Calcium in nature is always combined with another component; for example, carbonate, phosphate, lactate, or citrate. The calcium found in orange juice and other fortified juices is a mixed form called calcium citrate malate (CCM) and it was patented in the late 1980s. Supplements containing calcium carbonate are generally derived from oyster shells— plentiful and therefore inexpensive. CMM, being chemically synthesized, is more expensive than other calcium salts, but quite soluble and thus well suited for use in foods and beverages.
Researchers at Purdue University reviewed the evidence on the bioavailability and health benefits of CCM and found that CMM has benefits for people of all ages. CMM has been shown to help the body retain calcium, promote bone building in children and adolescents, and promote the maintenance of bone mass in adults. In the elderly, along with vitamin D, CMM decreases fracture risk and slows the rate of bone loss.
CMM is highly absorbable, making it a better source than calcium carbonate for individuals who have low stomach acid– particularly the elderly who produce less stomach acid or those who are on medications that reduce stomach acid. Researchers have also found that CCM does not appear to increase the risk of kidney stones, and may actually protect against them— it is the citrate component in CMM that does the protecting in the urine by making the calcium soluble.
Bottom line: CMM is well absorbed and a reasonable food supplement.