Calcium is stored in the bones. When there is not enough calcium in the blood to be used by the heart and other organs, your body will take the calcium from the bones to meet its day-to-day needs. This small amount of calcium keeps blood and cellular calcium levels within the proper range. Every cell in the body needs calcium, which helps maintain proper muscle contraction and relaxation, nerve transmission, blood clotting, and heart health. Magnesium is a nutrient needed for more than 300 biochemical reactions in the body and is fundamental to a number of important physical processes, including converting carbohydrates and protein into ATP (the body's energy source), blood clotting, activating B vitamins and relaxing muscles. Magnesium boosts the activity of several enzyme systems needed for neurochemical communication, and is needed to transport electrolytes like potassium and sodium in and out of cells for optimal cell health and functioning. Furthermore, magnesium assists in calcium and potassium absorption. Vitamin D, sometimes called "the sunshine vitamin," is a crucial nutrient that is normally obtained through the diet or produced by the skin after exposure to ultraviolet rays from the sun. Vitamin D promotes the body's absorption of calcium, which is essential for the normal development and maintenance of healthy teeth and bones. It promotes bone mineralization in concert with a number of other vitamins, minerals, and hormones.