Vitamin D and Immune Health

April 13, 2020

Vitamin D, often called the “sunshine vitamin” has a lot more to do with our health than just improving our mood on sunny summer days.  It is actually an incredibly important nutrient with impacts on just about every cell in our bodies.

Actions of Vitamin D

While we think of vitamin D as a vitamin, it actually acts in our bodies as a hormone!  We call it a vitamin because we can’t make it ourselves – at least not without sunlight.  But in our bodies it acts as a hormone – influencing bone health, insulin secretion, skin health and immune function.  But our heart, muscles, breasts, liver, and lungs all need vitamin D as well.

Vitamin D and the Immune System

The impact that vitamin D has on the immune system cannot be understated.  Without adequate vitamin D both the innate immune system and the acquired immune system cannot function normally.  Most of our immune cells have receptors for vitamin D and we recognize that vitamin D can regulate the function of these cells – as well as influencing how many of these cells our body makes. 

One of the most important things vitamin D does, especially notable during times of COVID 19 and other viral infections, is increase the production of defensins.  Defensins are small peptides on our skin and mucous membranes, that are involved in our first line of immune defense.  Defensins have both direct antimicrobial activity, and increase signaling amongst cells, alerting them to the presence of the invading virus.  Having high levels of defensins can help our immune system to destroy a virus on contact. 

Research is being done worldwide looking at the impact of vitamin D on preventing viral infections, and lessening severity of those infections.  Animal studies have shown significant benefit and human studies consistently show that low vitamin D is a risk factor for viral infections. 

Getting Enough D

For most Canadians our primary source of vitamin D is through supplements, or foods containing added vitamin D (mainly dairy).  In the winter months, I strongly suggest every Canadian, big and small, take a vitamin D supplement to support their immune health.  A general rule of thumb is to take 35IU per kg of body weight and to have your levels tested every winter to ensure you are getting enough.  Speak to your Naturopathic Doctor about increasing this dose during cold and flu season or during times when there is an increased risk of viral infections. 

Selected Resources

De Sa Del Fiol F, Barberato S, et al. Vitamin D and respiratory infections J Infect Dev Ctries 2015; 9(4):355-361. doi:10.3855/jidc.5711

Bilkle D. Vitamin D: Production, Metabolism, and Mechanisms of Action.  Endotext. 2017.  Available online at:

Teymoori-Rad M, Shokri F, Salimi V, Marashi SM. The interplay between vitamin D and viral infections.  Rev Med Virol. 2019 Mar;29(2):e2032. doi: 10.1002/rmv.2032. Epub 2019 Jan 6.

Dr. Lisa Watson believes that you don’t have to be perfect to be healthy.  Lisa encourages her patients to take a proactive approach to their health – taking meaningful steps towards achieving their goals for balanced and vibrant health.  An expert in women’s health and hormones, Lisa is a passionate advocate for women’s health and strives to educate all the women in her practice on how to achieve lifelong abundant health.  Dr. Watson practices at the Integrative Health Institute in downtown Toronto and writes regularly about women’s health on her website at

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