Abstract: European Journal of Clinical Nutrition www.nature.com/ejcn ARTICLE Nutrition in acute and chronic diseases The effect of high-dose parenteral vitamin D3 on COVID-19related inhospital mortality in critical COVID-19 patients during intensive care unit admission: an observational cohort study Mehmet Güven 1✉ and Hamza Gültekin 2 © The Author(s), under exclusive licence to Springer Nature Limited 2021 BACKGROUND: In many studies, vitamin D has been found to be low in COVID-19 patients. In this study, we aimed to investigate the relationship between clinical course and inhospital mortality with parenteral administration of high-dose vitamin D3 within the first 24 h of admission to patients who were hospitalized in the intensive care unit (ICU) because of COVID-19 with vitamin D deficiency. METHODS: This study included 175 COVID-19 patients with vitamin D deficiency [25(OH) D <12 ng/mL] who were hospitalized in the ICU. Vitamin D3 group (n = 113) included patients who received a single dose of 300,000 IU vitamin D3 intramuscularly. Vitamin D3 was not administered to the control group (n = 62). RESULTS: Median C-reactive protein level was 10.8 mg/dL in the vitamin D3 group and 10.6 mg/dL in the control group (p = 0.465). Thirty-nine percent (n = 44) of the patients in the vitamin D3 group were intubated endotracheally, and 50% (n = 31) of the patients in the control group were intubated endotracheally (p = 0.157). Parenteral vitamin D3 administration was not associated with inhospital mortality by multivariate logistic regression analysis. According to Kaplan–Meier survival analysis, the median survival time was 16 d in the vitamin D3 group and 17 d in the control group (log-rank test, p = 0.459). CONCLUSION: In this study, which was performed for the first time in the literature, it was observed that high-dose parenteral vitamin D3 administration in critical COVID-19 patients with vitamin D deficiency during admission to the ICU did not reduce the need for intubation, length of hospital stay, and inhospital mortality. European Journal of Clinical Nutrition (2021) 75:1383–1388; https://doi.org/10.1038/s41430-021-00984-5