For adults, dairy products supply 72% of the calcium in the U.S. diet, grain products about 11% and fruits and vegetables about 6%.¹ Milk drinkers get 80% more calcium in their diet compared to non-milk-drinkers.² Apart from total calcium content, foods and supplements should be evaluated in terms of the bioavailability of the calcium they contained (in other words, how much of it is actually absorbed and utilized by the body.) Calcium absorption from various dairy products is similar, at about 30%.³ However, many people choose alternatives to milk and dairy products for health reasons, such as the prevention of atherosclerosis or food allergies. A variety of calcium-fortified nondairy beverages are now available. However, the bioavailability of calcium in these beverages may differ from that of milk. A study of calcium-fortified soy milk found that the calcium in it was absorbed at only 75% of the efficiency of the calcium in cow’s milk.⁴ While cow’s milk and fortified soy milk are therefore not equivalent as calcium sources, the difference can easily be overcome by either consuming more of the fortified soy beverage, or by consuming soy beverages fortified with proportionally higher amounts of calcium.

Dietary supplements may contain one of several different forms of calcium. One difference between the various calcium compounds is the percentage of elemental calcium present. A greater percentage of elemental calcium means that fewer tablets are needed to achieve the desired calcium intake. For instance, in the calcium carbonate form, calcium accounts for 40% of the compound, while the calcium citrate form provides 24% elemental calcium.

Many medical doctors recommend calcium carbonate because it requires the fewest pills to reach a given level of calcium and it is readily available and inexpensive. For people concerned about cost and only willing to swallow two to three calcium pills per day, calcium carbonate is a sensible choice. Even for these people, however, low-quality calcium carbonate supplements are less than ideal. Depending on how the tablet is manufactured, some calcium carbonate pills have been found to disintegrate and dissolve improperly, which could interfere with absorption.⁵ The disintegration of calcium carbonate pills can be easily evaluated by putting a tablet in a half cup of vinegar and stirring occasionally. After half an hour, no undissolved chunks of tablet should remain at the bottom.⁶

Calcium carbonate may not always show optimal absorption, but it clearly has positive effects. For example, calcium carbonate appears to be as well absorbed as the calcium found in milk.⁷ In fact, some studies indicate that calcium carbonate is absorbed as well as most other forms besides calcium citrate/malate (CCM).^{8 9} For example, a recent study found absorption of calcium from calcium carbonate to be virtually identical to absorption of calcium from calcium citrate.¹⁰

For people willing to take more pills to achieve a given amount of calcium (typically 800 to 1,000 mg), calcium carbonate does not appear to be the optimal choice, because other forms have been reported to absorb better (however, they do require more pills per day because each pill contains less calcium). For this reason, some doctors recommend other forms of calcium, particularly CCM. Research shows that CCM is absorbed better than most other forms.^{11 12 13} CCM may also be more effective in maintaining bone mass, than some other forms of calcium supplements.¹⁴ Because of their similarity in both name and structure, CCM can be confused with calcium citrate, but they are not the same.

CCM is not the only form of calcium that might be absorbed better than carbonate. For example, most,^{15 16} though not all,¹⁷ studies suggest that calcium citrate might have some absorption advantage over calcium carbonate. However, no evidence suggests that calcium citrate is as well absorbed as CCM.

Microcrystalline hydroxyapatite (MCHC), a variation on bonemeal, has attracted attention because of studies reporting increases in bone mass in people with certain conditions¹⁸ and better effects on bone than calcium carbonate.¹⁹ Similar positive studies exist using CCM.²⁰ However, unlike CCM, MCHC has only occasionally been compared with other forms of calcium. In limited research that does make comparisons, MCHC fared poorly in terms of solubility, absorption, and effect on calcium metabolism.^{21 22}

Remarkably little is known about the relative efficacy of amino acid chelates (pronounced “kee-lates”) of calcium. In the only commonly cited trial, absorption was measured for an amino acid chelate called calcium bisglycinate and compared with absorption from citrate, carbonate, and MCHC.²³ In that trial, the amino acid chelate showed the best absorption and MCHC the worst. Although CCM was studied in that trial, it was taken under different circumstances than the chelate (with meals), so drawing definitive conclusions is not possible.

Recently, coral calcium has been claimed to be a vastly superior form of calcium, even though its calcium content is primarily calcium carbonate. One small, controlled human study reported that coral calcium was better absorbed than ordinary calcium carbonate.²⁴ However, the method used in this study to measure calcium absorption has been criticized as much less sensitive than other methods ²⁵ . No research has compared coral calcium to calcium citrate or to CCM. There is little evidence at this time that coral calcium is superior to other forms of calcium.

Whatever the form, calcium supplements typically are absorbed better when eaten with meals.²⁶ Moreover, research indicates that taking calcium with meals may reduce the risk of kidney stones and supplementing with calcium between meals might actually increase the risk.²⁷

Besides how to take calcium supplements, scientists have also been studying when to take them. Supplementing calcium in the evening appears better for osteoporosis prevention than taking calcium in the morning, based on the circadian rhythm of bone loss.²⁸ In order to not increase the risk of forming kidney stones, most doctors tell people to take calcium supplements only with food.

What is the relationship between calcium supplements and stomach acid? Years ago, researchers reported that people who do not make hydrochloric acid in their stomachs cannot absorb calcium adequately when the calcium is taken alone.²⁹ In that report, adding hydrochloric acid restored normal calcium absorption. Although researchers have subsequently confirmed these findings, they have also discovered that these same people absorb calcium normally if they take it with meals. In addition, researchers have noted that giving these people hydrochloric acid does not further improve absorption during meals.³⁰ Others have confirmed that hydrochloric acid, either from pills or from the stomach, is unnecessary for the absorption of calcium, as long as the calcium supplement is taken with meals.^{31 32 33 34}

Some doctors have expressed a concern that antacids that contain calcium (like Tums®) or calcium supplements that also act as antacids, interfere with the body’s absorption of calcium. However, this is not the case. Calcium carbonate, the principal ingredient in both Tums and many calcium supplements provides significant (though not optimal) absorbable calcium, as discussed above. Other forms of calcium that might be more bio-available, such as calcium citrate, also act as antacids. The form of calcium associated most consistently with best bio-availability, CCM, is itself, an antacid despite the fact it is used almost exclusively as a source of calcium.

Other concerns about the antacid effect of most calcium supplements (particularly when taken by people who do not need and are not seeking an antacid) are voiced by some doctors because stomach acid is needed to protect against bacterial infection and also to help digest protein. In theory, calcium supplements with antacid activity could at least temporarily interfere with these processes. However, to date, these concerns remain hypothetical.