Intended for healthcare professionals

  1. Peter Bergman, professor
  1. Department of Laboratory Medicine, Karolinska Institutet, Stockholm, Sweden
  1. peter.bergman{at}

Two new trials find no effect, but aren’t the final word

Vitamin D is an important regulator of calcium balance. In addition, it has important effects on the immune system, directly inducing antimicrobial peptides at mucosal surfaces and modulating the function of T cells.12 Observational studies from the pre-pandemic era found an association between low levels of vitamin D and an increased risk of respiratory tract infections.3 Results from randomised controlled trials were mixed, but two large meta-analyses found some evidence of a protective effect of vitamin D supplementation against respiratory tract infections, particularly in vitamin D deficient individuals.45 Could vitamin D help protect against covid-19?

At a mechanistic level, vitamin D boosts antiviral defences against other respiratory viruses, such as influenza A virus and rhinovirus.67 Data from observational studies suggest that low levels of 25-hydroxyvitamin D (25(OH)D) may be a risk factor for severe covid-19.8 However, this association could be due to reverse causality or confounding910: both covid-19 and vitamin D deficiency are independently associated with obesity, old age (>65 years), and male sex, for example. Two linked …

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