Australia is one of the sunniest countries on earth — and yet vitamin D deficiency affects around 1 in 4 Australians. The paradox is explained by a combination of sun avoidance, indoor work, high-SPF sunscreen use, and the fact that during winter in the southern states, UVB radiation is insufficient for vitamin D synthesis even at midday.
What Does Vitamin D Do?
Vitamin D is technically a hormone, not a vitamin. It's synthesised in the skin when UVB light hits cholesterol, then activated by the liver and kidneys. Nearly every cell in the body has vitamin D receptors — its effects extend far beyond bone health.
- Bone health — regulates calcium and phosphate absorption
- Immune function — modulates both innate and adaptive immunity
- Mood — linked to serotonin synthesis; deficiency associated with depression
- Muscle function — proximal muscle weakness is a classic symptom
- Cardiovascular health — low vitamin D associated with increased CV risk
Symptoms of Deficiency
- Fatigue and tiredness
- Bone pain or generalised aching
- Muscle weakness
- Low mood or depression
- Frequent illness (impaired immune function)
Testing Your Levels
A simple blood test (25-hydroxyvitamin D) is the standard measure. Optimal levels are generally considered to be 75–150 nmol/L. Below 50 nmol/L is deficient. Below 25 nmol/L is severely deficient and warrants prompt supplementation.
