"Vitamin D supplementation as a public health measure to improve outcomes is not supported by this study. Most importantly, our results suggest that investment in other therapeutic or preventative avenues should be prioritized for COVID-19 randomized clinical trials," say the authors.
To assess the relationship between vitamin D levels and COVID-19 susceptibility and severity, the researchers conducted a Mendelian randomization study using genetic variants strongly associated with increased vitamin D levels. They looked at genetic variants of 14,134 individuals with COVID-19 and over 1.2 million individuals without the disease from 11 countries.
In the study published in PLOS Medicine , the researchers found that among people who did develop the disease, there was no difference between vitamin D levels and a likelihood of being hospitalized or falling severely ill.
Studying the effects of vitamin D
Early in the pandemic, many researchers were studying the effects of vitamin D, which plays a critical role in a healthy immune system. But there is still not enough evidence that taking supplements can prevent or treat COVID-19 in the general population.
"Most vitamin D studies are very difficult to interpret since they cannot adjust for the known risk factors for severe COVID-19 such as older age or having chronic diseases, which are also predictors of low vitamin D," says co-author Guillaume Butler-Laporte , a physician and a fellow under the supervision of Professor Brent Richards at McGill University.
"Therefore, the best way to answer the question of the effect of vitamin D would be through randomized trials, but these are complex and resource intensive, and take a long time during a pandemic," he says.
By using a Mendelian randomization, the researchers say they were able to decrease potential bias from these known risk factors and provide a clearer picture of the relationship between vitamin D and COVID-19.
However, researchers noted that their study had some important limitations. It did not account for truly vitamin D deficient patients, consequently it remains possible that they may benefit from supplementation for COVID-19 related protection and outcomes. Additionally, the study only analyzed genetic variants from individuals of European ancestry. Future studies are needed to explore the relationship with vitamin D and COVID-19 outcomes in other populations, say the researchers.
"In the past Mendelian randomization has consistently predicted results of large, expensive, and timely vitamin D trials. Here, this method does not show clear evidence that vitamin D supplementation would have a large effect on COVID-19 outcomes," says Butler-Laporte, who is a microbiologist and an expert in infectious diseases.
About this study "Vitamin D and COVID-19 susceptibility and severity in the COVID-19 Host Genetics Initiative: A Mendelian randomization study" by Guillaume Butler-Laporte, Tomoko Nakanishi, Vincent Mooser, David R. Morrison, Tala Abdullah, Olumide Adeleye, Noor Mamlouk, Nofar Kimchi, Zaman Afrasiabi, Nardin Rezk, Annarita Giliberti, Alessandra Renieri, Yiheng Chen, Sirui Zhou, Vincenzo Forgetta, and J. Brent Richards is published in PLOS Medicine .