Nobody knows yet whether zinc and iron get in each other's way
The panel behind the US iron upper limit found no evidence of clinically significant harm from the pairing. Pooled trials in under-threes saw zinc move iron markers the wrong way in a subgroup.
What the study found
Whether zinc and iron get in each other's way is still open
The US iron upper-limit panel found no evidence of a clinically significant adverse effect from iron and zinc interacting. In pooled trials in under-threes, zinc pushed ferritin, haemoglobin and anaemia odds the wrong way in at least one subgroup. Whether spacing them helps is unknown.
The caveat · Neither designed to test the pairing
Neither source was designed to test the pairing: one is a set of marker outcomes graded very low to moderate certainty, the other an absence noted while setting an intake limit. The child trials used substantial doses of both minerals in populations where deficiency is common, not the amounts in an ordinary multivitamin. And the panel's finding was recorded in 2001 and has not been revisited.
What we don't know
A US panel found no evidence of clinically significant harm from iron and zinc interacting; pooled trials in under-threes saw zinc move iron markers the wrong way. Neither set out to test the pairing, so whether to space them is unknown.