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Vitamin D is an important vitamin with many benefits. It helps the body absorb dietary calcium, helps build strong bones and teeth, and helps maintain and support healthy immunity. Vitamin D deficiency has been linked to many different conditions, including heart disease,  high blood sugar levels, occasional low mood  and aches and pains.  Vitamin D deficiency is more common in the winter, with less sun exposure, with darker skin, with advancing age, and when living at northern latitudes. 
Vitamin D deficiency is very common in Canadians, and it is estimated that 90% of Canadians have inadequate intake of vitamin D from dietary sources alone. Even when supplements are considered along with diet, more than half of Canadians are still vitamin D deficient. The great news is that vitamin D deficiency can be addressed and prevented! While the body makes vitamin D when skin is exposed to the sun, dietary and supplemental sources are also excellent choices.
Vitamin D is incredibly important for healthy immune function. The immune system has two parts: the faster, innate immunity which we are born with, and the slower, but more specific adaptive immunity which develops over time. Both the innate immunity cells and the adaptive immunity cells have vitamin D receptors, and when there is not enough vitamin D to bind to them, function decreases and infections become more common. Studies show that infection rates are higher in vitamin D deficiency.  Vitamin D is particularly important for respiratory health, as studies have shown that people with low vitamin D levels are much more likely to catch a cold or flu.
Vitamin D may also play a role in autoimmunity, as vitamin D deficiency is common in autoimmune conditions such as multiple sclerosis and systemic lupus. Vitamin D helps regulate the immune system by promoting an anti-inflammatory response over a pro inflammatory response, and may even affect gene transcription. Larger, randomized controlled studies are needed to better understand the relationship between low vitamin D levels and autoimmunity.
The adequate intake(AI) to maintain skeletal health is 400 IU. The recommended dietary allowance for children and adults aged 1–70 years is 600 IU. More vitamin D may be needed depending on personal vitamin D deficiency status as well as individual health concerns. Just as it is important to take enough vitamin D, it is also important to avoid taking too much vitamin D. The tolerable upper intake level (UL), the highest safe dose that can be taken without potential side effects, is 4000 IU (100 mcg) for adults aged 9 and up. For immune support, some studies have used 1000–2000 IU per day, [10,11,12] though more may be needed depending on the degree of deficiency as determined by a vitamin D blood test and discussion with your health care provider.
You can read why Vitamin D is Important here, and learn more about foods with vitamin D, the best sources of vitamin D, causes of deficiency, and how to choose the best vitamin D supplement to support immunity and overall health.
Maxine Fidler, ND
Maxine Fidler is a licensed naturopathic doctor who loves to write about natural health.