Abstract: nutrients Article The Impact of Serum Levels of Vitamin D3 and Its Metabolites on the Prognosis and Disease Severity of COVID-19 Hani M. J. Khojah 1 , Sameh A. Ahmed 2, * , Sultan S. Al-Thagfan 1 , Yaser M. Alahmadi 1 1 2 3 * Citation: Khojah, H.M.J.; Ahmed, S.A.; Al-Thagfan, S.S.; Alahmadi, Y.M.; Abdou, Y.A. The Impact of Serum Levels of Vitamin D3 and Its Metabolites on the Prognosis and Disease Severity of COVID-19. Nutrients 2022, 14, 5329. https:// doi.org/10.3390/nu14245329 and Yasser A. Abdou 3 Department of Clinical and Hospital Pharmacy, College of Pharmacy, Taibah University, Al Madinah Al Munawarah 30001, Saudi Arabia Department of Pharmacognosy and Pharmaceutical Chemistry, College of Pharmacy, Taibah University, Al Madinah Al Munawarah 30001, Saudi Arabia Ohud Hospital, Al Madinah Al Munawarah 42354, Saudi Arabia Correspondence: shassan@taibahu.edu.sa; Tel.: +966-54-3110057 Abstract: Vitamin D is among the increasingly consumed dietary supplements during the COVID-19 pandemic. It plays a regulatory role in the immune system and moderates the renin–angiotensin system, which is implicated in infection pathogenesis. However, the investigation of serum levels of vitamin D3 forms and their relative ratios in COVID-19 patients is worth investigation to understand the impacts of disease severity. Hence, we investigated the serum levels of vitamin D3 (cholecalciferol) and its metabolites (calcifediol and calcitriol), in addition to their relative ratios and correlations with angiotensin-converting enzyme 2 (ACE2), interleukin-6 (Il-6), and neutrophil–lymphocyte ratio (NLR) in COVID-19 patients compared with healthy controls. Oropharyngeal specimens were collected from the study subjects for polymerase chain reaction testing for COVID-19. Whole blood samples were obtained for blood count and NLR testing, and sera were used for the analysis of the levels of the vitamin and its metabolites, ACE2, and IL-6. We enrolled 103 patients and 50 controls. ACE2, Il-6, and NLR were significantly higher in the patients group (72.37 ± 18.67 vs. 32.36 ± 11.27 U/L, 95.84 ± 25.23 vs. 2.76 ± 0.62 pg/mL, and 1.61 ± 0.30 vs. 1.07 ± 0.16, respectively). Cholecalciferol, calcifediol, and calcitriol were significantly lower in patients (18.50 ± 5.36 vs. 29.13 ± 4.94 ng/mL, 14.60 ± 3.30 vs. 23.10 ± 3.02 ng/mL, and 42.90 ± 8.44 vs. 65.15 ± 7.11 pg/mL, respectively). However, their relative ratios were normal in both groups. Levels of the vitamin and metabolites were strongly positively, strongly negatively, and moderately negatively correlated with ACE2, Il-6, and NLR, respectively. COVID-19 infection severity is associated with a significant decrease in vitamin D3 and its metabolites in a parallel pattern, and with a significant increase in ACE2, Il-6, and NLR. Higher levels of vitamin D and its metabolites are potentially protective against severe infection. Academic Editor: Andrea Fabbri Received: 21 November 2022 Accepted: 12 December 2022 Keywords: COVID-19; vitamin D; vitamin D3; cholecalciferol; calcifediol; calcitriol; angiotensinconverting enzyme 2; interleukin-6; NLR Published: 15 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license..