Abstract: Journal of Steroid Biochemistry and Molecular Biology 212 (2021) 105928 Contents lists available at ScienceDirect Journal of Steroid Biochemistry and Molecular Biology journal homepage: www.elsevier.com/locate/jsbmb The relationship between 25(OH) vitamin D levels and COVID-19 onset and disease course in Spanish patients Manuel Diaz-Curiel a, *, Alfonso Cabello b, Rosa Arboiro-Pinel a, Jose Luis Mansur c, Sarah Heili-Frades d, Ignacio Mahillo-Fernandez e, Antonio Herrero-González f, Marjorie Andrade-Poveda a a Department of Internal Medicine, Metabolic Bone Diseases, Instituto de Investigación Sanitaria Fundación Jiménez Díaz (IIS-FJD, UAM), Spain Department of Internal Medicine, Infectious Diseases Division, Hospital Universitario Fundación Jiménez Díaz, Spain Centro de Endocrinología y Osteoporosis, La Plata, Buenos Aires, Argentina d Lung Diseases Department, Hospital Universitario Fundación Jiménez Díaz, Spain e Epidemiology and Biostatistics Department, Hospital Universitario Fundación Jiménez Díaz, Spain f Department of Big Data Analytics, Instituto de Investigación Sanitaria Fundación Jiménez Díaz (IIS-FJD, UAM), Spain b c A R T I C L E I N F O A B S T R A C T Keywords: Vitamin D COVID-19 Hospital admission Intensive care unit (ICU) Mortality Objective: Currently, there are no definitive data on the relationship between low levels of vitamin D in the blood and a more severe disease course, in terms of the need for hospital admission, intensive care unit (ICU) stay, and mortality, in patients with coronavirus disease 2019 (COVID-19), the disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We aimed to study the association between levels of circulating 25-hydroxyvitamin D (25(OH)D) and adverse clinical outcomes linked to SARS-CoV-2 infection. We further aimed to observe the incidence of low, below-average, and normal levels of 25(OH)D in patients hospitalized for COVID-19 between March 12, 2020, and May 20, 2020, and assess whether these values differed between these patients and a normal population. Finally, we determined whether the need for transfer to the intensive care unit (ICU) and the mortality rate were related to low levels of 25(OH)D. Study Design: Retrospective observational study. Setting: Quironsalud Hospitals in Madrid, Spain. Participants: We analyzed 1549 patients (mean age, 70 years; range, 21–104 years); 835 were male (53.9 %; mean age, 73.02 years), and 714 were female (46.1 %; mean age, 68.05 years). Subsequently, infected patients admitted to the ICU (n = 112) and those with a fatal outcome (n = 324) were analyzed. Procedures: Serum concentrations of 25(OH)D were measured by electrochemiluminescence. Results: More hospitalized patients (66 %, n = 1017) had low baseline levels of 25(OH)D (<20 ng/mL) than normal individuals (45 %) (p < 0.001). An analysis by age group revealed that COVID-19 patients between the ages of 20 and 80 years old had significantly lower vitamin D levels than those of the normal population (p < 0.001). Patients admitted to the ICU tended to have lower levels of 25(OH)D than other inpatients (p < 0.001); if we stratified patients by 25(OH)D levels, we observed that the rate of ICU admission was higher among patients with vitamin D deficiency (p < 0.001), indicating that higher vitamin D levels are associated with a lower risk of ICU admission due to COVID-19. ICU admission was related to sex (higher rates in men, p < 0.001) and age (p <..