Most people know vitamin D for its important role in bone health and the increased absorption of calcium, but did you know this nutrient is critical to our overall health in many other ways?

Vitamin D is essential for the functioning of a healthy body. It controls blood calcium concentration, which keeps your heart beating and blood pressure levels normal.

Lower levels of vitamin D appear to be linked to some cancers, such as colon, prostate, breast, bladder, lung, pancreatic, skin and ovarian cancers, as well as lymphoma and leukemia. Lower levels are also associated with heart disease, as well as insulin resistance, which can lead to type 2 diabetes and speed its progression. Low vitamin D is also linked to lower levels of testosterone in men, which means it plays a role in fertility. Because of its role in helping to maintain a diverse gut microbiome, vitamin D is also connected to gut health and diseases, like irritable bowel disease.

Inadequate vitamin D levels are also correlated with mental health conditions such as depression, anxiety, autism, Alzheimer’s and Parkinson’s diseases. But maybe the most surprising role vitamin D plays in the body is in your immune system—your body’s first line of defence against infection and disease. Immune cells, such as B cells, T cells and antigen-presenting cells, all have vitamin D receptors, and when vitamin D binds to these receptors, the immune cells convert it into its biologically active form, calcitriol. Once in its active form, it acts as a hormone that regulates the gene expression of the immune cells. It helps to produce certain antimicrobial peptides that fight off different pathogens. In addition, the immune system produces both pro-inflammatory and anti-inflammatory cytokines (substances secreted by immune cells that play a role in signalling with other cells) in response to infections. Vitamin D helps reduce the “cytokine storm” by reducing the production of pro-inflammatory cytokines and increasing the production of anti-inflammatory cytokines, thereby reducing inflammation.

Vitamin D plays a role in our immune system and our body’s ability to fight off infection...and many of us aren’t getting enough of it.

In the middle of what feels like a never-ending pandemic, keeping healthy and fighting off COVID-19 is top of mind for many Canadians, so vitamin D has garnered a lot of attention. There is mounting evidence to suggest that vitamin D could play a key role in fighting off the virus. For example, a systematic review and meta-analysis published in International Journal of Clinical Practice in August 2021 found that 41 percent of COVID-19 patients had vitamin D deficiency (blood levels below 20 nmol/L) and 42 percent of COVID-19 patients were insufficient in vitamin D (blood levels below 50 nmol/L)— meaning 83 percent were low in vitamin D! The study reports that the odds of getting infected with COVID-19 are 3.3 times higher among individuals with vitamin D deficiency, and the chance of developing severe COVID-19 is about five times higher in patients with vitamin D deficiency.

This review and analysis found no significant association between vitamin D status and higher mortality rates. A separate systematic review and meta-analysis published in Clinical Endocrinology in July 2021 found that vitamin D deficiency was associated with greater severity of COVID-19 infection, as well as significantly higher mortality, higher rates of hospital admissions and longer hospital stays as compared to non-vitamin D deficient individuals. However, not all research shares these same findings.

Another meta-analysis published in Nutrition Journal in October 2021 found that vitamin D deficiency was not  significantly associated with risk of infection or death by COVID-19, and that vitamin D supplements did not significantly improve clinical outcomes in patients with COVID-19.

We obtain vitamin D in our diet from a few key food sources, such as fatty fish, like salmon and mackerel, eggs and fortified milk. But most Canadians don’t consume enough of these foods to get the recommended intake. We also produce vitamin D naturally when our skin is exposed to the sun. However, to create adequate levels, you’d need approximately 10 to 30 minutes of sun exposure on your face, arms, legs or back at least three to four times a week during times when the sun is strongest, between 10 a.m. and 3 p.m. In northern countries like Canada, most people don’t get enough sun exposure to meet their needs, especially in winter, and because many of us spend most of our time indoors. It’s important to keep in mind that your skin must be exposed to the sun without any sunscreen so the skin’s absorption of UVB rays isn’t blocked, and clearly, this contrasts recommendations to protect your skin from the sun because of the risk of skin cancer.

The only way to be sure of your vitamin D status is through a blood test, which Canadians typically have to pay for out of pocket (about $39) unless you have osteopenia/ osteoporosis, celiac disease or other malabsorption diseases. The National Academy of Medicine, which Health Canada is part of, considers a blood level of 50 to 125 nmol/L to be adequate. Statistics Canada data from 2013 finds that 32 percent of Canadians had low blood levels of vitamin D (below 50 nmol/L).

In winter this rose to 40 percent, compared to 25 percent in summer. Some research indicates this could be even higher. Health Canada recommends Canadians meet the recommended dietary allowance of 600 international units (IU) for individuals aged one to 70, and 800 IU for those 71 and older, and that anyone over the age of 50 take a daily supplement of 400 IU.

However, as mentioned previously, most people don’t get enough through diet or from the sun, so a  supplement really is a good idea for everyone. A dosage of 600 IU to 800 IU will typically only bring blood levels to 50 nmol/L, which is “adequate” but not optimal. Low vitamin D levels are more common in non-white Canadians compared to white Canadians, because the darker the skin pigmentation, the more sunlight that is needed to produce enough vitamin D. The likelihood of deficiency is also higher among people living in more northern regions with less sunlight exposure, those with larger bodies, elderly people (especially those living in nursing homes), and people who work indoors. Depending on these factors, supplementation with vitamin D3 (cholecalciferol) with anywhere from 1000 to 4000 IU per day is recommended to reach optimal levels. But don’t go above 4000 IU unless advised by your doctor. At the end of the day, when it comes to COVID-19, some promising research is showing that vitamin D may be beneficial in preventing infection, decreasing severity of symptoms and decreasing likelihood of death. But it is still inconclusive; therefore we can’t make a definitive conclusion at this point. However, we know that, in general, vitamin D plays a role in our immune system and our body’s ability to fight off infection, and many of us aren’t getting enough.

Vitamin D is inexpensive and widely accessible, and since we’re in the middle of winter in Canada during a pandemic, we should all consider taking a vitamin D supplement.