en تغذيه Nutrition پژوهشي Original Article <div style="text-align: justify;"><strong>Background and Purpose</strong>: This study aimed to determine the association between vitamin D serum levels and the severity of COVID-19 in Iran.&nbsp;<br> <strong>Materials and Methods</strong>: This cross-sectional study was done on 121 COVID-19 patients referred to Kowsar Hospital of Semnan City, Iran, in 2022. The patients were assigned to the mild/moderate (n=80) and severe (n=41) COVID-19 groups. The data collection tool was a checklist that included demographics, clinical variables, and laboratory parameters extracted from the medical file and interviews with the patient. Also, patients&rsquo; serum vitamin D level was measured using the liquid chromatography-tandem mass spectrometry method. Data were entered in Stata software, version 14 and analyzed by multivariate logistic regression. P<0.05 were considered significant.<br> <strong>Results</strong>: The mean age of severe and mild/moderate groups were 67.66&plusmn;16.15 and 48.5&plusmn;18.7 years, respectively. About 61% and 55% of mild/moderate and severe groups were male. The multivariate logistic regression model showed vitamin D (odds ratio [OR]=0.55; 95% CI, 0.42%, 0.83%) is the most important factor predicting the severity of COVID-19 so that, for one ng/mL increase in vitamin D level, the odds of contracting the severe form of COVID-19 decreases by about 45%.&nbsp;<br> <strong>Conclusion</strong>: The present study shows that low vitamin D levels may increase the risk of severe COVID-19. However, further studies with larger sample sizes are recommended.</div> Vitamin D, COVID-19, Cross-sectional study, Iran 57 64 http://jhs.mazums.ac.ir/browse.php?a_code=A-10-1070-1&slc_lang=en&sid=1 Shahrzad Izadi Shahrzadi55@gmail.com 100319475328460012422 100319475328460012422 No Department of Surgery, Faculty of Medicine, Semnan University of Medical Sciences, Semnan, Iran. Masoume Bitaraf Bitaraf2023@gmail.com 100319475328460012423 100319475328460012423 No Department of Internal Medicine, Faculty of Medicine, Semnan University of Medical Sciences, Semnan, Iran. Fatemeh Rahaei Rahaeifatemeh@yahoo.com 100319475328460012424 100319475328460012424 No Clinical Research Development Unit, Valiasr Hospital, Zanjan University of Medical Sciences, Zanjan, Iran. Seyedeh Pardis Naghibi Rokni spnrokni20@gmail.com 100319475328460012425 100319475328460012425 No Department of Surgery, Faculty of Medicine, Semnan University of Medical Sciences, Semnan, Iran. Kamyar Mansori kamyarmansori@yahoo.com 100319475328460012426 100319475328460012426 No Clinical Research Development Unit, Valiasr Hospital, Zanjan University of Medical Sciences, Zanjan, Iran. Mohammad Memarian dr.memarian20@gmail.com 100319475328460012427 100319475328460012427 Yes Department of Internal Medicine, Faculty of Medicine, Semnan University of Medical Sciences, Semnan, Iran.