Calcium naturally occurs bound to other substances—such as carbonate, citrate, or gluconate—never alone. Each form contains a different percentage of “elemental” calcium. Calcium carbonate contains the most elemental calcium by weight—40 percent. The more calcium, the fewer pills you need. Some studies have found that calcium citrate is best absorbed. Calcium carbonate is cheapest, and one pill typically supplies 500 or 600 milligrams of elemental calcium. Other forms provide much less calcium per pill.
There’s little added benefit from taking calcium in doses greater than 500 milligrams at a time, since the body absorbs larger amounts less efficiently. It’s best to spread your calcium intake throughout the day—which is another reason why food sources are preferable. Take calcium carbonate with meals, since stomach acid secreted during digestion enhances its absorption. Calcium citrate is well absorbed without stomach acid, so you can take it anytime.