively. Which means it may be preserving some of what you've just made, while also blocking the wavelength most responsible for photoageing and offering no benefit at all. The two aren't purely in opposition. On the UVA side, they're on the same team.

The Food You Can Actually Eat

Very few foods contain vitamin D naturally, and I'd rather be honest about that than pretend a salad will fix it. But the ones that do are worth knowing, and one of them is genuinely delightful.

The mushroom trick, which I love

Mushrooms contain ergosterol — a compound structurally close to the 7-dehydrocholesterol in your own skin. It undergoes exactly the same UV conversion, producing vitamin D2.

Which means you can do this at home. Slice mushrooms, lay them gills-up on a tray, and leave them in direct sunlight for fifteen to thirty minutes before cooking. Studies have found this can raise their vitamin D content from negligible to several hundred IU per serving. The vitamin survives cooking and storage.

It has to be real sunlight, not light through a window — the same UVB rule applies to fungi as to skin. There is something very satisfying about a food that makes its vitamin D the same way you do.

We've spent a century treating this as something to swallow, when the body was quietly waiting for something much simpler: to be taken outside at midday, with its arms uncovered, a few times a week.

What I Actually Do

Why the Reframe Matters

Calling it a vitamin makes it sound like a small thing you might be slightly short of — a gap to be plugged, ideally with a purchase.

Calling it what it is changes the picture. This is a hormone your body manufactures from cholesterol and light, converts across two organs, and then uses to regulate the expression of a thousand genes in nearly every tissue you have. It is part of your endocrine system. It has been part of it for the entire history of the species, in bodies that spent their days outdoors.

The deficiency now common across the industrialised world isn't really a nutritional gap. It's what happens when a species that evolved outside relocates indoors and behind glass. The mechanism is intact and working perfectly. It just isn't being given the one input it requires.

Which is oddly hopeful, I think. The fix isn't a product. It's fifteen minutes outside in the middle of the day — the least medical intervention imaginable, for something that turns out to be running most of the body.