Vitamin C, Vitamin D and COVID-19

It seems more important than ever that our immune system is functioning well, yet not much is said about it in the media. There are very simple things you can do to ensure that your immune system is given the best chance to fend off infections, be it viral or bacterial. Having a healthy immune system also helps with injury recovery which is something we have a keen interest in!

Vitamin C, aka Ascorbic Acid – An essential vitamin; meaning that our bodies can’t make it, we must get it from what we eat or drink. The recommended daily dose is 100-200mg/day for general well-being.

It contributes to immune defense by supporting various cellular functions such as helping stop pathogens (viruses) getting through our skin cells, and promotes the restorative process of the skin. It builds up within our immune cells which help fight against bacteria and other disease-causing microbes.

Vitamin C deficiency = impaired immunity, poor wound healing and higher chance of getting infections – have you ever seen Scurvy before? An example of Vitamin C deficiency seen in sailors, pirates and malnourished populations.

Supplementation with Vitamin C appears to be able to both prevent and treat respiratory and systemic infections (in general).

People at high risk of Vitamin C deficiency:

  • Smokers - Both smokers and passive smokers have lower vitamin C levels than non-smokers. As low as one-third less than non-smokers, and it has been recommended that smokers should consume an additional 35 mg/day of vitamin C to ensure there is sufficient ascorbic acid to repair oxidant damage.

  • Diabetes Sufferers - Individuals with diabetes are at greater risk of common infections, including influenza, pneumonia, and foot infections, which are associated with increased morbidity and mortality. A decrease in plasma vitamin C levels has been observed in studies of type 2 diabetes, due to a high level of oxidative stress caused by high blood sugar (hyperglycemia).

  • The Elderly - Elderly people are particularly susceptible to infections due to decreased immune cell function. A lower mean vitamin C status has been observed in free-living or institutionalized elderly people.

How to boost your Vitamin C intake:

  • The old faithful ORANGE = 70-95mg/serve (1 medium fruit or ¾ cup juice)

  • KIWI FRUIT and STRAWBS = 49-64mg/serve (1 medium fruit and ½ cup fresh)

  • BROCCOLI = 51mg/serve (1 cup cooked)

  • Many others you can look up here

Vitamin D, aka Calcitriol - is actually a hormone that consists of 5 different vitamins, the most important forms being vitamin D2 and vitamin D3. Did you know that we get this from the sun!?

Ultraviolet rays from the sun shine onto our skin to convert a cholesterol precursor in the skin to previtamin D3, which is converted to vitamin D3. Further technical processes happen in our body to end up with Calcitriol which is the active hormone form of vitamin D.

Vitamin D stimulates natural immune responses by promoting white blood cells (immune cells) to kill foreign bacteria/viruses during infection.

It also strongly reduces inflammation at the site of infection by supressing the immune response to allow for balance in our system. Much like when we take anti-inflammatories (ibuprofen) when an injury is causing a lot of pain. i.e. we want some inflammation to occur for healing, but we also need to be able to function as well without pain and swelling.

Furthermore, several studies suggest that lower Vitamin D levels are associated with an increased risk of respiratory infection at all ages.

Deficiency results in abnormalities in calcium, phosphorus, and bone metabolism. This causes a generalized decrease in bone mineral density (BMD), resulting in osteopenia and osteoporosis.

People at high risk of Vitamin D deficiency:

  • Poor sun exposure - In the northern hemisphere during the Winter months there can be days with very minimal sunlight for most of the day. People from ethnic and religious backgrounds who cover up their skin due to cultural and religious reasons tend to have lower vitamin D. In a study that included 100 University students in Istanbul they found that the prevalence of vitamin D deficiencies was 55.0% for covered and 20.0% for uncovered students.

  • Interestingly, in Australia we have a lot of sun exposure but because we don’t want to risk getting skin cancer we tend to ‘slip, slop, slap’ a little too much. That is why Australia ranks quite high in terms of Vitamin D deficiency. Research says that wearing sunscreen with a sun protection factor of 30 reduces vitamin D synthesis in the skin by more than 95%.

  • Darker skin tone - People with a naturally dark skin tone have natural sun protection in their skin and require at least three to five times longer exposure to make the same amount of vitamin D as a person with a lighter skin tone.

  • Obesity - Greater amounts of subcutaneous fat captivate more of the vitamin and alter its release into the circulation.

  • The elderly – Particularly those in residential care, of middle-eastern origin and with poor mobility and sun exposure.

How to boost your Vitamin D:

  • It’s FREE from the sun! It doesn’t have to be ‘sunny’ to get the ultraviolet rays from the sun, but it does mean that on cloudy days it would take longer to get the same amount of absorption into the skin.

  • As far as getting it from food sources, very little foods contain Vitamin D and the ones that do don’t come close to the amount we get from the sun.

  • Some examples are; oily fish – such as salmon, sardines, herring and mackerel, red meat, liver, egg yolks, fortified foods – such as some fat spreads and breakfast cereals.

What does this mean for COVID-19?

A large review that compiled multiple studies together (called a systematic review) in July 2021 included confirmed COVID-19 patients and the role of Vitamin D. In total they compiled 23 separate studies including 11901 participants. They found that 41% of the COVID-19 patients were suffering from vitamin D deficiency/insufficiency, with only 19% showing normal levels.

Also, there is about three times higher chance of getting infected with SARS-CoV-2 among vitamin-D-deficient individuals and about five times higher probability of developing the severe disease in vitamin-D-deficient patients.

Lastly, Vitamin D deficiency showed no significant association with mortality rates in this population.

To get information on your general health and Vitamin D/C levels, consult your General Practitioner.

Sources of information for this post:


- Buyukuslu, N., Esin, K., Hizli, H., Sunal, N., Yigit, P., & Garipagaoglu, M. (2014). Clothing preference affects vitamin D status of young women. Nutrition research, 34(8), 688-693.

- Ghasemian, R., Shamshirian, A., Heydari, K., Malekan, M., Alizadeh-Navaei, R., Ebrahimzadeh, M. A., ... & Shamshirian, D. (2020). The Role of Vitamin D in The Age of COVID-19: A Systematic Review and Meta-Analysis. MedRxiv.

- Carr, A. C., & Maggini, S. (2017). Vitamin C and immune function. Nutrients, 9(11), 1211.

- Determination of optimal vitamin C requirements in humans. Levine M, Dhariwal KR, Welch RW, Wang Y, Park JB Am J Clin Nutr. 1995 Dec; 62(6 Suppl):1347S-1356S.

- Carr, A. C., & Frei, B. (1999). Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans. The American journal of clinical nutrition, 69(6), 1086-1107.

- Nair, R., & Maseeh, A. (2012). Vitamin D: The "sunshine" vitamin. Journal of pharmacology & pharmacotherapeutics, 3(2), 118–126.

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