Calcium carbonate is the most commonly used form of calcium in supplements. Contrary to popular belief, most research has indicated it is at least as well absorbed as calcium from milk. Although some research has shown that calcium citrate and calcium citrate malate are better absorbed than calcium carbonate, other research has found that calcium carbonate is absorbed at least as well as calcium citrate, calcium citrate malate, calcium lactate, calcium gluconate, and oyster shell calcium, and that it is significantly more bioavailable than calcium phosphate. At this time, there is no conclusive evidence indicating that one form is preferable to another. However, people with hypochlorhydria have difficulty absorbing calcium carbonate, unless it’s taken with food.5
Another important issue when selecting a calcium supplement is tablet dissolution. Some tablets (not capsules or liquid preparations) do not dissolve well in the stomach, resulting in poor absorbability. In addition, some bone meal, dolomite, and oyster shell preparations have been found to contain significant amounts of lead or aluminum.6