Iron absorption can be reduced by other vitamins and minerals found both in supplements and foods.
Multiple studies have found that calcium is a strong inhibitor of iron absorption, and avoiding the simultaneous use of calcium and iron supplements may be beneficial. In one study, calcium phosphate inhibited iron absorption from a ferrous sulfate supplement, whether taken with or without food. In contrast, calcium carbonate only inhibited iron absorption when both supplements were consumed with food, suggesting that if a combined iron and calcium carbonate supplement is required, it should be taken between meals.[1] Calcium citrate, on the other hand, reduced iron absorption both with and without food, but not to a statistically significant degree in one study[1], and reduced absorption of non-heme iron without food in another study.[2]. The impact of calcium chloride on iron absorption seems to vary depending on whether it’s taken with food[3] or without food.[4]
Furthermore, a wide variety of beverages with a high antioxidant content, including coffee and tea, have some acute inhibitory effects on non-heme iron absorption. Coffee may reduce iron absorption,[5] possibly due to the presence of chlorogenic acid, a known iron chelator.[6] This mechanism can be extended to green coffee extract, which is an even richer source of chlorogenic acid. Tea, whether green[7] or black,[5] might inhibit iron absorption, possibly due to the presence of catechins[7] and theaflavins.[8] Infusions of chamomile, lime flower, pennyroyal, peppermint, and vervain may also reduce non-heme iron absorption.[5]
Rosemary (a source of rosmarinic acid), rich in phytic and phenolic acid, has also been shown to reduce non-heme iron absorption.[7]
Additionally, in one randomized controlled trial, people with beta-thalassemia who took 500 mg of quercetin (a chelator of iron[9]) per day for 12 weeks alleviated iron overload (a common complication of beta-thalassemia), notably reducing serum iron, ferritin, and inflammatory markers.[10]
Lastly, although zinc has the potential to reduce the absorption of iron, the effect is inconsistent, and the strength of this effect is unclear, with different studies reporting varying results.[11][12][13][14][15] The interaction between zinc and iron seems to be linked to competition for transporters in the liver, such as divalent metal transporter 1 (DMT1), human copper transporter 1 (hCTR1),[16] and ZRT/IRT-like protein 14 (Zip14).[17] Taking zinc between meals is likely a good way to prevent any interference with iron absorption.
It’s important to note that the typical Western diet is complex and rich in both iron absorption enhancers and inhibitors, whose effects may both be attenuated when these substances are consumed simultaneously.[18]