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The A to Z of Vitamin D.

Vitamin D is a unique vitamin in that the physiologically active form (1,25(OH)2D) acts as a hormone. The calcium handling effects of vitamin D make it crucial to bone health and athletic performance, yet there’s much, much more to be gained from vitamin D. It plays a role in cardiovascular function, insulin and thyroid stimulating hormone secretion, cell cycle regulation and moderating inflammation. Vitamin D also modulates the innate and adaptive immune responses, so a deficiency in vitamin D is associated with increased autoimmunity as well as increased susceptibility to infection.

Vitamin D and Immune Health

The evidence surrounding vitamin D and immune modulation is compelling. A number of studies have shown that taking vitamin D, particularly in people who are deficient, reduces the chances of developing acute respiratory infection, including influenza.

One study in particular shows that those with the highest vitamin D (125 – 150 nmol/L or 50 – 60 ng/ml) have the best protection against common colds and flu. So much so that there were 43% fewer cases of flu-like illness when compared to those with Vitamin D levels less than 50 nmol/L (20 ng/ml).

Early studies being done on vitamin D in the case of COVID-19 suggest that raising levels above 100 -150 nmol/L (40-60 ng/mL) should be the goal. It’s thought that vitamin D defends against COVID-19 infection through two primary mechanisms:

- it can lower viral replication rates.

- it reduces concentrations of pro-inflammatory cytokines that produce the inflammation that leads to injury of the lining of the lungs and eventually pneumonia.

Rates of Vitamin D Deficiency

In Australia, 50 nmol/L or less is considered a state of mild deficiency. Using this definition, The Australian Health Survey done in 2011 - 2013 showed that 23% of Australians are vitamin D deficient.

Rates are obviously going to vary depending on the criteria for deficiency. If evidence suggests that >100nmol/L supports the best defence against COVID-19 and 125 – 150 nmol/L for the common cold or flu then that would suggest sub-optimal levels are actually far more prevalent. In my clinical experience it’s rare to see a client with greater than 75 nmol/L (I celebrate when I do).

If vitamin D insufficiency is defined as levels ≤ 80 nmol/L, then research shows the prevalence in winter to spring could be as great as 82% up in Queensland, Australia and 97% down in Tasmania, Australia.

Who’s at Risk of Vitamin D Deficiency?

Regardless of what defines a deficiency, it’s clear a significant portion of the population are falling short.

- Those most at risk are, quite simply, those that don’t expose their skin to the sun. In Australia, with such a strong ‘slip, slop, slap’ message and long winters in the southern states, it’s no surprise rates of deficiency are as they are. UVB rays are typically not strong enough in the winter months to impact vitaminD levels.

- Other populations with little exposure to the sun include those that work indoors 9 - 5pm, the elderly, the disabled those that choose to wear a veil.

- Dark skinned people are at greater risk.

- Those with certain medical conditions or who are on medication.

- Infants born to vitamin d deficient mothers who are exclusively breast fed.

This begs the question, are the plant based community at greater risk? It’s true that anyone on a plant based diet will lack dietary intake of vitamin D, but studies show that vegans aren’t at greater risk than the general population.

Supporting Optimal Vitamin D Status

Vitamin D is synthesised in the skin when UVB rays (from the sun) come into contact with cholesterol (in the skin). As little as 15 – 30 minutes/day with a small amount of skin exposed (face, hands & forearm) between the hours of 10am – 3pm contributes about 600 IU/day. That’s triple the Australian Adequate Intake (AI) for young adults.

Alarmingly, the AI is said to be enough to maintain a serum vitamin D level of 27.5 nmol/L. What do you think? That sounds like an Inadequate Intake (II!) goal to me.

Dietary sources of vitamin D include the flesh of oily fish (trout, sardines, salmon), fish oils and, in much smaller amounts, beef liver, cheese and eggs. For those on a pescatarian diet, each portion of fish offers around 750IU vitamin D and eggs, 50 IU.

For those on a purely plant based diet, mushrooms may offer traces though sun is your primary source. Work towards that 15 – 30 minutes of skin exposure to the arms, legs or chest each day. Not enough to get burned, but enough to get vitamin D synthesis underway.

Are Supplements Necessary?

Supplements may be a requirement at times. Especially in those groups at most risk of deficiency and with very little means of increasing sun exposure safely. Before supplementing, I recommend getting a baseline understanding of serum vitamin D levels and working with a practitioner who can guide you as to how long and with how much you should be supplementing. They can also work with you on understanding which form of supplement is appropriate – D3 or D2.

It’s important to get your supplement schedule right as vitamin D is a fat-soluble vitamin which means it does get stored and in excess can become toxic. Similarly, a conservative supplement schedule mightn’t be enough to support required change.

Reach out to me if you’d like assistance with understanding your vitamin D levels or relative risk of inadequacy. In the meantime, let your hair down, roll your sleeves up and enjoy 10 minutes in the sun today!



Hamilton B. 2010. Vitamin D and Human Skeletal Muscle. Scand. J. Med. Sci. Sports 20, 2, 182Y90.

Aranow C. 2011. Vitamin D and the Immune System. J Investig Med 59, 6, 881–886.

Rose S, et al. 2019. Ensuring Adequate Vitamin D Status for Patients on a Plant Based Diet. Orthopedics and Rheumatology Open Access Journal, 15, 3.

Dunlop E, et al. 2017. Vitamin D3 and 25-Hydroxyvitamin D3 Content of Retail White Fish and Eggs in Australia. Nutrients, 9, 7, 647.

Kimlin M, et al. 2003. Comparison of human facial UV exposure at high and low latitudes and the potential impact on dermal vitamin D production. Photochemical and Photobiological Sciences, 2, 4, 370-375.

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