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Introduction

1.1 Vitamin D biology

Vitamin D intake knowledge perception is required for intestinal absorption of calcium, magnesium, and phosphorous metabolism and therefore it plays an important role in skeletal health (Haroon and FitzGerald 2012). It can be obtained from sun exposure, dietary sources (vitamin D2 and D3), and supplements. The two most important forms of Vitamin D are Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol) (Christakos et al. 2012).

Vitamin D3 is produced in the skin from UV irradiation in the form of 7- dehydrocholesterol, Vitamin D intake knowledge perception it helps in breaking the ring of vitamin D to form Pre D3 which eliminate from the skin and bound to DBP (Vitamin D binding protein) (Roth et al 2018). The liver is the organ that metabolizes Vitamin D either from the skin or in the form of oral ingestion to 25OHD (25-hydroxyvitamin D) and it is the major circulating metabolite of Vitamin D2 and D3 which is largely used as a biomarker of vitamin D status (Holick 2017).

The 25OHD form of Vitamin is further metabolized in the kidney with the help of certain enzymes CYP2R1 (Christakos et al. 2012); some of the other ones include CYP27B1 and CYP24A1.  Further, the vitamin circulated and absorbed by epithelial cells, immune system, and parathyroid glands (Christakos et al. 2012). The metabolites of the vitamin are transported in the blood bound to albumin and the vitamin is circulated in the blood (Christakos et al. 2012). The metabolism of Vitamin D occurs with the help of cytochrome P450 and converted the Vitamin D into 25-hydroxylation, 1 a- hydroxylation, and 24- hydroxylation (Holick 2017). The primary source of 25-OH is Ultraviolet B (UVB) sunlight from exposing an epidermal layer of skin surfaces that contain adequate ultraviolet B radiation which known as cholecalciferol (Bikle 2017).

Then, it synthesized from the derivate of steroid of cholesterol presented in the skin, the liver functions to convert this into calcidiol which later converted in the active form vitamin known as calcitriol in the kidney (Roth et al 2018).  From the kidney, the active form of Vitamin D is transported to immune cells, bones, and intestine (Holick 2017). Thus, sunlight is considered as the main source for the majority of people in the UK (SACN 2016). Figure 1 shows the synthesis of Vitamin D and where it gets stored after the formation….