Can Vitamin D Help Protect The Immune System?

Vitamin D, also known as the sunshine vitamin, sure is a hot topic right now. 

As the days are getting shorter and the nights colder, there are plenty of reasons why it should be on your mind, and I am here to shed some light on why... 

What is vitamin D?

Vitamin D, also referred to as calciferol, is an essential nutrient which supports a range of functions within the body including calcium absorption, bone mineralisation and immune system function. Dubbed the sunshine vitamin, vitamin D is produced naturally when the skin is exposed to sunlight. Small amounts of vitamin D can also be found in foods such as fatty fish, eggs, butter and liver. However, it is difficult to get adequate amounts from the diet alone. 

Vitamin D comes in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). While both are effectively absorbed into the bloodstream, they differ in the roles they play within the body due to the way they are metabolised by the liver. D2 is converted into 25-hydroxyvitamin D2 and vitamin D3 into 25-hydroxyvitamin D3. Both end up as calcifediol - the key nutrient that is measured in the blood when testing for vitamin D. 

So which is more effective? Vitamin D3 is much more effective as raising blood calcifediol levels. In fact, multiple studies[1][2] have found it to be nearly 2x more effective than D2. 

Where is vitamin D found?

We already know that the best source of vitamin D is sunlight, but when it comes to food sources; D3 is only found in animal sources, and vitamin D2 mainly from plants. 

Sources of Vitamin D3

  • Oily fish (salmon, mackerel, sardines).
  • Liver
  • Egg yolk
  • Dietary supplements

Sources of Vitamin D2

  • Mushrooms (grown in UV light)
  • Fortified foods
  • Dietary supplements

Given that vitamin D occurs in minimal amounts within food sources, it is recommended that those spending large amounts of time indoors, or living in countries with low sunlight exposure (northern hemisphere) should supplement from Sept-April. One of the main reasons for this is due to the role vitamin D plays within the immune system - which is why there are so many more instances of cold and flu in the winter months. 


What role does it play in immunity? 

Although it’s more commonly known for its role in teeth and bone health, Vitamin D is actually one of the most important vitamins when it comes to helping the immune system to fight off cold and flu. So much so that many doctors, nutritionists and now even the UK government are recommending supplementing with it during the COVID-19 pandemic to help protect against the virus.

So, how does it work? Vitamin D has receptors present on immune cells within the body which help regulate both the innate and acquired immune response. It works by enhancing the functioning of T-cells and macrophages that help protect your body against pathogens  - in particular within respiratory tract infections[3].

Can vitamin D help lower your risk of COVID-19?

We all know social distancing and proper hand washing techniques could help lower your risk of contracting the virus, but could vitamin D help you fight back should you come into contact with COVID-19? Let’s look at the research...

Interest in the role of vitamin D in the prevention of acute respiratory infections is nothing new, in fact, it dates back to the 1930s. More recent studies, such as a systematic review of 11 placebo controlled trials, carried out on 5560 participants, found vitamin D supplementation to significantly reduce the risk of acquiring a respiratory infection. This study also found that daily supplementation (300-2000 IU) had the biggest effect vs larger doses taken on a single or monthly basis (100,000-200,000 IU per month) - concluding that regular, daily supplementation is most effective[4].

Another meta analysis of 25 randomised, double-blind controlled trials found vitamin D supplementation to positively help protect against acute respiratory tract infections in all participants but especially in those who showed signs of deficiency[5].

Many other studies have found low vitamin D levels to be a risk factor for a range of other respiratory diseases such as asthma, tuberculosis, chronic obstructive pulmonary disease and the flu virus[6] [7][8]. 

The latest interest in vitamin D and covid-19, however, came about as researchers started to find a trend in COVID-19 cases with those with low vitamin D levels. One study found that 8 in 10 patients tested had a vitamin D deficiency, concluding that a lack of vitamin D was four times more likely in hospitalised covid patients. While it’s still very early days to come to any conclusions, here’s what other studies so far have found:

Study 1:

Who? Tel Aviv University, Israel[9].

What? They tested vitamin D levels for 782 people who tested positive for coronavirus retrospectively and compared them to healthy individuals.

Findings: People with vitamin D levels below optimal - were 45% more likely to test positive and 95% more likely to be hospitalised.

Limitations: They checked their vitamin D levels after, not during the time of infection. 


Study 2: 

Who? The University of Chicago.

What? Researchers compared vitamin D levels of 500 volunteers with the amount that later caught COVID-19. 

Findings: Low vitamin D levels correlated with a 60% per cent higher rate of Covid-19.

Limitations: Compounding factors were not accessed such as participants' age, job and location as these could have all had an impact on the chance of contracting the virus. 


Study 3: 

Who? Tehran University, in Iran, and Boston University[10].

What? They analysed data from 235 hospitalised patients with Covid-19.

Findings: Patients who had sufficient vitamin D levels were 51.5% less likely to die from the disease. They also had a significantly lower risk of falling seriously ill or needing ventilation. Patients who had good levels of vitamin D had less inflammation - a negative side effect of Covid-19.

Limitations: Confounding factors, such as smoking, and socioeconomic status were not accounted for that would have affected participants' health.   


Study 4: 

Who? Cordoba University in Spain[11].

What? 50 hospital patients with Covid-19 were given vitamin D as a treatment. Researchers then compared their health and recovery with 26 volunteers in a control group.

Findings: Only one of the 50 patients needed intensive care and none died. Half of 26 virus sufferers who did not take vitamin D were later admitted to intensive care and two died

Limitations: Small sample size.


Study 5

Who? Inha University, South Korea[12].

What? 50 hospital patients with Covid-19 were checked for levels of all vital vitamins and compared to a control group.

Findings: 76% were deficient in vitamin D and a severe vitamin D deficiency was found in 24% of Covid-19 patients and just 7% in the control group.

Limitations: Small sample size.


Study 6

Who? Brussels University [13].

What? They compared vitamin D levels in almost 200 Covid-19 hospital patients with a control group of more than 2,000 healthy people.

Findings: Men who were hospitalised with the infection were significantly more likely to have a vitamin D deficiency than healthy men of the same age. Deficiency rates were 67% in the COVID-19 patient group, and 49% per cent in the control group. The same was not found for women.

Limitations: Independent scientists say blood vitamin D levels go down when people develop serious illness, so it is difficult to determine whether the illness is causing the low vitamin D levels or the other way round. 


Although there are some limitations with the studies, recent research is definitely proving promising. As a result there are plenty of studies in the pipeline in the next 6-12 months to dig into this area more, see full list click here.


Should I take a supplement?

Despite playing such an obvious and important role within the immune system, vitamin D is actually one of the most common deficiencies in the world. This is mainly due to the lack of sunlight in the northern hemisphere and the amount of time currently spent indoors - something which is especially topical at the moment due to the pandemic restrictions and self-isolating measures. 

It is therefore recommended by health professionals including the NHS that everyone should take a supplement during the COVID-19 pandemic, and generally, in the months Sept-April due to the lack of sunlight. 

They also advise that ethnic minority groups with dark skin, from African, Afro-Caribbean and South Asian backgrounds, should consider taking a vitamin D supplement all year round. Click here for more information on the NICE guidelines on Vitamin D: supplement use in specific population groups. 

How much should you take?

The RNI is 400 IU per day - however if you are already deficient or you do not eat any foods containing D3, 1000+ IU a day would be more suitable to ensure optimal blood levels, seek advice from your doctor or health care professional if you are unsure on your recommended dose.

The bottom line is, Vitamin D supplements are low cost, low risk, easily accessible and effective at protecting the immune system. If found to be successful, they could massively aid the fight against the pandemic and many other flu like diseases in the future.

Click here to get yours and stay protected this winter!



[1] https://pubmed.ncbi.nlm.nih.gov/22552031/

[2] https://pubmed.ncbi.nlm.nih.gov/18492750/

[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6305614/

[4] https://www.bmj.com/content/356/bmj.i6583

[5] https://www.ncbi.nlm.nih.gov/books/NBK536320/pdf/Bookshelf_NBK536320.pdf

[6] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4214003/


[7] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2759054/

[8] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4776550/

[9] https://febs.onlinelibrary.wiley.com/doi/full/10.1111/febs.15495

[10] https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0239799

[11] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7456194/

[12] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7418699/

[13] https://www.medrxiv.org/content/10.1101/2020.05.01.20079376v2