The most recognized roles for vitamin D in supporting a healthy immune system include reinforcing the barrier provided by epithelial cells (like skin cells), enhancing the cell’s ability to produce vitamin D at the site of infection, increasing other key defenses of innate immunity and decreasing inflammatory markers. At the site of infection vitamin D has antibacterial and antiviral benefits.12-14 Vitamin D influences some of the genes involved in the innate and adaptive immune systems and in inflammation and therefore affects susceptibility to and severity of bacterial and viral infections. 15,16
The immune system has two parts that overlap, adaptive and innate. Innate immunity is the human body’s first defense against infection. It includes physical barriers including the skin, mucus and lining of the lungs and intestines, as well as cells with receptors that quickly recognize pathogens. The adaptive immune system fights against pathogens or antigens very specifically, but it takes longer to develop. It is activated by exposure to pathogens. 17,18
Observational studies report consistent independent associations between low serum concentrations (amount in the blood) of 25-hydroxyvitamin D (the major circulating vitamin D metabolite) and susceptibility to acute respiratory tract infection. In an analysis of 25 studies, vitamin D supplementation reduced the risk of acute respiratory tract infection among over 11,000 subjects who were supplemented. Daily or weekly supplementation benefited the subjects more than just a large single dose. Those that were most deficient benefited the most. 19
While scientists have not yet determined an exact answer to this question, mechanisms have been proposed. It is known that inflammation and what is called a cytokine storm, which is when an infection triggers your immune system to flood your bloodstream with inflammatory proteins called cytokines, play a critical role in COVID-19 patient outcomes. This dramatic inflammatory reaction can kill tissue and damage organs. Adequate vitamin D levels as well as a healthy balanced diet, particularly the Mediterranean Diet, have been identified as beneficial to offset this inflammatory response. Additionally, vitamin D may exhibit antiviral effects by interfering with viral replication and through its ability to stimulate the immune system and reduce inflammation. 20
The Institute of Medicine recommends a dietary intake of Vitamin D of 600 IU (15 mcg)/day for all ages over 1 year (and for pregnant and lactating women), with increases to 800 IU (20 mcg)/day for individuals over 71 years. 21
Food sources of vitamin D include fatty fish (salmon, mackerel, sardines, cod liver oil) and some types of mushrooms (Shiitake), especially if sundried, contain cholecalciferol (vitamin D3). Because vitamin D not easily obtained from food, many countries, including the US and Canada, encourage the fortification of commonly consumed foods, such as milk, orange juice, other dairy products and cereals with vitamin D. Many people have inadequate sun exposure to meet vitamin D requirements and few people are able to obtain enough vitamin D from food sources alone, therefore supplementation is important. How much to supplement is less clear and will vary based on vitamin D status. Deficient and insufficient individuals will need a different supplemental strategy than those who are not. There is no agreed upon dose globally, recommendations often range from 400-2000 IU (10-50 mcg)/day and your doctor may recommend more. However, a common dose of 1000 IU (25 mcg) of vitamin D3, has been identified to be safe and beneficial for health. 22
While there is no global consensus on optimal serum vitamin D levels, the Institute of Medicine recommends keeping levels consistently above 50 nmol/L (20 ng/mL)23, and the Endocrine Society Clinical Practice Guidelines recommend maintaining blood levels of 25(OH)D consistently above 75 nmol/L (30 ng/ml). 24 Some research indicates that a daily vitamin D dose of 800 IU (20 mcg) can achieve a target 25(OH)D level of at least 50 nmol/L (or 20 ng/mL) in most healthy individuals, and 2000 IU (50 mcg) is sufficient to achieve a level of at least 75 nmol/L (or 30 ng/mL).25
To convert IU to mcg or mg, the Council for Responsible Nutrition offers a conversion calculator at www.belabelwise.org/conversion-calculator.