Inhibition of iron absorption by calcium in rats and dogs: effects of mineral separation by time and enteric coating
1990
It is known that in rats and humans, calcium markedly inhibits the absorption of iron when both minerals are present in the small intestine. The interaction between iron and calcium was studied in two species, rats and dogs. Female rats were orally dosed with 6 mg calcium as calcium citrate-malate followed by 70 ug iron as ferrous sulfate labeled with [59]Fe. Doses were separated by 0 to 180 min. Iron absorption was measured by using the whole-body isotope retention technique. We determined that iron and calcium must be separated by at least one hour to prevent a negative effect of calcium on iron absorption. This information was used to design tablets containing iron (radiolabeled with [59]Fe) and calcium. Iron tablets were made by enteric coating iron with or without ascorbic acid (an enhancer of non-heme iron absorption). The enteric coat delayed the release of iron into the small intestine by preventing iron dissolution in the stomach for at least one hour. Iron absorption from these tablets and uncoated tablets was measured in dogs using an assay of [59]Fe incorporation into hemoglobin. Calcium inhibited the absorption of uncoated iron-ascorbic acid by about 50%. However, when the enteric-coated iron-ascorbic acid tablets were dosed with calcium tablets, iron absorption was not inhibited. We conclude that calcium inhibits iron absorption in rats and dogs and that this inhibition can be overcome when the two minerals are separated by at least one hour, which may be achieved by enteric-coating the iron source.
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