Found naturally only in the form of its compounds, never uncombined. In its pure state, it ignites in air, and reacts with water, alcohols, and dilute acids with evolution of hydrogen. Principal commercial source is limestone. Calcium is the most abundant mineral and the fifth most abundant substance in the body after carbon, hydrogen, oxygen, and nitrogen. Calcium salts are consumed in the diet and through supplementation.
Some forms are more absorbable that others.
People who exercise infrequently or who are bedridden absorb less and excrete more calcium and are more prone to bone disorders than are active people. Increased intake of the mineral often alleviates or improves the condition. The first bones to be affected are the backbone and jaw. Pregnant women who consume little calcium or excess phosphorus are a greater risk for developing tetany. Infants who are fed undiluted cow's milk also might develop tetany.