How is vitamin D deficiency diagnosed?

Vitamin D is a fat soluble vitamin that regulates calcium balance and is essential for bone health. Most of the vitamin D in the body is made in the skin. This only occurs when it is exposed to sunlight, specifically ultraviolet‐B (UVB) radiation. It is then metabolized in the liver and kidneys to a metabolically active form, which is involved in hundreds of skeletal and other biological functions.1

Small amounts of vitamin D is also found naturally in a few foods including oily fish such as salmon, tuna and mackerel, fish liver oil, beef liver, cheese and egg yolks. Other foods are artificially fortified with vitamin D and these supply most of the vitamin D we consume through diet. These foods often include orange juice, milk, yoghurt, margarine and cereals.2

Despite our predominantly sunny climate, vitamin D deficiency in Australia is much higher than previously thought. This is largely due to modern lifestyles, long work hours, and concerns about the risks of skin cancer associated with sun exposure.3 People who are deficient in vitamin D may have no symptoms at all, or they may be vague and hard to pinpoint. Potential warning signs include feeling tired, a general unwell feeling, subtle aches and pains4, muscle weakness5, and sometimes bone pain from unnoticed hairline fractures.4

People at risk of developing a vitamin D deficiency include:

  • Those with little to no sun exposure.6
  • Hospitalized people, those in aged care or housebound individuals.6
  • Those who wear clothes that cover the whole body for religious or cultural reasons.6
  • Elderly people, especially women. In fact, 57 percent of women over the age of 75 years in Australia have been found to be deficient. This is particularly important, as vitamin D deficiency is a key risk factor for falls and fractures in the elderly.7

Although a vitamin D deficiency does not cause many symptoms, it is easily diagnosed with a simple blood test. The test measures a metabolite of vitamin D in the blood, called 25‐hydroxyvitamin D (25‐OHD).8 This is the most accurate way of assessing how much vitamin D is in your body.9

It is recommended that serum concentrations of 25‐OHD remain equal to or above 50 nmol/L. When they are deficient, they may be classified in the following ways:10

  • Mild deficiency is 30‐49 nmol/L
  • Moderate deficiency is 12.5‐29 nmol/L
  • Severe deficiency is less than 12.5 nmol/L

If you think you may be at risk of having low vitamin D levels, speak to your healthcare professional about having a blood test. The best time to have your levels tested is at the end of winter or early spring, when your vitamin D levels are at their lowest.11