Calcium in supplements (and our diet) is always bonded with another substance, such as carbon (calcium carbonate) or citric acid (calcium citrate). Most labels will list the amount of calcium alone (without the other carrier substance) to allow consumers to compare products based on "elemental" calcium. But this is somewhat misleading, as different forms of calcium may have different effects on the body. For example, calcium carbonate may be poorly absorbed (especially by older people) becasue it interferes with the production of the very digestive juices required to prepare calcium for absorption. In contrast, calcium found in MCHC or calcium citrate may be better absorbed. So 500 mg of "elemental" calcium from calcium carbonate may not provide the same health benefits as 500 mg of elemental calcium from MCHC or calcium citrate.