In adults, some studies show that when zinc and iron are co-ingested, zinc can reduce the intestinal absorption of iron or the incorporation of iron into red blood cells, particularly when zinc is ingested in higher amounts.[1][2][3] In women with iron deficiency and in infants, some evidence also shows that supplementation with zinc can lower plasma ferritin (a biomarker for the body’s iron stores that is used to diagnose iron deficiency).[4][5] Consequently, it is sometimes claimed that excess zinc intake can cause iron deficiency.
However, zinc appears to impair iron absorption only when zinc and iron are co-ingested in an aqueous solution.[1][2][3] In contrast, when zinc and iron are mixed with food, there is no inhibitory effect of zinc on intestinal iron absorption or the incorporation of orally ingested iron into red blood cells.[6][1][7][8][9][10] Additionally, the majority of human studies in adults find no effect of orally ingested zinc on plasma ferritin (body iron stores).[1][10] Therefore, the risk of zinc-induced iron deficiency is probably negligible in most people.
It’s important to note that reduced absorption of dietary iron is not the same as iron deficiency (low iron stores as indicated by low plasma ferritin) because the body stores iron to help regulate iron absorption in line with the body’s need for iron. Also, note that iron deficiency is not the same as anemia, a condition characterized by low red blood cell count and low hemoglobin concentrations. Indeed, iron deficiency can cause anemia — specifically iron deficiency anemia — but it is not always sufficient to do so.