What is Calcium, Magnesium & D3? Calcium is the most abundant, essential mineral in the human body. Of the two to three pounds of calcium contained in the average body, 99% is located in the bones & teeth. Magnesium is also an essential mineral meaning that we cannot manufacturer it within the body yet we cannot survive without it. What Does Vitamin D3 Do? Vitamin D’s most important role is maintaining blood levels of calcium, which it accomplishes by increasing absorption of calcium from food & reducing urinary calcium loss. Both effects keep calcium in the body & therefore leave the calcium that is stored in bones. Vitamin D may help to maintain healthy bones & teeth. May help to maintain a healthy immune system.