**1. Introduction**

Vitamin D, "sunshine" vitamin, is a vital topic that has attracted great attention of many researchers and the public over the past decades, because a large proportion of the world's population is deficient in this nutritious element [1]. Vitamin D was first discovered at the beginning of the twentieth century in children with rickets [2].

Vitamin D is a prohormone steroid and belongs to the fat-soluble vitamins. It is responsible for endocrine, paracrine, and autocrine functions [1]. Vitamin D is also essential for calcium absorption, bone mineralization, calcium and phosphorus homeostasis, hormonal release, nerve conduction, and neuromuscular function [3, 4].
