I wasn't planning to write about vitamin D. It felt too basic. Like writing about water, or sleep, or flossing. Everyone kind of knows they need it. Nobody's particularly excited about it.

Then I looked at the data.

One in four Australian adults is vitamin D deficient. Not insufficient. Deficient. Below 50 nanomoles per litre, which is the clinical threshold. That's roughly four million people, according to the Australian Bureau of Statistics. And during winter in Victoria, ACT, and Tasmania, that number climbs to nearly one in two.

In one of the sunniest countries on earth.

I'll admit that stat threw me. I grew up in Brisbane. I figured vitamin D deficiency was something that happened in Scandinavia, or maybe London. Not here. Not in a country where we've been told to slip-slop-slap since primary school. But that's part of the problem, actually. And I'll get to that.

The other thing that changed my mind about writing this was my own test result. Last year, mid-winter, I tested at 41 nmol/L. Technically deficient. I work in an office. I commute in the dark and come home in the dark for about three months of the year. I wear sunscreen religiously because... well, because I'm Australian and my mum drilled it into me. Turns out that combination of indoor work, sun protection, and winter is enough to drop your levels below the threshold. Even in Brisbane.

So. Maybe vitamin D isn't as boring as I thought. Maybe it's one of those markers that's worth understanding properly, especially if you're someone who assumes they're fine because they live in Australia.

A note before we get into it

The Australian paradox: sunny country, widespread deficiency

This is the bit that genuinely surprised me. The more I researched it, the more it made sense.

Australia has a complicated relationship with the sun. We have the highest rate of skin cancer in the world. The SunSmart campaign (slip, slop, slap, seek, slide) has been one of the most successful public health interventions in the country's history. And rightfully so. Melanoma is serious, and sun protection saves lives.

But the unintended side effect of four decades of effective sun safety messaging is that a lot of Australians now get very little unprotected UV exposure at all. And UV exposure is how your body makes vitamin D. Specifically UVB radiation, which triggers a chemical conversion in your skin that eventually becomes the active form of the hormone.

So we've ended up in this slightly absurd situation. We live in a sun-drenched country but spend most of our time indoors. When we do go outside, we're covered in SPF 50. Which is sensible. But it also means a large chunk of the population is quietly running low on a nutrient that most people assume they're getting plenty of.

The ABS data from the Australian Health Survey tells the story pretty clearly. Nationally, about 23% of adults are deficient year-round. But the seasonal variation is dramatic. In summer, deficiency rates drop to around 14%. In winter, they rise to 36% nationally, and much higher in the southern states. Victoria and the ACT hit 49% deficiency in winter. Tasmania sits at 43%.

Even Queensland, sun-blessed subtropical Queensland, has a 15% deficiency rate in winter. I did not expect that.

The geography matters because UVB availability changes with latitude and season. In southern Australia during winter, the sun sits too low in the sky for UVB to efficiently reach your skin, even if you spend time outdoors. It's not a willpower problem. It's physics.

What vitamin D actually does (and why it matters more than you'd expect)

Vitamin D isn't really a vitamin. It's technically a hormone, or more accurately a prohormone, that your body synthesises from sunlight and then converts through two stages (liver first, then kidneys) into its active form.

Its most established role is in calcium regulation and bone health. Vitamin D helps your intestines absorb calcium efficiently. Without adequate vitamin D, you can eat all the calcium you want and your body will struggle to use it. Long-term deficiency increases the risk of bone density loss, osteoporosis, and fractures, especially in older adults.

But it does more than just bones.

Vitamin D receptors are found throughout the body: muscle tissue, the immune system, the brain, the nervous system. Research has linked deficiency with muscle weakness (especially in the legs and hips), increased risk of falls in older adults, fatigue, and mood changes. There's also a growing body of observational research exploring associations between low vitamin D and conditions including autoimmune disease, cardiovascular health, and metabolic function.

Now, I should be careful here. "Associated with" is not the same as "causes." A lot of the more exciting vitamin D claims you see online are based on observational data. They show a pattern, not a proven mechanism. Large intervention trials have produced mixed results for many of these associations. The relationship between vitamin D and immunity, for example, is real at a biological level, but the clinical significance is still debated.

What we can say with confidence is that vitamin D matters for bone health, muscle function, and calcium regulation. And that deficiency is common, often silent, and usually easy to fix once identified.

That should be enough reason to check.

Who's most at risk in Australia

Not everyone is equally likely to be deficient. Some groups carry much higher risk, and it's worth knowing whether you fall into one of them.

People who spend most of their time indoors. Office workers, shift workers, people who are housebound or in residential care. If your daily sun exposure is limited to walking from your car to the building and back, you may not be synthesising enough vitamin D even in summer. This was my situation. I didn't think of myself as "indoors all day," but when I actually mapped my sun exposure, it was embarrassingly small.

People living in southern Australia. Victoria, Tasmania, ACT, and southern NSW and SA get far less UVB during winter. From May to August, it's difficult to maintain adequate vitamin D levels from sunlight alone in these regions, regardless of how much time you spend outside.

People with darker skin. Melanin, the pigment that gives skin its colour, reduces the skin's ability to synthesise vitamin D from UV exposure. People with naturally darker skin need considerably more UVB to produce the same amount of vitamin D as someone with lighter skin. This is a well-documented biological difference, not a lifestyle factor.

People who cover their skin for cultural or medical reasons. Clothing that covers most of the body limits UV skin exposure. Some religious and cultural dress codes mean very little skin is exposed to sunlight, which can contribute to deficiency.

Older adults. The skin's capacity to synthesise vitamin D decreases with age. Combine that with reduced outdoor activity and dietary changes, and it's no surprise that older Australians are disproportionately affected.

People with conditions affecting absorption. Certain gastrointestinal conditions (coeliac disease, Crohn's disease, inflammatory bowel disease) can impair the body's ability to absorb vitamin D from food and supplements. Some medications also affect vitamin D metabolism.

People who are obese. Vitamin D is fat-soluble, meaning it can be sequestered in body fat rather than circulating freely. Research shows that obese individuals tend to have lower circulating vitamin D levels even with comparable sun exposure.

If you recognise yourself in more than one of these categories (say, an office worker in Melbourne with darker skin), the compounding effect is real. Testing is just sensible at that point.

How the test works

The standard vitamin D test measures serum 25-hydroxyvitamin D (25-OH-D). That's the storage form of vitamin D, and it reflects your overall status from both sun exposure and dietary intake.

It's a simple blood draw. No fasting required. You can order it through Bloody Good, attend a pathology collection centre, and get your results through your secure dashboard. That's it.

A couple of things worth knowing about the test:

Timing matters. Vitamin D levels fluctuate with the seasons. If you test in August and get a low result, it might reflect winter depletion rather than a year-round problem. Conversely, testing in February might give you a falsely reassuring number that masks a winter dip. If you can only test once, winter or early spring gives you a more conservative (and arguably more useful) picture.

One test is a snapshot. Like most biomarkers, vitamin D is most informative when you can compare results over time. A single reading tells you where you are right now. Two or three readings, across seasons or before and after supplementation, tell you much more.

Medicare coverage is limited. In Australia, Medicare generally only covers vitamin D testing for people who meet specific criteria (certain medical conditions, high-risk groups, or clinical indication for testing). For everyone else, it's an out-of-pocket test. This is one of the reasons a lot of people who should probably check their levels never do. They don't have a specific medical reason to ask, and their GP may not order it routinely.

Understanding your results

The levels used in Australia to classify vitamin D status are based on the position statement from Osteoporosis Australia, the Endocrine Society of Australia, and the Australasian College of Dermatologists:

Below 30 nmol/L — Moderate to severe deficiency. Associated with increased risk of bone problems and may require higher-dose supplementation under clinical guidance.

30–49 nmol/L — Mild deficiency. May be associated with some health effects, especially in winter or if levels are trending downward.

50 nmol/L and above — Generally considered sufficient for bone and overall health in most people.

Above 75 nmol/L — The position statement notes that some people, especially those at the end of summer, may benefit from levels in this range to provide a buffer against seasonal decline.

A few caveats. These are population-based guidelines, not individual prescriptions. Some people feel noticeably better at 70 nmol/L compared to 55 nmol/L, despite both being "sufficient." Others feel no difference. Reference ranges provide a framework. Your experience is the other half of the equation.

Also: don't chase extremely high levels. Vitamin D toxicity is rare but real, and it comes almost exclusively from over-supplementation. Your body self-regulates production from sunlight, so sun exposure alone won't cause toxicity. More isn't always better.

The sun exposure balancing act

This is where the conversation gets genuinely tricky, because you're trying to optimise for two competing risks: skin cancer from too much UV, and vitamin D deficiency from too little.

The Cancer Council's position, which I think is the most balanced guidance available, is that sensible sun protection does not put most people at risk of vitamin D deficiency. For most Australians, a few minutes of incidental sun exposure on most days (when the UV index is 3 or above) is enough to maintain adequate levels. You don't need to sunbake. You don't need to burn. Brief exposure to forearms, hands, and face during daily activities is generally sufficient in summer.

The challenge is winter, especially in the south. Across much of southern Australia from May to August, UV levels drop below 3. Getting a vitamin D-effective dose from sunlight becomes difficult regardless of how long you spend outside. During these months, the advice shifts: spend more time outdoors in the middle of the day when UV is highest, expose more skin where practical, and consider supplementation if your levels are low.

I'll be honest. I found this confusing at first. We spend decades being told "avoid the sun" and then the guidance is "actually, you need some sun, but only at certain times, in certain amounts, and not too much." It makes sense when you understand the science, but the messaging is genuinely hard to follow for most people.

If you're protecting your skin diligently (which you should be), make sure you know where your vitamin D sits. Especially heading into winter.

Supplementation: what the evidence says

If your levels are low, your healthcare provider may recommend supplementation. A few things to be aware of:

Vitamin D3 (cholecalciferol) is preferred over D2 (ergocalciferol) for supplementation. D3 is more effective at raising and maintaining blood levels. Most supplements available in Australia are D3.

Dosing depends on your starting level and your clinician's assessment. Standard maintenance doses for mild deficiency are typically in the range of 1,000–2,000 IU per day. Moderate to severe deficiency may require higher loading doses for a period, followed by maintenance. Your GP can advise based on your results.

It takes time. Vitamin D levels don't shift overnight. Most clinicians recommend retesting after at least 3 months of consistent supplementation to assess whether the dose is adequate.

Vitamin D works alongside other nutrients. Calcium, magnesium, and vitamin K all play roles in the pathways vitamin D supports. So "just take vitamin D" isn't always the complete picture. A balanced diet matters alongside supplementation.

Don't mega-dose without guidance. Vitamin D is fat-soluble, which means excess amounts are stored rather than excreted. Toxicity can cause elevated calcium levels, kidney issues, and other complications. This is extremely rare at standard supplement doses but becomes a risk with very high-dose, long-term supplementation. Always follow your clinician's recommendations.

When to retest

Timing your retest matters. Here's a general guide:

After starting supplementation: Retest at 3 months to check whether the dose is adequate and your levels are responding.

Seasonal monitoring: If you tested low in winter, a follow-up in late summer can tell you whether your levels recover naturally with increased sun exposure, or whether you need year-round supplementation.

Ongoing tracking: For people in high-risk groups, an annual vitamin D check (ideally at the end of winter, when levels are lowest) is a reasonable habit to build into your health routine.

After a change in circumstances: Moved from Brisbane to Melbourne? Started working night shifts? Developed a gut condition? These are all reasons to re-check, because the variables that determine your vitamin D status have changed.

Tests to consider through Bloody Good

Related tests worth considering if you're investigating fatigue or bone health alongside vitamin D:

If you'd rather do a broad baseline:

The Bloody Good Test covers 100 biomarkers in one test, including vitamin D alongside cholesterol, liver function, kidney function, blood sugar, and more. If you've never done a broad health check, this covers the vitamin D question plus everything else in a single blood draw.

You can find a collection centre near you here.

What to do with your results

If your level is 50 nmol/L or above: You're likely in a good range for general health. If you tested in summer, keep in mind that your winter levels may be lower. Some clinicians suggest aiming for 60–75 nmol/L at the end of summer to provide a seasonal buffer.

If your level is 30–49 nmol/L: Mild deficiency. Your GP may recommend a daily supplement (typically 1,000–2,000 IU of vitamin D3) and increased safe sun exposure where possible. Retest in 3 months.

If your level is below 30 nmol/L: Moderate to severe deficiency. This usually warrants discussion with your GP about a higher-dose supplementation protocol, plus investigation of any underlying factors (absorption issues, medication interactions, lifestyle factors). Retest at 3 months to monitor your response.

Regardless of your result: Vitamin D is one marker. It doesn't exist in isolation. If you're dealing with fatigue, mood changes, muscle weakness, or bone pain, consider testing other relevant biomarkers alongside it: iron, thyroid function, B12, HbA1c. Sometimes what looks like a vitamin D problem is actually a vitamin D problem plus something else.

And if everything comes back fine but you still don't feel right, keep looking. Talk to your GP. The answer might be in a different test, a different conversation, or a different aspect of your health entirely.

Explore more biomarkers

If you want to go deeper into any of the markers mentioned here, the Bloody Good biomarker directory has detailed pages on what each test measures and how to think about results in general terms.

Browse the Bloody Good Biomarker Directory