Iron, zinc and copper interactions: chronic versus acute responses of rats

Several methodologies were evaluated to study iron, zinc and copper interactions. In three studies, rats were fed diets containing adequate levels of iron (33-35 micrograms Fe/g diet) and zinc (15-25 micrograms Zn/g diet) and diets with excessive levels of zinc (2441-2470 micrograms Zn/g diet) or iron (1408-3042 micrograms Fe/g diet). Consumption of excess iron for several weeks or in one test meal did not alter tissue levels of zinc or apparent absorption of 65Zn. In contrast rats fed excess zinc throughout the three studies had depressed hematocrits and serum copper levels, apparently absorbed 59Fe less efficiently and retained less 59Fe in livers and tibias than control animals. Nutritional status with regard to iron or zinc, not just the intakes of iron and zinc, also influenced the interactions between zinc and iron. Rats fed excess iron for several weeks retained less 65Zn from a test meal in their tibias than rats fed excessive iron in only the test meal. Although chronic ingestion of excess zinc depressed apparent absorption of 59Fe from a test meal, the ingestion of one test meal with excess zinc did not. Investigators studying nutrient interactions should not rely solely on studies in which unadjusted humans or animals are given a single dose of test substances because acute responses do not reflect all the changes induced by chronic feeding.