In the human body, vitamin D is produced due to the effect of ultraviolet light (sunshine) on the skin, or is taken in via supplements or fortified food sources. It’s most well known function is to regulate calcium concentrations in the body, but more and more research is showing that it plays a much larger and more important role, especially within the immune system. Studies have found that the protective aspects of vitamin D begin to be seen at serum concentrations of 20-30 nm/mL, but according to conventional standards normal range is between 25 and 80 ng/mL. As concentrations in the blood increase, the protective association in regards to immune function does as well. The optimal range for COVID-19 prevention appears to be between 40 and 60 ng/mL, which for many in the United States and other northern hemisphere counties is hard to reach. For the US and European population to reach this level, most individuals would have to be supplementing between 2000 and 5000 IUs per day of vitamin D3.