Abstract: F1000Research 2023, 12:254 Last updated: 09 MAR 2023 RESEARCH ARTICLE The effect of Vitamin D levels on the course of COVID-19 in hospitalized patients – a 1-year prospective cohort study [version 1; peer review: awaiting peer review] Darko Siuka1, Rajko Saletinger2,3, Jure Uršič1, Kristina Jevnikar4, Rado Janša1,2, David Štubljar 5, Joško Osredkar 6,7 1Department of Gastroenterology, University Medical Centre Ljubljana, Ljubljana, 1000, Slovenia 2Faculty of Medicine, University of Ljubljana, Ljubljana, 1000, Slovenia 3Department of Infectious Diseases and Febrile Illnesses, University Medical Centre Ljubljana, Ljubljana, 1000, Slovenia 4Department of Ophthalmology, University Medical Centre Ljubljana, Ljubljana, 1000, Slovenia 5Research and Development, In-Medico, Metlika, 8330, Slovenia 6Institute of Clinical Chemistry and Biochemistry, University Medical Centre Ljubljana, Ljubljana, 1000, Slovenia 7Department of Clinical Biochemistry, Faculty of Pharmacy, Ljubjana, 1000, Slovenia v1 First published: 09 Mar 2023, 12:254 https://doi.org/10.12688/f1000research.131730.1 Latest published: 09 Mar 2023, 12:254 https://doi.org/10.12688/f1000research.131730.1 Open Peer Review Approval Status AWAITING PEER REVIEW Any reports and responses or comments on the Abstract Background: The aim of the current study was to assess the patients with COVID-19 and the impact of vitamin D supplementation on the course of COVID-19. Methods: This prospective cohort study included patients hospitalized due to COVID-19 between December 2020 and December 2021. Patients' demographic, clinical, and laboratory parameters were analysed. Results: 301 participants were enrolled in the study. 46 (15,3%) had moderate, and 162 (53,8%) had severe COVID-19. 14 (4,7%) patients died, and 30 (10,0%) were admitted to the ICU due to disease worsening. The majority needed oxygen therapy (n=224; 74,4%). Average vitamin 25(OH)D3 levels were below optimal at the admittance, and vitamin D deficiency was detected in 205 individuals. More male patients were suffering from vitamin D deficiency. Patients with the more severe disease showed lower levels of vitamin 25(OH)D3 in their blood. The most severe group of patients had more symptoms that lasted significantly longer with progressing disease severity. This group of patients also suffered from more deaths, ICU admissions, and treatments with dexamethasone, remdesivir, and oxygen. Conclusion: Patients with the severe course of COVID-19 were shown to have increased inflammatory parameters, increased mortality, and higher incidence of vitamin D deficiency. The results suggest that the vitamin D deficiency might represent a significant risk factor for a severe course of COVID-19. article can be found at the end of the article. Page 1 of 15 F1000Research 2023, 12:254 Last updated: 09 MAR 2023 Keywords vitamin D; COVID-19; severity; supplementation Corresponding author: Joško Osredkar (josko.osredkar@kclj.si) Author roles: Siuka D: Conceptualization, Validation, Writing – Original Draft Preparation, Writing – Review & Editing; Saletinger R: Formal Analysis; Uršič J: Data Curation, Investigation, Methodology; Jevnikar K: Data Curation, Investigation, Methodology; Janša R: Formal Analysis, Validation; Štubljar D: Data Curation, Formal Analysis, Investigation; Osredkar J: Conceptualization, Supervision, Writing – Review & Editing Competing interests: No competing interests were disclosed. Grant information: The author(s) declared..