Vitamin D is a prohormone that can be photochemically produced in the skin as well as ingested through diet, as a dietary supplement or as a medication. The two main forms of vitamin D, ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) are, although in small quantities, naturally present in some foods. However, the majority of vitamin D is acquired via the action of sunlight on the skin, converting 7 dehydrocholesterol to vitamin D3. Vitamin D is metabolized first to calcidiol, 25(OH)D, and then to the hormonal form calcitriol, 1,25(OH)2D. Calcitriol is the ligand for the vitamin D receptor (VDR), a transcription factor, that binds to sites in the deoxyribonucleic acid (DNA) called vitamin D response elements (VDREs). There are thousands of these binding sites regulating hundreds of genes in a cell-specific way. The identification of a VDR in most tissues and cells and the observation that a multitude of genes may be directly or indirectly regulated by 1,25(OH)2D have provided a rationale for the health benefits of vitamin D. Due to the modern way of life, we live in a global vitamin D deficiency, which not only negatively affects the skeletal system but also facilitates the development and progression of several different diseases, including cardiovascular diseases, diabetes, autoimmune diseases, respiratory diseases, neurological diseases, cancer among others. Yet, despite the growing number of randomized controlled clinical trials, which could support linking the non-skeletal health benefits of vitamin D, meta-analyses of these studies have drawn inconsistent conclusions. Based on theoretical research of the literature and studies, the aim of this paper is to determine to what extent vitamin D affects human health and certain diseases.
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