Calcium carbonate from crushed oyster shells, or from dolomite (bone meal) is poorly absorbed. Pharmaceutical grade calcium carbonate, and chelated (micro-bound) forms of calcium are better absorbed. Some sources say that the carbonate form must be taken with food, as it requires stomach acid to be absorbed, while the citrate, and other chelate forms can be absorbed on an empty stomach.

I chose a formula that has a blend of calcium that can be absorbed in either situation, since I take 2 doses with meals, and the last dose at bedtime on an empty stomach.

Doreen