It must be taken on a full stomach to be properly absorbed, since hydrochloric acid is excreted in response to eating, which enhances absorption. In some individuals, calcium carbonate may cause gas or constipation. In such cases, the usual measures such as increased fluid intake and increased activity are recommended. If these measures are unsuccessful, then switching to another calcium formulation is recommended. The only disadvantages are the higher cost and the lower percent of elemental calcium.
Calcium citrate is the salt that is used to fortify breakfast cereals and orange juices.
Calcium phosphate is an well-absorbed supplement that does not cause gas and constipation. It can be taken without regard to food. Calcium phosphate tends to be more expensive than calcium carbonate, but less expensive than calcium citrate. How To Take Calcium Supplements Calcium is best absorbed if consumed throughout the day. However, taking it all at once is better than not taking it at all.