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Not Technically a Vitamin

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artistinerokeitsi

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What vitamin D actually is,

and why the internet is wrong about almost all of it.

A confused lamb holding a bottle of vitamin D

What it is, where it comes from, and why a sheep is involved

Vitamin D is not actually a vitamin. It is a prohormone, meaning your body converts it into an active hormone. The “vitamin” part is a historical accident that stuck.

The D, incidentally, is also an accident of history. In the early twentieth century, biochemist Elmer McCollum was systematically naming newly discovered vitamins in alphabetical order. A and B had already been claimed, C followed shortly after. When the factor preventing rickets was isolated, it simply got the next available letter. McCollum had initially suspected it might be a new form of vitamin B, and when it turned out to be something distinct, it was bumped into its own category and given a D. No grand reasoning. No clever symbolism. Just the alphabet, running out of better options.

And while we are clearing up misconceptions, there are actually two main forms worth knowing about: D2 and D3.

D2 comes from plants, fungi and yeast. D3 is what your body produces when your skin meets sunlight, and it is also found in animal-based foods. Both raise your vitamin D levels, but D3 does it more effectively and keeps levels up for longer. For most people, D3 is the one worth looking for on the label.

Now here is where it gets genuinely interesting. Most D3 supplements are made from lanolin, the natural wax found in sheep’s wool. The process mimics exactly what happens in your skin on a sunny day: a compound called 7-dehydrocholesterol is exposed to UV radiation and converted into cholecalciferol, which is the chemical name for D3. The sheep are not harmed. The lanolin is a byproduct of normal shearing, and without this use it would largely go to waste. The end result is chemically identical to what your body makes itself.

For vegans, there is now a plant-based D3 option derived from lichen, a slow-growing organism somewhere between algae and fungi. Same molecule, no animal involvement, though noticeably pricier to produce.

As for getting your D from sunlight, that is the conventional wisdom and it is broadly true. With one catch. I am a redhead with skin type I, which means I burn faster than I can say “D is not actually a vitamin.” Any meaningful sun exposure ends with me retreating to the shade. For people like me, a supplement is not a backup plan. It is the plan.

Ginger bearded Keitsi holding sunscreen tube while standing in very bright sunlight.

What it does in your body

So what does this prohormone actually do once it gets inside you?

For decades, the answer was simple and boring. Bones. Vitamin D helps absorb calcium, calcium builds bones, end of story. Textbooks were written. Doctors nodded. Rickets was defeated. Everyone went home.

Then researchers started finding vitamin D receptors in places that had nothing to do with bones. The receptor turned up in skin, the pancreas, the placenta, colon cancer cells, activated T cells. This was unexpected enough to cause some serious head-scratching in research departments. If vitamin D is only about bones, why does it have a dedicated receptor in your immune cells?

It does not, it turns out, have much interest in staying in its lane.

Bones and calcium metabolism remain the most solid ground. This is where the evidence is clearest, the mechanisms are understood, and the clinical recommendations are not going to change anytime soon.

Muscle function is nearly as well supported, particularly in older adults. Low vitamin D levels are consistently associated with weakness, poor grip strength, and increased risk of falls. Not glamorous, but genuinely important.

Immunity is where things get interesting. Vitamin D plays a role in both innate and adaptive immune responses, influencing how cells proliferate, differentiate, and deal with threats. The evidence on respiratory infections is solid enough that it has survived multiple large reviews without falling apart, which in nutritional science is something of an achievement.

Mood and depression show a consistent association with low vitamin D levels. Whether supplementing actually lifts mood in people who are not deficient is a different and messier question. The trials are mixed. Worth knowing, not worth overclaiming.

Testosterone is genuinely interesting. A 2024 meta-analysis covering seventeen randomised controlled trials found that supplementation did raise total testosterone levels in men, though the effect on free testosterone, the form your body actually uses, was not significant. So something is happening, but whether it matters clinically remains unclear. The wellness industry has naturally decided it is the most important finding in human history.

Cancer, cardiovascular disease, autoimmune conditions, dementia. All show intriguing associations in observational studies. All have so far been frustratingly modest in intervention trials. The pattern is consistent enough to take seriously and inconsistent enough to resist any sweeping conclusions. More research is, as always, ongoing. Researchers are, as always, cautiously optimistic.

The honest summary is this. Vitamin D almost certainly does more than keep your skeleton from collapsing. Exactly how much more, and for whom, is a question science is still working through without the drama that the supplement industry prefers.

How much, and how do you know

This is where things get refreshingly undramatic, at least compared to what the wellness industry would have you believe.

Below 50 nmol/L is universally considered deficient. Above 125 nmol/L you are not gaining anything extra and may be edging toward harm. The sweet spot most guidelines agree on sits somewhere in between, though exactly where depends on which organisation you ask and what year their guidelines were last updated.

The Nordic Nutrition Recommendations make one point worth highlighting: supplementation does not appear to provide additional health benefits when your levels are already sufficient. The benefits are real, but they belong to people who actually need them. Taking more when you are already fine is not optimisation. It is just expensive urine.

For most healthy adults, somewhere between 25 and 50 micrograms daily through autumn and winter is a reasonable starting point. You will see the same amount written as 1000 to 2000 IU on some labels, since one microgram equals 40 IU. Finnish supplement labels typically use micrograms, which is arguably the more sensible unit, though neither tells you much without knowing where your levels actually are.

The 2024 Endocrine Society update stepped back from recommending routine screening for healthy adults without risk factors, noting that clinical trials have not established a clear threshold tied to specific health benefits beyond bone health. In plain language: if you feel fine, have no risk factors, and eat a reasonably varied diet, you probably do not need to rush to your doctor for a blood test. If you have bone pain, persistent fatigue, muscle weakness, or any of the conditions associated with deficiency, testing makes sense.

The test itself measures 25-hydroxyvitamin D in your blood, which is the most reliable indicator of your actual status. It captures both what you eat and what your skin produces, which is convenient given that one of those sources is a Finnish winter sky in February.

If you do get tested and your levels are low, do not panic. This is one of the more fixable nutritional deficiencies in existence. A modest daily supplement, a bit of patience, and a retest in three months is usually all it takes.

Can you take too much

Yes, though you have to work fairly hard at it.

The generally accepted upper limit is 100 micrograms per day, or 4000 IU. Most standard over-the-counter supplements contain between 10 and 25 micrograms, so staying within safe territory is not difficult if you are reading labels.

The problem with overdoing it is not immediate drama. Vitamin D toxicity is rare, and acute symptoms typically require blood levels above 375 nmol/L sustained over time. What actually goes wrong is a slow buildup of calcium in the blood, a condition called hypercalcemia, because vitamin D’s main job is to help your body absorb calcium and at high enough doses it does this rather too enthusiastically.

Symptoms of toxicity include nausea, vomiting, increased thirst, frequent urination, confusion, muscle weakness, and in serious cases kidney stones. None of these are pleasant, but they also do not tend to appear from sensible supplementation. The cases in the medical literature that make for grim reading involve people taking tens of thousands of IU daily for months, sometimes based on advice from the darker corners of the internet.

There is also a subtler finding worth mentioning: one study found that older adults at risk for falls actually had an increased fall risk when taking higher doses of 50 to 100 micrograms daily, compared to those on lower doses. More is not always better, and in this case it can be actively worse.

The practical takeaway is the same as most things in nutrition. A sensible amount does real good. An absurd amount does real harm. The gap between those two points is wide enough that most people have nothing to worry about.

Letter D and a questionmark
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So, should you actually take it

Here is the unsexy conclusion that no supplement brand will ever put on their packaging.

If you live somewhere with a real winter, work indoors, are over 60, have darker skin, or belong to the subset of humanity that bursts into flames at the mere suggestion of sunshine, the case for a modest daily supplement is genuinely solid. Not miraculous. Not life-extending in the dramatic sense that certain Reddit accounts would have you believe. Solid. Supported. Sensible.

If you are already getting adequate sun exposure, eat oily fish regularly, and your blood levels are fine, an extra supplement is unlikely to do much beyond keeping the supplement industry comfortable.

The honest version of this story is less exciting than the version where a single tiny capsule unlocks longevity, supercharges your testosterone, defeats cancer, and reorganises your gut microbiome into a thriving ecosystem of wellness. Vitamin D is not that. It is a prohormone your body genuinely needs, that a significant chunk of the population is short on, that is cheap and safe to supplement at sensible doses, and that does real measurable good when the people who actually need it start taking it.

It is made from sheep wool, or failing that, lichen. It is not technically a vitamin. And for those of us who treat direct sunlight as a personal threat, it is probably worth taking from October to April without much agonising about it.

That is it. No miracle. No hype. Just a small capsule doing a quiet, useful job while the rest of the wellness internet argues about whether you should be taking 50,000 IU and meditating facing magnetic north.

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