Vitamin D3 is also called a sunshine vitamin, and it is a compound with a steroid structure traditionally classified to vitamins, but it plays the role of a prohormone the human body.
It is synthesized from cholesterol in the skin under the influence of sunlight, UV-B. Then, it gets transformed into calcitriol (the active form of vitamin D, a hormone controlling the calcium-phosphate economy in the organism), as a result of metabolic reactions in the liver and kidneys.
In many tissues and cells it connects to its nuclear receptor – VDR, while controlling more than 200 genes. It interacts with vitamin A, vitamin K2 and calcium.