**4. Synthesizing and metabolizing physiology of vitamin D**

Vitamin D is a secosteroid hormone that is either absorbed by dietary intake or manufactured by the ultraviolet beam (UVB) rays ranging from 290 to 310 nm reaching the epidermis of the skin. In the presence of epidermal 7-dehydrocholesterol, the absorbed vitamin D gets converted into pre-vitamin D3. Within the epidermis, a thermal reaction occurs to convert the pre-vitamin D3 into vitamin D3 also known as cholecalciferol [10, 11]. Vitamin D3 further, in process, moves to bind to the vitamin D-binding plasma proteins. Vitamin D3 is transported via vitamin D-binding proteins to the liver where it is metabolized into 25(OH) vitamin D (calcifediol) and an inert form of vitamin D. Calcifediol is tightly regulated by parathyroid hormone and converts it into 1,25-dihydroxy vitamin D also known as calcitriol [1]. Calcitriol is the active form of vitamin D, which binds to VDRs in the intestines, bones, and kidney and other extrarenal tissues to enhance the absorption of calcium from the intestines, promotes calcium deposition in bones, and decreases parathyroid hormone concentrations (PTH) [3, 6]. In the process, calcitriol binds to vitamin D receptors, the receptors from the nuclear receptor superfamily that regulates the cellular function in several tissues located in the body including brain neurons [1].
