# Exploring Vitamin D Synthesis and Function in Cardiovascular Health: A Narrative Review
###### Abstract  
Vitamin D plays a crucial role in the human body, influencing a wide range of physiological processes from bone health to immune function. The complex biochemical pathways involved in the synthesis, metabolism, and action of Vitamin D are explored, emphasizing its importance in nutrition and food technology. This review also investigates the regulatory mechanisms that control Vitamin D metabolism and its systemic effects on calcium homeostasis, cell proliferation, differentiation, and immune modulation. The role of Vitamin D3 in regulating blood pressure and atherosclerosis in the onset of cardiovascular disorders is discussed. Given the importance of Vitamin D in food science and technology, the regulatory mechanisms that control Vitamin D metabolism and its systemic effects on calcium homeostasis are also investigated, integrating innovative approaches and advanced technologies to improve human health through nutrition. Additionally, the review assesses the influence of food processing on Vitamin D levels and discusses cutting-edge technologies as innovative strategies to mitigate Vitamin D loss during food processing. This comprehensive exploration aims to improve our understanding of the biochemical pathways of Vitamin D and its relevance to food science, contributing to the development of new strategies for food fortification and the promotion of optimal health through diet. Vitamin D; food fortification; nutritional biochemistry; Vitamin D metabolism; clinical nutrition 2020 May 202020  
sectors including infections, immunity, cancer, and cardiovascular diseases. Vitamin D plays a role in regulating the immune system. It works by controlling the activity of suppressor T lymphocytes, cytokine synthesis, and the cell death process [4]. In particular, Vitamin D also stimulates the absorption of phosphate in the intestine and prevents its excretion through the kidneys [5]. Therefore, Vitamin D represents a molecule with a large pleiotropic functional profile. The term Vitamin D refers to a significant group of fat-soluble substances with the main chain made up of cholesterol rings. 25(OH)D (calcifediol or calcidiol) is the main circulating compound, while 1,25(OH)2D (calcitriol) represents the active compound and interacts with the Vitamin D/hormone D receptor (VDR) [6]. 1,25(OH)2D binds to the VDR to initiate or inhibit gene transcription, causing the heterodimerization of the VDR and the retinoid \(X\) receptor [7]. When the heterodimer translocates into the nucleus, the complex binds to the Vitamin D consensus sequences, and gene transcription is modulated. At the circulatory level, Vitamin D (2 and 3) attaches to Vitamin D binding protein (DBP) and is then accumulated in adipose tissue [8]. Subsequently, metabolization into 25(OH)D occurs in the liver by hydroxylase (cytochrome P450(CYP)2R1 and CYP27A1). This process can also occur in other tissues in an autocrine and/or paracrine manner [9]. Hydroxylation then occurs in the renal tubule to produce the active molecule 1,25(OH)2D. During this process, cublin and megalin, two proteins that allow the DBP-25(OH)D complex to enter through the cellular receptors of the renal tubule [10], intervene. A decrease in cublin and megalin determines a greater urinary loss of 25(OH)D. Furthermore, the cells of the renal tubule present two hydroxyles belonging to the cytochrome P450 family, i.e., 1-alpha-hydroxylase (CYP27B1) and 24-alpha-hydroxylase (CYP24A1), which, by hydroxyling 25(OH)D, produce the active form of Vitamin D (1,25(OH)2D-calcitriol) or the inactive metabolite 24,25(OH) 2D [10]. The main production site of Vitamin D is the skin [6]. Vitamin D3 (cholecalciferol) is produced in the skin when exposed to UVB rays with a wavelength of 290-315 nm. These radiations convert 7-dehydrocholesterol (7-DHC) into pre-Vitamin D, which in turn is converted into Vitamin D3 in a thermosensitive but non-catalytic process, which involves the formation of three double bonds [11]. Both UVB intensity and skin pigment levels influence the rate of D3 formation.