Calcium Carbonate
Calcium carbonate is converted to calcium chloride by gastric acid. Some of the calcium is absorbed from the intestines but about 85% is reconverted to insoluble calcium salts, such as the carbonate and is excreted in the faeces.
Magnesium Carbonate
Magnesium carbonate reacts with gastric acid to form soluble magnesium chloride and carbon dioxide in the stomach. Some magnesium is absorbed but is usually excreted rapidly in the urine.
Sodium Bicarbonate
Administration of sodium bicarbonate by mouth causes neutralisation of gastric acid with the production of carbon dioxide. Bicarbonate not involved in that reaction is absorbed and in the absence of a deficit of bicarbonate in the plasma, bicarbonate ions are excreted in the urine that is rendered alkaline with an accompanying diuresis.