Vitamin D Deficiency: Blood Tests That Matter

Vitamin D deficiency is one of the most common nutritional deficiencies in Australia, affecting approximately 23% of adults — rising to over 36% during winter and in southern states [1]. Despite Australia's reputation as a sun-drenched country, many Australians have insufficient vitamin D levels. Blood testing provides the only reliable way to know your vitamin D status.

Overview

Vitamin D deficiency occurs when blood levels of 25-hydroxyvitamin D fall below the threshold considered adequate for bone health and general wellbeing. Vitamin D is a fat-soluble vitamin that functions more like a hormone in the body. It is essential for calcium absorption, bone mineralisation, immune function, and muscle function [2]. The body produces vitamin D when skin is exposed to UVB sunlight, and smaller amounts come from dietary sources such as oily fish, eggs, and fortified foods.

According to the Australian Bureau of Statistics' Australian Health Survey (2011-12), approximately 23% of Australian adults have vitamin D levels below 50 nmol/L — the threshold generally considered adequate [1]. Prevalence rises significantly in winter and in southern states, with over 36% of adults in these groups affected [1]. People with darker skin, those who cover their skin for cultural or religious reasons, office workers with limited sun exposure, and older adults are at particularly high risk.

Vitamin D deficiency often produces no obvious symptoms in its early stages, and when symptoms do appear — fatigue, muscle weakness, bone pain — they are non-specific and easily attributed to other causes [3]. Blood testing is the only reliable way to determine your vitamin D status. A single test provides your current level, and annual testing tracks seasonal patterns and the effectiveness of any supplementation or lifestyle changes over time.

Relevant Biomarkers

When to Consider Testing

• Living in southern Australian states (Victoria, Tasmania, southern parts of SA, WA, and NSW) where UVB intensity is lower in winter [1]
• Spending most of the day indoors, including office workers and shift workers
• Darker skin pigmentation — melanin reduces the skin's ability to produce vitamin D from sunlight
• Covering skin for cultural, religious, or sun-protection reasons
• Being over 65 years of age — the skin's ability to produce vitamin D declines with age [5]
• Obesity — vitamin D is sequestered in fat tissue, reducing circulating levels
• Conditions affecting fat absorption such as coeliac disease, Crohn's disease, or cystic fibrosis
• Exclusively breastfed infants and their mothers

While vitamin D deficiency itself is not directly inherited, certain genetic variations can affect vitamin D metabolism, including how efficiently your body converts sunlight to vitamin D and how it processes the vitamin. Family members often share lifestyle and dietary habits, skin type, and geographic location, all of which influence vitamin D status. If vitamin D deficiency is common in your family, regular testing is particularly worthwhile.

Mild vitamin D deficiency may produce no symptoms. As levels drop further, symptoms can include persistent fatigue, generalised muscle weakness, muscle aches and cramps, bone pain (particularly in the lower back, hips, and legs), and frequent infections [3]. Severe, prolonged deficiency can lead to osteomalacia (softening of bones) in adults and rickets in children [2]. Because these symptoms overlap with many other conditions, blood testing is the only reliable way to confirm vitamin D status.

What Results Mean

Blood test results should always be interpreted by a qualified healthcare provider. The information below is general in nature and should not replace professional medical advice.

In Australia, a 25-hydroxyvitamin D level of 50 nmol/L or above is generally considered adequate for bone and general health [4][6]. Levels between 30-49 nmol/L are considered mildly deficient, and levels below 30 nmol/L are considered moderately to severely deficient [6]. Some practitioners consider levels above 75 nmol/L as optimal, though this varies. Your healthcare provider will interpret your result in the context of your overall health, symptoms, and risk factors. Seasonal variation is normal — levels typically peak in late summer and dip in late winter.