The What, Why, and How of Vitamin D

Vitamin D Sunshine Vitamin

Vitamin D

It’s the end of November, and many parts of the country are under snow. If you work a typical 9-5, you might not see the sun at all Monday through Friday. Maybe you’ve heard that Canadians should be taking vitamin D daily from October to March, but you’re not sure why. Stick with me, and we’ll go over the what, the why, and the how of vitamin D. 

What is it?

Vitamin D is an essential fat-soluble vitamin. It’s sometimes called the “sunshine vitamin” because our body makes it from cholesterol embedded in the skin in response to exposure to UVB rays. Unlike other vitamins, vitamin D is a steroid hormone in the body, which means it’s formed in one part of the body and acts on another part. Every cell in the body has receptors for Vitamin D. 

Vitamin D comes in two different forms: D2 and D3. There are small differences in their chemical structure. Vitamin D2 (ergocalciferol) is the plant-based form, while vitamin D3 (cholecalciferol) is from animals. 

So what does it do?

It’s mostly known for its ability to help build strong bones. But, vitamin D is also essential for a healthy immune system, digestive system, heart health, mental health, blood sugar regulation, fertility, and resistance to cancer.

  • Regulates bone mineralization and metabolism
  • Modulates the immune system
  • Has anti-inflammatory properties
  • Controls the expression of over 200 genes
  • Regulates the release of hormones such as serotonin (needed for mental health as well as gut health)
  • Controls cell growth

What does deficiency look like?

Studies show that up to 80% of people don’t get enough vitamin D. This deficiency is so common that some researchers have called it a “public health concern” and a “global problem.”

Deficiency might manifest as tiredness or fatigue, muscle weakness, aches and pain, frequent illness or infection, hair loss, osteomalacia (or more severely osteoporosis).

You can see why it’s so important that our levels are optimal. 

Vitamin D, the immune system, and inflammation

Several studies have shown a link between low levels of vitamin D and immune-related conditions like rheumatoid arthritis, systemic lupus erythematosus, type 1 diabetes, multiple sclerosis, celiac disease, psoriasis, and inflammatory bowel disease. 

In the lab, vitamin D seems to have anti-inflammatory and antioxidant properties. It can reduce immune response and inflammatory markers. Some studies in people with immune conditions (e.g. cystic fibrosis, multiple sclerosis), show that supplementing with vitamin D reduces inflammatory markers in the blood, although not all studies agree.

Some researchers think vitamin D, due to its effects on the immune system, may also help with severe food allergies. A few small studies show that children with low vitamin D levels have an increased risk of food allergies. More research is needed.

Vitamin D and digestive diseases

Since vitamin D is fat-soluble, it’s absorbed along with fat in the diet. So, people who don’t eat or absorb enough fat are at risk of lower vitamin D levels. This can include people with many digestive issues such as celiac disease, inflammatory bowel diseases (IBD) like Crohn’s & colitis, as well as people who have had gastric bypass surgery. 

Also, a healthy vitamin D status seems to go hand-in-hand with a healthy gut. For example, there is a link between sub-optimal vitamin D, gut microbiome status, gut inflammation, and diseases of the gut like IBD and colon cancer. 

Vitamin D and blood sugar

Low vitamin D levels are associated with higher levels of insulin resistance in people without diabetes. It may also increase the risk of developing diabetes.

Supplementing with vitamin D may help improve blood sugar management in some people with diabetes.

How do I get it?

At this point, you may be thinking, ‘Ok, ok, I get it, Amanda. I need vitamin D. How do I get it?”

Let’s go through the three primary sources of vitamin D – sun exposure, foods, and supplements.

Sun exposure

 Our skin contains “pre” vitamin D. When exposed to UV rays from the sun, this “previtamin” is converted into vitamin D (calciferol).

In Canada, due to our northern latitude, the sun is too low to generate vitamin D from approximately October to March. This is one reason vitamin D levels decline in people throughout the winter.

Knowing that too much UV radiation can contribute not only to skin cancer but also to dryness and other cosmetic changes in the skin over time, does this mean we should avoid the sun and supplement instead? 

Nope. It just means we need to be smart about sun exposure. From late spring to early fall plan for 15-20 minutes of unprotected sun exposure before 10 am or after 3 pm. Between those times, you’re at a higher risk of burning, so it’s important to cover up, find shade, or use a safe sunscreen.  

I like to use the D Minder app to see if the sun is high enough to make vitamin D and how long I need to be out to hit my targets. It also has a timer to alert you when you are likely to burn (but be smart, it’s an app and has limitations).


 Vitamin D is not naturally found in many foods. The best sources (although still not high amounts) include cold-water fish like salmon, sardines, and mackerel, and cod liver oil. It is also found in smaller quantities in beef liver, and egg yolks. You’re unlikely to get enough vitamin D from diet alone, especially if you live at a northern latitude, so supplementation may be necessary in the winter. 


High-quality supplements can be a huge help in correcting a deficiency or in certain people maintaining adequate levels through the winter.

Before you go ahead and start supplementing (or if you’ve been supplementing for a longer period of time), it’s best to have your vitamin D levels tested. Because vitamin D is fat-soluble, any excess vitamin D is stored in the liver rather than excreted in the urine like water-soluble vitamins. With high levels of supplementation, there is a risk of toxicity. If you are at risk for vitamin D deficiency, your health care provider can test your blood levels of 25(OH)D and recommend a course of action specifically for you.

If you do need to supplement it’s best to choose a high-quality vitamin D3 (cholecalciferol) in an oil base, which will help with absorption. Supplementing with vitamin D alone is not a solution; it needs to be done in combination with other fat-soluble vitamins, vitamin A and K2, and plant-based antioxidants.

Supplements can be helpful, but when we look at supporting autoimmune health, it’s important to remember that we need to address gut health, hormone regulation, stress, and other lifestyle factors as well. With a holistic approach, we can identify and address the factors that contribute to disease rather than put a band-aid over them.

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