Calcium carbonate is entirely insoluble. Calcium citrate is only slightly soluble in water.
Calcium carbonate will react with stomach acid to form calcium chloride (along with CO2 and water). When given intravenously, calcium chloride is perfectly bioavailable. However, within the small intestines, it has very poor uptake, both through the intestines' active and passive mechanisms. Any remaining calcium carbonate has no uptake at all.
Calcium carbonate will react with stomach acid to form calcium chloride (along with CO2 and water). When given intravenously, calcium chloride is perfectly bioavailable. However, within the small intestines, it has very poor uptake, both through the intestines' active and passive mechanisms. Any remaining calcium carbonate has no uptake at all.