H Council of Italy (IFC-CNR), 56124 Pisa, Italy; [email protected] Institute for Biomedical Study and Innovation

H Council of Italy (IFC-CNR), 56124 Pisa, Italy; [email protected] Institute for Biomedical Study and Innovation (IRIB), National Analysis Council of Italy (CNR), 98164 Messina, Italy; [email protected] Division of Clinical and Experimental Medicine, School and Operative Unit of Allergy and Clinical Immunology, University of Messina, 98125 Messina, Italy; [email protected] Correspondence: [email protected]; Tel.: +39-0902-212-364 These authors contributed equally.Citation: De Martinis, M.; Allegra, A.; Sirufo, M.M.; Tonacci, A.; Pioggia, G.; Raggiunti, M.; Ginaldi, L.; Gangemi, S. Vitamin D Deficiency, Osteoporosis and Impact on Autoimmune Illnesses and Hematopoiesis: A Overview. Int. J. Mol. Sci. 2021, 22, 8855. https://doi.org/ 10.3390/ijms22168855 Academic Editor: Valentina Morelli Received: 23 June 2021 Accepted: 16 August 2021 Published: 17 AugustAbstract: Vitamin D (VD) is crucial for bone homeostasis, nevertheless it can also be involved in pleiotropic effects on various organs and tissues. In adults, VD deficiency can cause or exacerbate osteoporosis and induce osteomalacia. NTR1 Agonist drug Nonetheless, just about every tissue and cell within the body features a VD receptor, which includes the brain, heart, stomach, pancreas, skin, gonads, and immune cells, and also a deficiency may perhaps modify the function of these organs. Therefore, the wide-ranging actions of VD support to clarify why a reduction in VD quantity has been correlated with many chronic illnesses. In actual fact, VD deficiency increases the risk of osteoporosis and various other illnesses and complications characterized by impaired bone metabolisms, including autoimmune ailments, inflammatory bowel diseases, allergy, endocrinological diseases, hematological malignancies, and bone marrow transplantation. This review aims to investigate the link involving VD deficiency, osteoporosis, and its concomitant ailments. Additional epidemiological and mechanistic research are important so as to ascertain the real role of hypovitaminosis in causing the reported diseases; however, sufficient vitamin supplementation and restoration of metabolic normality may very well be beneficial for far better management of these pathologies. Keywords: vitamin D; osteoporosis; autoimmune illnesses; allergy; endocrinological ailments; monoclonal gammopathies; bone marrow transplantationPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.1. General Considerations on Vitamin D VD is actually a fat-soluble substance of steroidal nature, its sources might be endogenous, and its production is UVB mTORC1 Inhibitor Biological Activity dependent or exogenous by way of nutritional intake and supplements [1,2]. Cholecalciferol, the inactive form of VD, calls for two hydroxylations for activation: the initial takes location within the liver where it can be converted into calcidiol or 25-hydroxycholecalciferol (25(OH)D), and the second inside the kidney, beneath strict manage of PTH plus the phosphaturic hormone fibroblast growth factor 23 (FGF-23), where calcidiol is metabolized into calcitriol or 1-,25-dihydroxycholecalciferol (1,25(OH)2D) [3]. Serum 25(OH)D levels are positively influenced by ultraviolet radiation in sunlight, although inside the liver, kidney, and/or intestine diseases, various drugs and obesity negatively regulate VD levels [4]. 25(OH)D deficiencyCopyright: 2021 by the authors. Licensee MDPI, Basel, Switzerland. This short article is an open access report distributed under the terms and conditions with the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/license.