What is vitamin D?
Vitamin D is a fat-soluble vitamin and pro-hormone that plays a central role in bone health, immune function, muscle strength and overall metabolism. Your body makes most of its vitamin D when your skin is exposed to UVB rays from sunlight, with smaller amounts coming from foods like oily fish, egg yolks and fortified dairy. Blood tests measure the storage form, 25-hydroxyvitamin D (25(OH)D), reported in nmol/L in Australia.
Vitamin D is one of the most commonly ordered nutrient tests because deficiency is widespread, even in sunny climates like Australia. Despite our sunshine, around one in four Australian adults has vitamin D deficiency, and rates can climb to almost half in southern states during winter. You can read more in our guide to vitamin D deficiency in Australia.
Why does vitamin D matter for long-term health and wellbeing?
Vitamin D helps your body absorb calcium and phosphate, which are essential for strong bones, healthy teeth and proper muscle function. It also acts on receptors found in almost every tissue in the body, influencing immune function, inflammation, insulin sensitivity, cardiovascular health, mood and even how genes are expressed. Low levels have been linked with osteoporosis, fractures, muscle weakness, recurrent infections and increased risk of chronic conditions.
Because vitamin D touches so many systems, deficiency can quietly contribute to fatigue, low mood, poor recovery and long-term health risks well before any obvious symptoms appear. Checking your level gives you a clear, actionable starting point that is easy to correct with the right approach. It is one of the nutrient markers Vively tracks as part of its baseline health testing.
What is an ideal vitamin D level?
In Australia, vitamin D is generally interpreted using ranges from the Australian and New Zealand Bone and Mineral Society and Osteoporosis Australia. Below 30 nmol/L is considered deficient (severe below 12.5 nmol/L), 30 to 49 nmol/L is mild deficiency or insufficiency, and 50 nmol/L or above is sufficient for bone health, with levels ideally higher at the end of summer to buffer winter decline. Many clinicians consider an optimal range for wider health benefits to sit around 75 to 125 nmol/L, though this is not an official cut-off.
Levels above 250 nmol/L are considered potentially toxic. Interpretation depends on your age, sex, ethnicity, pregnancy status, medications, chronic conditions and season, so results are best reviewed in context. A single reading is more meaningful when interpreted alongside your history and, ideally, tracked across seasons.
What influences vitamin D levels?
Sun exposure is the biggest driver, and it depends on time of day, season, latitude, cloud cover, sunscreen use and how much skin is exposed. Skin tone matters too, since higher melanin reduces vitamin D synthesis and darker skin needs longer sun exposure to make the same amount. Age also plays a role, with older adults producing less vitamin D in the skin and often having lower dietary intake.
Body composition affects vitamin D, since it is fat-soluble and can be sequestered in body fat, lowering circulating levels in people with obesity. Diet contributes through oily fish, egg yolks, mushrooms exposed to UV light, fortified dairy or plant milks and cod liver oil. Malabsorption conditions such as coeliac disease, inflammatory bowel disease, cystic fibrosis and post-bariatric surgery can also reduce levels.
Liver and kidney health influence vitamin D too, since both organs are needed to activate it. Medications including corticosteroids, anti-epileptic drugs, some antiretrovirals, cholestyramine and orlistat can lower vitamin D. Pregnancy, breastfeeding and season are further important factors, and in Australia levels typically fall in winter and rise across summer.
What are the symptoms of low vitamin D?
Mild vitamin D deficiency often has no symptoms, which is one of the main reasons it goes undetected. When symptoms are present, they can include persistent fatigue or low energy, muscle weakness, aches or cramps, bone pain (particularly in the hips, lower back or legs), frequent infections and slow recovery from illness. Some people also notice low mood, brain fog, difficulty concentrating, hair thinning or slow-healing wounds.
In children, severe deficiency can cause rickets, and in adults it can cause osteomalacia (soft, weakened bones). Symptoms are rarely specific to vitamin D, so testing is far more reliable than trying to interpret how you feel. Any persistent symptoms are worth discussing with your GP alongside a full clinical picture.
What causes low vitamin D?
Low vitamin D is common in Australia and is usually driven by limited sun exposure, which can result from indoor lifestyles, shift work, high SPF sunscreen use, covering clothing or living in southern states. Darker skin tones naturally require more sun exposure to synthesise vitamin D, and older age reduces the skin's ability to produce it. Obesity is another common contributor, since vitamin D is stored in body fat and less is available in circulation.
Malabsorption disorders such as coeliac disease, inflammatory bowel disease and cystic fibrosis, as well as post-bariatric surgery, can significantly reduce vitamin D absorption. Chronic liver or kidney disease impair the activation of vitamin D, and medications such as corticosteroids, anti-epileptic drugs and orlistat can lower levels. Exclusive breastfeeding without vitamin D supplementation is another common cause of low levels in infants.
What are the symptoms of high vitamin D?
High vitamin D is far less common than deficiency and almost always related to over-supplementation rather than sun or food intake. Symptoms usually appear when vitamin D causes blood calcium to rise (hypercalcaemia), and can include nausea, vomiting, poor appetite, constipation or abdominal pain. Excessive thirst, frequent urination, muscle weakness, confusion, drowsiness and, in severe cases, kidney stones or an irregular heartbeat may also occur.
These symptoms need prompt medical review, since untreated hypercalcaemia can become dangerous. If you take vitamin D supplements and develop any of these signs, stop supplementation and see your GP.
What causes high vitamin D?
High vitamin D almost always reflects excessive intake from supplements rather than sun or food. Common causes include taking high-dose supplements without medical supervision, combining multiple supplements that contain vitamin D (such as multivitamins, fish oil and bone health formulas), and prescribed high-dose loading doses without appropriate follow-up. Rare medical conditions such as sarcoidosis, some lymphomas and other granulomatous diseases can also increase conversion of vitamin D to its active form.
Sun exposure and food alone are extremely unlikely to cause toxicity, as the body has natural mechanisms to regulate skin production. This is why testing before starting high-dose supplements and retesting after 8 to 12 weeks is a sensible approach.
What does it mean if vitamin D is outside the optimal range?
A low vitamin D level suggests your body has insufficient supply to support bone, muscle and immune function, and long-term deficiency may increase the risk of osteoporosis, fractures and other health issues. It does not diagnose any single condition on its own, but it is a signal worth acting on, particularly if it is persistent or severe. Mild insufficiency often responds well to safe sun exposure, dietary changes or supplementation.
A very high vitamin D level, particularly above 250 nmol/L, may indicate over-supplementation or an underlying condition affecting vitamin D metabolism. High vitamin D combined with high calcium requires medical review. As with all markers, patterns over time and the context of your other results matter more than any single reading.
Can vitamin D be normal but something still be wrong?
Yes, a "normal" vitamin D level does not always rule out other issues. Some people feel symptoms of deficiency (fatigue, muscle aches, low mood) even with a level considered technically sufficient, particularly if other nutrients like iron, B12, magnesium or thyroid hormones are also suboptimal. Symptoms usually have multiple contributors, and vitamin D is only one piece.
Vitamin D testing also has limitations, including seasonal variation, assay differences between labs and biological variability between individuals. A single reading may not capture your year-round pattern, especially if tested at the end of summer versus end of winter. This is why vitamin D is best interpreted alongside related markers and your broader health context.
What other markers should be checked with vitamin D?
Vitamin D works closely with calcium, so calcium and ionised calcium are important companions, and phosphate is often checked alongside them. Parathyroid hormone (PTH) typically rises when vitamin D and calcium are low, giving further insight into how well the system is working together. Magnesium is another useful marker, since it is needed to activate vitamin D.
Albumin is worth checking too, as it affects how calcium is measured and reflects nutritional status, and alkaline phosphatase (ALP) can flag bone turnover issues. Because fatigue and low mood often have multiple causes, iron studies, ferritin, B12 and thyroid function tests (TSH, free T4) are frequently checked at the same time. Inflammation markers such as hs-CRP add further context, as covered in our article on how the CRP blood test reveals hidden inflammation, and you can see the full set of markers Vively looks at through our tests page and shop tests page.
How can you improve vitamin D to a healthier level?
Safe, regular sun exposure is the most natural way to support vitamin D, ideally aiming for a few minutes on arms, face and legs most days in summer and longer periods in winter, outside peak UV hours. Including vitamin D-rich foods such as oily fish (salmon, sardines, mackerel), egg yolks, fortified milks and cereals, and UV-exposed mushrooms can help support levels. A daily vitamin D3 (cholecalciferol) supplement may be recommended by your GP or dietitian if your level is low, sun exposure is limited or you have higher needs.
Addressing underlying issues such as malabsorption, obesity or medication effects with your GP can make a significant difference, since simply adding vitamin D without treating the root cause may not lift levels effectively. Retesting after 8 to 12 weeks of any changes is a good way to see how your body is responding. Not every marker can or should be self-optimised, and Vively's how it works page explains how testing, monitoring and dietitian coaching combine to make change practical.
When does vitamin D need medical review?
See your GP if your vitamin D is significantly low (below 30 nmol/L) or is not improving with basic changes, or if your level is very high (above 250 nmol/L) or you have symptoms of toxicity. Any symptoms such as bone pain, muscle weakness, frequent fractures, kidney stones, ongoing fatigue or unexplained low mood are worth reviewing with a clinician rather than self-managing.
Clinical input is also important if you have chronic conditions such as osteoporosis, chronic kidney disease, malabsorption, liver disease, sarcoidosis or a history of gastric bypass surgery. Pregnancy, planning pregnancy, breastfeeding, and taking medications known to affect vitamin D or calcium metabolism are all reasons to check in. Vitamin D should not be self-diagnosed based on symptoms alone, as testing is the only reliable way to know your level and the right dose for you.
How does Vively help you understand vitamin D?
Vitamin D is one of the nutrient markers included in the Vively Baseline Health Check, alongside metabolic, cardiovascular, liver, kidney, hormonal and inflammation markers, adding up to more than 60 biomarkers in total. Rather than looking at vitamin D in isolation, Vively interprets it in context with your other results, symptoms, lifestyle, medications and season. That means a low result is not treated as a one-size-fits-all fix but as a signal to explore what may be contributing to it.
An accredited practising dietitian reviews your results with you one on one, helping you understand what they mean and what changes may support your health goals. Because your markers are retested over time, you can see how sun exposure, nutrition, supplementation and lifestyle are actually shifting your vitamin D and broader health profile. Start at the Vively homepage or explore the full range of tests in the Vively shop.
References
- Nowson CA, McGrath JJ, Ebeling PR, et al. Vitamin D and health in adults in Australia and New Zealand: a position statement. Medical Journal of Australia. 2012;196(11):686 to 687.
- Osteoporosis Australia and Healthy Bones Australia. Vitamin D. https://healthybonesaustralia.org.au/
- Royal College of Pathologists of Australasia. RCPA Manual: 25-hydroxyvitamin D. https://www.rcpa.edu.au/Manuals/RCPA-Manual/Pathology-Tests/V/Vitamin-D
- Healthdirect Australia. Vitamin D blood test. https://www.healthdirect.gov.au/vitamin-d-test
- Better Health Channel, Victorian Department of Health. Vitamin D. https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/vitamin-d
- Australian Bureau of Statistics. Vitamin D. https://www.abs.gov.au/articles/vitamin-d
- Royal Australian College of General Practitioners. Guidelines for preventive activities in general practice (Red Book). https://www.racgp.org.au/clinical-resources/clinical-guidelines/key-racgp-guidelines/view-all-racgp-guidelines/red-book
- Cancer Council Australia. Position statement: Risks and benefits of sun exposure. https://www.cancer.org.au/
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism. 2011;96(7):1911 to 1930.
- National Institutes of Health, Office of Dietary Supplements. Vitamin D fact sheet for health professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
- Cleveland Clinic. Vitamin D deficiency. https://my.clevelandclinic.org/health/diseases/15050-vitamin-d-vitamin-d-deficiency
- Mayo Clinic. Vitamin D. https://www.mayoclinic.org/drugs-supplements-vitamin-d/art-20363792
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