Vitamin D is widely accepted as important for bone health and immune function, and its benefits are bolstered by a 2017 meta-analysis finding that vitamin D supplementation reduces the risk of respiratory infections, especially in deficient individuals. This raises the question: might low vitamin D levels increase the risk or severity of COVID-19 infection? Two large studies from Israel and the United Kingdom investigated this hypothesis but came to different conclusions.

The Israeli researchers conducted a retrospective, population-based study over three months last spring, using data on more than 14,000 members of Leumit Health Services, a large health maintenance organization. They compared vitamin D levels of subjects who had at least one positive test for COVID-19. After multivariate analysis, the investigators found a significant association between low vitamin D levels and greater likelihood of both COVID-19 infection and hospitalization. By contrast, researchers in the United Kingdom analyzed available data on 348,958 people from Public Health England and found no association between vitamin D levels and COVID-19 infection.

An editorial in the Lancet Diabetes and Endocrinology in September posited that both studies may have been limited by how vitamin D levels were measured (prior to exposure to COVID-19 infection) and also by confounding factors. The editorialists and the authors of the Israeli study suggest that more trials would help to clarify the effects of vitamin D supplementation regarding COVID-19 infection.

In the meantime, clinicians are encouraged to follow the Recommended Dietary Allowances for adults to keep patients' vitamin D levels in the optimal range. For details, visit: https://ods.od.nih.gov/factsheets/VitaminD-Consumer.-Gail M. Pfeifer, MA, RN