The sunshine molecule: A guide to vitamin D
We often call vitamin D a “vitamin,” but that is actually a biological misnomer. In reality, vitamin D acts more like a pro-hormone—a vital chemical messenger that your body can manufacture all on its own, provided it gets the right ingredients.
Despite our sun-drenched climate, vitamin D deficiency is surprisingly common in Australia, affecting roughly a quarter of the population. Let’s dive into what this incredible molecule does, how your body creates it, and how to keep your levels optimal.
What vitamin D does in the body
For decades, vitamin D was celebrated primarily for its role in bone health. It acts as the biochemical key that unlocks your gut’s ability to absorb calcium and phosphorus. Without enough of it, your body cannot build or maintain strong bones, leading to conditions like rickets in children or osteomalacia (bone softening) and osteoporosis in adults.
However, modern science has revealed that nearly every tissue in your body features vitamin D receptors. It plays a massive role in:
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Immune function: It helps your immune system fight off viruses and bacteria while preventing chronic inflammation.
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Muscle strength: Proper levels improve muscle function and reduce the risk of falls, especially in older adults.
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Mood and mental health: Low vitamin D is heavily linked to seasonal affective disorder (SAD) and low mood
How your body converts vitamin D
Your body does not just absorb vitamin D; it manufactures it through a multi-step chemical assembly line.
It all starts in your skin. When ultraviolet B (UVB) radiation from the sun hits your skin, it interacts with a form of cholesterol and converts it into vitamin D3 (cholecalciferol). This form is biologically inactive. To become useful, it travels through your bloodstream to your liver, where it is converted into 25-hydroxyvitamin D, or calcidiol. Finally, it moves to your kidneys, where it undergoes a final transformation into 1,25-dihydroxyvitamin D, or calcitriol—the fully active hormone your body can actually use.
Seasonality and skin colour
Getting your vitamin D from the sun seems simple, but it is governed by a complex equation of geography, biology, and timing.
1. “Vitamin D winter”
Depending on where you live in Australia, the sun might not be capable of producing vitamin D for part of the year. If you live in southern cities like Melbourne, Hobart, or Adelaide, the winter sun sits very low in the sky. The atmosphere filters out the crucial UVB rays, meaning you could stand outside at length in July and synthesise very little vitamin D. In northern locations like Darwin or Cairns, UVB levels remain high enough for synthesis year-round.
2. Skin colour
Melanin—the pigment that gives skin its colour—acts as a natural sunscreen. It protects the skin from dangerous UV damage, but it also slows down vitamin D synthesis. If you have deeply pigmented skin, you may need anywhere from 3 to 7 times longer in the sun to produce the same amount of vitamin D as someone with very fair skin.
Does sunscreen block vitamin D?
Historically, older reviews showed that while sunscreen theoretically blocks vitamin D synthesis in strict laboratory settings, real-world community use showed very little impact because people rarely apply enough, miss spots, or stay out longer when wearing it.
However, an important Australian clinical trial (the Sun-D Trial) looked at the modern reality of using high-protection SPF 50+ sunscreen daily. The trial found that routinely applying SPF 50+ on days when the UV index is 3 or higher does result in lower blood vitamin D levels and increases the risk of deficiency compared to discretionary use.
Because Australia has the highest incidence of skin cancer in the world, you should not stop using sunscreen. Instead, health bodies advise balancing the risks by using targeted, short bursts of safe sun exposure or utilising supplements if you are a rigorous sunscreen user.
How to get enough sun exposure safely
To maintain your vitamin D without increasing your risk of skin cancer, you want to achieve “sub-erythemal” exposure—meaning you get enough UV light to trigger vitamin D production without ever turning red or burning.
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When the UV index is below 3 (typically southern winter): Sun protection is generally not required unless you are outdoors for extended periods. Spend time outdoors in the middle of the day with some skin eg arms, exposed.
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When the UV index is 3 or above (summer, and year-round in the north): Mid-day sun is highly efficient at making vitamin D. For fair skin, exposing the arms or legs for just a few minutes outside of peak UV hours on most days of the week is plenty. Darker skin types will require significantly longer to achieve the same result.
Dietary vitamin D
While the sun is our primary source, you can get modest amounts of vitamin D from your diet, typically less that 10% of daily requirements. True dietary sources are limited, but they include:
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Fatty fish: Wild-caught salmon, mackerel, sardines, and herring.
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Cod liver oil: A single tablespoon packs a significant dose.
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Egg yolks and mushrooms: Egg yolks contain small amounts, as do mushrooms that have been exposed to UV light.
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Fortified foods: In Australia, certain brands of milk, margarine, and yoghurts are artificially enriched with vitamin D to support public health.
Measure your levels
If you suspect your levels are low, do not guess—test. The gold standard for checking your status is a simple pathology blood test called the 25-hydroxyvitamin D test. In Australia, the normal range is above 50 nmol/L but above 75 nmol/L may be preferable, especially of bone health is a problem.
Note: For Australians with specific risk factors like deeply pigmented skin, institutional living, signs of osteoporosis or chronic medical conditions, this test is fully supported under the Medicare Benefits Schedule. Routine screening costs about $70-100.
How to supplement safely
If you work long hours indoors, have dark skin, strictly apply SPF 50+ every day, or have documented low levels, supplementation is an excellent, safe way to keep your levels up.
1. D3 and K2
When buying a supplement at the chemist, look for a formula that couples vitamin D3 with vitamin k2 (specifically in the MK-7 form). These two work as an essential biological team. While vitamin D increases calcium absorption in your gut, vitamin k2 acts as the traffic cop—activating proteins like osteocalcin to ensure that calcium is deposited safely into your bones and teeth, rather than hardening into your arteries or kidneys.
2. Capsule vs spray
Traditional caps and liquid drops are the most common methods.
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Traditional oral supplements: Because vitamin D is a fat-soluble nutrient, standard capsules or drops should always be taken with a meal containing healthy fats (like avocado, eggs, or olive oil) to ensure your gut can absorb them efficiently.
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Oral sprays: Sprayed inside the cheek or under the tongue, these skip traditional digestion by absorbing directly through the vascular tissue in your mouth. They are a brilliant option for anyone with gut absorption issues like coeliac disease or IBS.
Rather than taking massive “mega-doses” infrequently, Australian endocrine guidelines suggest a “slow and steady” approach of moderate daily or weekly oral doses (e.g., 1000 IU to 4000 IU daily) as the safest and most effective strategy for your body.
Mason, R. S. (2025). Vitamin D: an update. Endocrinology Today, 14(1), 12–18.
