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Vitamin D – deficiency and the effect on your health

by dietitian Clare Wood

What is Vitamin D?

Vitamin D is a pro-hormone that is mainly produced in the skin from direct UVB sunlight rays, with a very small amount coming from naturally occurring and fortified food sources.

Vitamin D plays a vital role in the following:

A deficiency in Vitamin D is known to be linked with a disease called Rickets in children and Osteomalacia in adults. It is also important for immune function and inflammation, and low levels have recently been linked to several diseases:

However, we still don’t fully understand if low levels of vitamin D actually cause these diseases, scientists need more research to determine this.

sunshine getting a dose of vitamin D

Vitamin D deficiency

The best indicator of vitamin D status is the concentration of 25-hydroxyvitamin D 25(OH)D in the blood, although there is still some confusion about what the ideal levels of Vitamin D are for optimal health.


Vitamin D deficiency is well recognized as a pandemic and a global public health problem with many high risk populations around the globe with deficiency even occurring in places like Australia where there is plenty of sun.

Vitamin D deficiency is becoming more prevalent due to cautious sun exposure, think “Slip Slop Slap”, the increasing obesity problem, and a reduction in foods that are naturally high, or fortified with vitamin D. Generally there are seasonal differences with lower Vitamin D levels seen in winter.

Effect on health

There are mixed reports in the literature, with some showing a link between low vitamin D status and increased risk of both upper and lower respiratory tract infections, plus longer durations of sickness. However, there is less evidence to show that supplementation will reduce the incidence or severity of sickness.

Should we Supplement?

There are many factors effecting vitamin D status, including:

With the high prevalence of vitamin D deficiency and insufficiency in the general population, plus the potential health benefits of supplementation, improving vitamin D status seems warranted. Current policies for vitamin D supplementation and fortification are possibly outdated requiring new guidelines.

The improvement of Vitamin D levels can be made using the following strategies:

Risk of toxicity

Vitamin D toxicity can occur at concentrations of 25(OH)D reaching > 200 ng/mL, achieved with chronic intake of extremely high doses for several months. The result is high levels of calcium and phosphate in the blood, which if untreated can lead to kidney failure, soft tissue calcification and ultimately death. However, a prolonged intake of 10,000 International Units per day of vitamin D(3) is unlikely to pose a risk of adverse effects in almost all individuals in the general population.


There is enough evidence to suggest that deficiency and insufficiency are widespread in the general population. Priority should be given to determining levels for optimal health and then standardizing the classification of vitamin D deficiency, insufficiency and sufficiency. First line strategy would be implementation of food fortification programs in high risk areas on a population level, to effectively achieve population improvements. Specific targeted strategies, such as a screening tool, should be implemented to identify at risk individuals who are deficient/insufficient. This will allow appropriate assessment, followed up with blood testing in the winter/autumn months of the year (when levels are lowest) where required. Then, appropriate supplementation on an individual level can occur to increase basal levels mainly to improve bone health, and possibly also to reduce illness, inflammation and to optimize muscle function. There is not enough evidence yet to prescribe vitamin D for prevention of chronic disease.

Selected References

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