Calcium carbonate depresses iron bioavailability in rats more than calcium sulfate or sodium carbonate

Calcium carbonate supplements depress iron bioavailability when consumed with meals. Our objective was to determine whether this effect is due to the calcium, the carbonate or a combination of the two. A rat hemoglobin repletion assay and an in vitro digestion procedure were used to assess the effects of four salts (calcium carbonate, calcium sulfate, sodium carbonate and sodium sulfate) on iron bioavailability. The salts were added to purified rat diets (for the hemoglobin repletion study) or to iron-fortified infant formula (for the in vitro study) at three different levels. Calcium carbonate had the greatest depressive effect on iron bioavailability, depressing hemoglobin iron gain in a dose-related manner. Calcium sulfate and sodium carbonate also depressed hemoglobin iron gain but to a lesser extent and only in rats fed diets containing the highest level of these two salts. Sodium sulfate did not affect hemoglobin iron gain. Significant interactions between cation, anion and salt concentration were found, suggesting that both the cation and the anion in calcium carbonate contribute to the iron absorption-depressing action of this salt. Results from the in vitro experiments were similar to the in vivo results.