Vitamin D: More Than Just Bone Health

When most people think about vitamin D, they think about bones — and for good reason. Vitamin D plays an essential role in calcium absorption and maintaining healthy bone mineralisation.

But vitamin D receptors are found in tissues throughout the body, and its biological roles extend well beyond the skeleton.

Vitamin D and immune function

Vitamin D is involved in both the innate and adaptive immune systems. Immune cells including macrophages, dendritic cells and lymphocytes can respond to vitamin D, where it contributes to antimicrobial defence and regulation of inflammatory and immune responses.

This doesn't mean that taking large amounts of vitamin D will “boost” your immune system — a phrase I generally dislike. Immune function is about appropriate regulation, not simply making the immune response stronger.

However, ensuring adequate vitamin D status is one piece of supporting normal immune function.

Muscle function matters too

Vitamin D also plays an important role in skeletal muscle function.

Deficiency can be associated with muscle weakness, aches and impaired physical function, particularly when deficiency is significant or prolonged.

This is one reason I think vitamin D deserves consideration when investigating symptoms such as unexplained weakness or fatigue — while recognising that these symptoms are nonspecific and can have many possible causes.

What about vitamin D and mood?

This is where things become more nuanced.

Low vitamin D status has been associated with depression and poorer mental health outcomes in observational research. Vitamin D receptors are also present in areas of the brain involved in mood regulation.

But an association doesn't automatically mean that low vitamin D causes depression, and clinical trials of vitamin D supplementation for improving depressive symptoms have produced mixed results.

In other words: vitamin D deficiency is worth identifying and correcting, but vitamin D isn't an antidepressant.

Mental health symptoms deserve comprehensive assessment and appropriate care.

How do we know if vitamin D is low?

Vitamin D status is assessed through a blood test measuring 25-hydroxyvitamin D [25(OH)D].

In Australia, levels below 50 nmol/L are generally considered deficient, although interpretation should also consider the individual and seasonal variation. Levels naturally tend to be lower toward the end of winter.

People at greater risk of deficiency can include those with limited sun exposure, darker skin, older age, certain malabsorptive conditions and people who cover most of their skin outdoors.

More isn't always better

Vitamin D is fat-soluble, meaning excessive supplementation can accumulate and potentially cause toxicity.

That's why I'm not a fan of blindly taking increasingly large doses because a nutrient is “good for you”.

Test when appropriate. Supplement when indicated. Use an appropriate dose. Then reassess.

That's a much more useful approach than assuming more must be better.

Vitamin D may have earned its reputation through bone health, but its role throughout the body makes maintaining adequate vitamin D status an important part of the bigger health picture.