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1. Introduction
Vitamin D deficiency (VDD) is a global health epidemic which affects almost one billion people worldwide and constitutes almost 15 per cent of the world’s total population (Mithal et al., 2009; Palacios and Gonzalez, 2014; Wahl et al., 2012). This means that people have either a VDD (<20 ng/mL) or an insufficiency (20-30 ng/mL) (Palacios and Gonzalez, 2014). Spending less time in the sun, inadequate UV-B exposure, excess melanin in the skin, geographical location and use of sunscreen are proposed reasons for VDD (Mithal et al., 2009). However, some studies suggest that VDD is prevalent in regions where there is ample sunshine (Holick and Chen, 2008) and in the FirstWorld countries that have long-running programs to fortify foods with vitamin D (Lips, 2007; Calvo and Whiting, 2003; Black et al., 2012; Nowson and Margerison, 2002). Therefore, the reasons for such widespread VDD are still unclear, but it could be because of our own evolving physiology or genetic makeup.
Vitamin D is a fat-soluble vitamin necessary for calcium homeostasis in the blood, bone and soft tissues. Vitamin D exists in nature as vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Although both these forms have vitamin D activity, vitamin D3 is biologically more potent than vitamin D2 (Moorani et al., 2019). It is frequently referred to as the “sunshine vitamin” as it can be synthesized in the keratinocytes present in the skin (Bikle, 2018). Briefly, 7-dehydrocholesterol (7-DHC) (the precursor to cholecalciferol) reacts with UV-B radiation between the wavelengths of 270 and 300 nm. This causes 7-DHC to photolyze into pre-vitamin D3 which then spontaneously undergoes thermal isomerization to vitamin D3 or cholecalciferol (Khundmiri et al., 2016).
Important functions of vitamin D include maintenance of calcium homeostasis in the body by promoting calcium uptake by the intestinal cells, increasing the number of osteoclasts and resulting in bone resorption and maintenance of serum calcium and phosphorus levels, resulting in bone development and aiding the parathyroid gland in further establishing adequate calcium levels in the body (Bikle, 2009; Bouillon, 2018). Apart from its classical functions, the vitamin D receptor (VDR) can affect the transcription of almost 200 genes, some of which include the genes that are responsible for cellular differentiation, growth regulation, apoptosis, cellular adhesion,...