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Vitamin D and Your Immune System

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Medically approved by Dr Earim Chaudry
Chief Medical Officer
iconLast updated 11th May 2021
In 30 seconds…

It won’t just give you strong bones – there’s evidence Vitamin D boosts your immune system, too.

Studies into Vitamin D’s role in immunity are new and there’s plenty left to uncover. But it seems to boost immune speed and accuracy.

Vitamin D won’t preventscribble-underline you from catching disease (so make sure you keep safe) but it could help you fight it.

Chances are, you’re already vaguely familiar with Vitamin D’s importance in maintaining bone health.

The classical action of Vitamin D is to promote calcium homeostasis, and we get our supply from sunlight, as the skin makes Vitamin D in response to UV B radiation – hence its nickname, “the sunshine Vitamin”.

But Vitamin D gets more interesting than that — and it also plays a key role in our immune system, as new research is continuing to discover. In this article, we’ll look at how Vitamin D boosts your immune system and helps your body fight disease.

Vitamin D’s Link to Immunity

When looking to boost your immune system, you’re probably not thinking of your Vitamin D levels — but perhaps you should be.

Scientists first spotted a link between Vitamin D and the immune system over a century ago, when Dr Nils Finsen won the Nobel Prize for Medicine or Physiology for curing a form of tuberculosis (TB) with concentrated light. This led to the discovery that ultra-violet light exposure causes the skin to synthesise Vitamin D, which in turn led to doctors prescribing Vitamin D supplements to treat diseases including leprosy with great success. However, 1928 brought with it the discovery of modern antibiotics, so studies into Vitamin D’s role in immunity took a back seat.

In recent years, however, scientists resumed study into how Vitamin D helps the immune system. Why was it that causing the skin to produce Vitamin D helped patients fight bacterial infections? TB and leprosy are diseases we mostly left behind in the 20th Century, but could Vitamin D help our immune system in other ways — or even help fight modern diseases? 

When you consider that up to 50% of the world’s population is deficient in Vitamin D, the link between Vitamin D and immunity is an important consideration in the here and now.

Vitamin D and Your Immune System

The immune system helps protect your body from bacteria, viruses, parasites, harmful substances or faulty cells (collectively known as antigens), while not attacking your own healthy cells. To understand how Vitamin D may help the immune system, it’s important to know how it works.

A well-functioning immune system consists of several types of immune cells:

  1. White blood cells (leukocytes) found throughout the body. When they encounter a pathogen, they multiply and signal that something is wrong.
  2. Phagocytes surround pathogens and eat them. There are several types that play various additional roles, including healing wounds and removing dead cells.
  3. B lymphocytes (B cells) produce antibodies, which are proteins that help fight specific antigens, and T lymphocytes (T cells) destroy infected cells and alert leukocytes.

There are two kinds of immune system response: innate and adaptive. The innate immune system acts as a general defence against any harmful infection, while the adaptive system uses antibodies to “remember” how it fought that infection, so your body can respond quicker next time (this is how vaccines work).

A large factor in how well your body fights disease, therefore, is how accurately your body can recognise an infection, communicate it, and respond. This is what researchers believe Vitamin D supports. Exactly how it does this, however, is a question yet to be fully answered by science. 

Vitamin D: What We Know

It’s understood that Vitamin D binds to a Vitamin D receptor (VDR) in cells, allowing it to regulate gene transcription. This is how Vitamin D helps maintain calcium levels. This is also how Vitamin D helps modulate different cellular processes such as apoptosis (cell death) and cell proliferation. This helps to explain why Vitamin D has shown to reduce the growth of certain tumours in laboratory animals. 

But it’s not just able to fight off our own cancerous cells. Very recent discoveries of VDRs on cells in the immune system led to renewed interest in the function of Vitamin D, as it suggests it also helps trigger an immune response to foreign pathogens. 

Some clues were found in a study on the immune system of mice when exposed to E. coli bacteria. The Vitamin D-deficient mice still produced phagocytes in response, but these “ate” significantly fewer bacteria. Another study carried out DNA analysis into gene expression in human leukocytes after their VDRs were exposed to a pathogen found that Vitamin D helped trigger expression.

The importance of Vitamin D’s role in the immune system becomes more evident still when studying Vitamin D deficiency. Generally, this is linked with increased susceptibility to infection and even autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, diabetes mellitus, and inflammatory bowel disease. 

One study found that people low in Vitamin D are more likely to suffer from respiratory tract infections. Several more studies have found an association between Vitamin D deficiency and increased infection rates, including influenza. In fact, a study of Japanese children found that a daily dose of Vitamin D for 15+ weeks during the winter lowered the rate of influenza infection by 42%.

That’s not all. Observational studies have noted low levels of Vitamin D in HIV-infected patients, and in another observational study, adults aged 65+ with low Vitamin D had a 45% higher risk of death – although this could be because illness and immobility prevented them from going outside.

That’s not to say that having low Vitamin D levels causes any of these diseases, but it proves a correlation between having sufficient Vitamin D and the body’s increased ability to fight them. 

Key Takeaways

While the research into how Vitamin D helps immune system functionality is still developing, evidence for its ability to help the speed and accuracy of immune response is significant and growing.

For any virus to spread, it must first get into a healthy human cell, where it “tricks” it into making many copies of the virus, which are released to more cells around the body. Most bacteria reproduce by essentially doubling themselves, meaning infection grows exponentially. This is why your body must defend itself fast, whatever the disease may be.

Furthermore, our immune system must know what to kill. A trigger-happy approach that also kills our healthy cells can end up causing additional harm. We encounter the same problem in autoimmune diseases, where the body’s immune system loses the ability to tell its own cells from harmful invaders and starts attacking itself.

In the modern age, overuse of antibiotics is breeding antibiotic-resistant bacteria. New viruses can emerge and spread (at a predicted rate of one new disease a year), before vaccines are developed. This is why research into Vitamin D holds such fascination; it’s not a cure, but evidence suggests it can boost our immune system’s natural response.

As for how to make sure you’re getting enough, it’s important to note that, while you can get your fix from the sun, wearing SPF actually prevents your body from making Vitamin D. And that’s when there’s sun outside — most of us don’t get enough sunshine throughout the year to get adequate Vitamin D. Sunbathing without it is a known skin cancer risk, so supplements are your safest — and most reliable — bet.

While we’re on the subject of safety, make sure you’re keeping safe by taking scientifically advised steps to avoid infection. Vitamin D won’t prevent you from catching a disease, but it could certainly help you fight it.


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While we've ensured that everything you read on the Health Centre is medically reviewed and approved, information presented here is not intended to be a substitute for professional medical advice, diagnosis, or treatment. It should never be relied upon for specific medical advice. If you have any questions or concerns, please talk to your doctor.

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