Helen:  There is reason to believe that the iron in the formula may reduce
the bioavailability of the iron in the mother's milk, as iron-fortified
solids have been shown to do (don't have the reference handy).  There is
also the theoretical possibility of the excess free iron allowing the
growth of pathogenic bacteria in baby's gut. However, these issues
actually have not been studied in babies receiving mixed feeds, to my
knowledge.  Hope this helps, Alicia Dermer, MD, IBCLC.