• Dr. Courtenay Boer

Vitamin D’s role in immunity and autoimmunity

Vitamin D is often called “the sunshine vitamin” due to our ability to synthesize it from cholesterol in our skin and UV light (from sunlight exposure). It actually refers to a group of fat-soluble compounds including vitamin D2 (ergocalciferol), vitamin D3 (cholecalciferol), and the most metabolically active form calcitriol (1,25-dihydroxycholecalciferol).

In Canada, for many people it’s nearly impossible to maintain sufficient Vitamin D levels throughout the wintertime, and it is one of the most common nutrient deficiencies in the northern hemisphere. In fact, Statistics Canada found that 40% of Canadians had insufficient Vitamin D levels in the winter time (1). This is mostly due to the lack of adequate sunshine (and UV light) throughout the winter months (and importantly, our ability to produce Vitamin D from the sun, which declines with age), as well as the limited availability of Vitamin D in food sources.

The name Vitamin D is a bit of a misnomer: it is actually a hormone, and not a vitamin! There are specific Vitamin D receptors in almost all tissues in the body. The functions of Vitamin D in the body are diverse, and include:

  • Regulation of metabolism

  • Calcium and magnesium regulation, and bone health

  • Gene transcription

  • Cardiovascular health

  • Insulin sensitivity

  • Mood (especially seasonal affective disorder)

  • Prevention of breast and colon cancer

  • Hormonal regulation

  • Immune system regulation

I want to focus on that last one, immune system regulation. Because of its modulatory effects on our adaptive and innate immune systems, Vitamin D plays a vital role in strengthening our defenses and there are several implications associated with a deficiency:


A deficiency of serum Vitamin D has been linked to increased risk of respiratory infections in people of all ages(2). So, can supplementing with Vitamin D prevent or treat respiratory infections? A recent study(3) noted that Vitamin D supplementation reduced the incidence non-influenza respiratory viral infections, which confirms earlier studies showing prevention through supplementation(4). Another recent study (5) found Vitamin D supplementation to be preventative for seasonal influenza, as well as decrease duration and severity of symptoms. Supplementing with Vitamin D to correct common deficiencies can help in both prevention of cold and flu as recovery.


Atopic diseases such as allergies and asthma (as well as atopic dermatitis, or eczema) represent an imbalanced immune system and predisposition to hypersensitivity reactions. One recent study (6) found Vitamin D deficiency in childhood to be associated with increased risk of persistent asthma, while a second study (7) found low Vitamin D levels to have a strong association with asthma exacerbations in adulthood. This confirms the results of a recent meta-analysis (8), which found that Vitamin D supplementation reduced the rate of asthma exacerbations and the need for systemic corticosteroids. Clearly, Vitamin D plays an important role in balancing the immune system and supplementation may be indicated for atopic conditions.


Just as Vitamin D has profound effects on decreased immune function and hypersensitivity reactions, it has been well-researched and implicated in autoimmune disease processes as well(9).

Hashimoto’s Thyroiditis:

Hashimoto's Thyroiditis (HT) is an autoimmune condition in which antibodies target and attack the thyroid gland, and is a leading cause of hypothyroidism. Several recent studies have shown lower serum Vitamin D levels in those with HT as compared to healthy individuals (9, 10). A recent randomized, double-blind, placebo-controlled trial (11) suggested Vitamin D supplementation improved TSH levels in hypothyroid patients while another study found supplementation decreased anti-TPO levels (the antibodies involved in HT) in those who were deficient (12). Vitamin D supplementation is an important consideration for those with HT.

Multiple Sclerosis:

Multiple Sclerosis (MS) is an autoimmune condition of the central nervous system, in

which an immune response is mounted against myelin (the insulation surrounding nerve cells), creating lesions in the white matter of the brain and spinal cord. Vitamin D is known to be deficient in patients with MS (13). Interestingly, incidence of MS increases in northern latitudes and some studies suggest that geographical location, connected to lower levels of Vitamin D, may play a role in the pathogenesis of the disease (14). Correcting this deficiency with supplementation has been shown to decrease the relapse rate and improve health outcomes in those with the condition (15, 16).

Because of the importance of Vitamin D in many physiological processes and the frequency of deficiency in Canada, it is important to have your Vitamin D levels checked and take appropriate supplementation to ensure that you are maintaining sufficient Vitamin D levels. Particularly as we enter the winter months, Vitamin D is an important part of immune system support. The recommended dosage of Vitamin D will vary with your current blood levels as well as other health concerns (autoimmunity, chronic digestive issues, etc.), and should be monitored once starting supplementation.

Consider talking to your health care provider about having your levels checked and to discuss if supplementation might be beneficial for you.


1. Janz, T. et al. (2013). Vitamin D Blood Levels of Canadians. Statistics Canada Catalogue no-82-625-X.

2. Miraglia Del Giudice, M. et al. (2018). Vitamin D: Immunomodulatory Aspects. Journal of Clinical Gastroenterology, 52(1).

3. Loeb, M. et al. (2018). Effect of Vitamin D supplementation to reduce respiratory infections in children and adolescents in Vietnam: a randomized controlled trial. Influenza Other Respir Viruses, doi: 10.1111/irv.12615.

4. Bergman P. et al. (2015). Vitamin D Supplementation to Patients with Frequent Respiratory Tract Infections: a Post Hoc Analysis of a Randomized and Placebo-Controlled Trial. BMC Res Notes, 8:391.

5. Zhou, J. et al. (2018). Preventive Effects of Vitamin D on Seasonal Influenza A in Infants: A Multicenter, Randomized, Open, Controlled Clinical Trial. Pediatric Infectious Disease Journal, 37(8): 749-754.

6. Hollams, E. et al. (2017). Vitamin D over the first decade and susceptibility to childhood allergy and asthma. Journal of Allergy and Clinical Immunology, 139(2): 472-481.

7. Confino-Cohen, R. et al. (2014). Vitamin D, asthma prevalence and asthma exacerbations: a large adult population‐based study. Allergy, 69(12): https://doi.org/10.1111/all.12508

8. Jolliffe, D. et al. (2017). Vitamin D supplementation to prevent asthma exacerbations: a systematic review and meta-analysis of individual participant data. The Lancet, 5(11): 881-890.

9. Orbach, H. et al. (2007). Novel biomarkers in autoimmune diseases: prolactin, ferritin, vitamin D, and TPA levels in autoimmune diseases. Annals of New York Academy of Science, 1109: 385-400.

10. Tamer, G. et al. (2011). Relative vitamin D insufficiency in Hashimoto’s thyroiditis. Thyroid, 21(8): 891-896.

11. Talaei, A. et al. (2018) The Effects of Vitamin D Supplementation on Thyroid Function in Hypothyroid Patients: A Randomized, Double-blind, Placebo-controlled Trial. Indian Journal of Endocrinology and Metabolism, 22(5): 584-588.

12. Mazokopakis, E. et al. (2015). Is vitamin D related to pathogenesis and treatment of Hashimoto's thyroiditis? Hellenic Journal of Nuclear Medicine, 18(3): 222-227.

13. Pierrot-Deseilligny, C. (2009). Clinical Implications of a Possible Role of Vitamin D in Multiple Sclerosis. Journal of Neurology, 256(9): 1468-1479.

14. Ramagopalan, S. et al (2009). Expression of the Multiple Sclerosis-Associated MHC Class II Allele HLA-DRB1*1501 Is Regulated by Vitamin D. PLOS Genetics, doi: 10.1371/journal.pgen.1000369

15. Jelinek, G. et al. (2015). Latitude, sun exposure and vitamin D supplementation: associations with quality of life and disease outcomes in a large international cohort of people with multiple sclerosis. BMC Neurology, 15:132, https://doi.org/10.1186/s12883-015-0394-1

16. Laursen, J. et al. (2016). Vitamin D supplementation reduces relapse rate in relapsing-remitting multiple sclerosis patients treated with natalizumab. Multiple Sclerosis and Related Disorders, 10: 169-173