COVID-19 is a respiratory illness caused by the SARS-CoV-2 virus. A growing number of studies have demonstrated the importance of vitamin D in regulating the immune system and controlling COVID-19 disease severity. Authors of a new study found that vitamin D supplementation shortened SARS-CoV-2 infection in vitamin D-deficient people.

Vitamin D is a hormone that affects the expression of many genes important for immune function. Vitamin D deficiency is associated with greater risk of influenza and other respiratory infections. Unfortunately, one 2012 report estimates that approximately half of all people worldwide have vitamin D insufficiency, and 1 billion have vitamin D deficiency, defined as blood levels less than 12 nanograms per milliliter

The study involved 40 adults who had a positive SARS-CoV-2 RNA test; mild or no COVID-19 symptoms; and vitamin D deficiency (blood level less than 20 nanograms per milliliter). The authors assigned 16 study participants to take 60,000 international units of vitamin D3 (cholecalciferol) orally until they reached a blood level greater than 50 nanograms per milliliter.

At baseline, the average serum vitamin D level in the treatment group was 8.6 nanograms per milliliter. After two weeks of supplementation, 75 percent of the participants achieved a vitamin D blood level greater than 50 nanograms per milliliter. Those who corrected their deficiency were more likely to clear the virus by day 21 of the study. They also experienced a decrease in serum fibrinogen, a marker of inflammation.

The authors concluded that short-term high dose oral vitamin D supplementation is effective in correcting deficiency and promotes the clearance of SARS-CoV-2. They encouraged cholecalciferol supplementation as a way to decrease viral transmission.

• Link to full study.

• Learn more about how vitamin D supplementation may reduce susceptibility to COVID-19-associated lung injury in this clip from a Q&A with Dr. Rhonda Patrick.