We’ve been interested in the role vitamin D plays in our overall health for a while—particularly as it emerges that so many of us are quite deficient, particularly in the age of sitting inside all day and sunscreen. “Vitamin D” is sort of a misnomer because it doesn’t really behave like a vitamin; rather it functions more like a hormone. The form you typically consume (in food or supplements, or indirectly via sunshine) is vitamin D3, but your body converts this into a steroid hormone, called calcitrol. Once vitamin D is turned into this active form, it travels throughout the body and plays a part in a number of diverse (and vital) functions: It builds bones and muscles; it also has anti-inflammatory effects, and helps to make enzymes and proteins that prevent diseases; it affects aging. High levels of vitamin D have been linked to stronger immune systems,
Source: Vitamins & Supplements – Goop
