Oral iron, dietary ligands and zinc absorption

The effect of iron on zinc absorption in humans was investigated by using 65Zn and whole-body counting after 2 wk. Increasing the molar ratio of ferrous iron (with ascorbic acid) to zinc from 1:1 to 2.5:1 did not affect absorption of zinc from water when given in a fasting state; 59 and 58% was absorbed, respectively. However, at an Fe:Zn ratio of 25:1, zinc absorption from water decreased significantly to 34%. When oral iron in the same ratio to zinc was given with a meal, no inhibitory effect was observed (25, 23 and 22%, respectively). Addition of the zinc ligand, histidine, to the water solution decreased the inhibitory effect of the higher dose of iron, resulting in a zinc absorption of 47%. Two weeks of iron preloading did not affect zinc absorption from water. The results demonstrate that when a multimineral supplement is taken on an empty stomach, excessive iron levels can negatively affect zinc absorption. Intake of the supplement with a meal or with a zinc ligand (such as histidine) may overcome this inhibitory effect.