Calcium carbonate is the least expensive and requires the fewest tablets. However, it may cause constipation and bloating and must be taken with meals for adequate absorption. Calcium citrate is often more expensive than calcium carbonate and requires more tablets to get the desired dose.

While all three forms—calcium citrate, hydroxyapatite, and carbonate—contain the same amount of elemental calcium (1 gram), there are differences in how much your body absorbs calcium when you take each form.

♦ Calcium hydroxyapatite and calcium citrate are both forms of calcium supplements, but they have some key differences. Calcium hydroxyapatite is derived from natural sources. Calcium citrate, conversely, is a synthetic form of calcium derived from citric acid.

Some well-known calcium carbonate products include Caltrate, Viactiv Calcium Chews, Os-Cal, and Tums. Calcium citrate supplements are absorbed more easily than calcium carbonate. They can be taken on an empty stomach and are more readily absorbed by people who take acid-reducing heartburn medications.

More than 99% of calcium in the body is in the form of calcium hydroxyapatite, an inorganic matrix of calcium and phosphate that is stored in the bones and teeth [1,4,5]. Unlike teeth, ... Calcium carbonate appears to cause more of these side effects than calcium citrate, especially in older adults who have lower levels of stomach acid .