Hi,
     I know we have talked about this before but here we go again.
     When breastmilk and formula are mixed the lactoferritin from the
breastmilk ( the protein that carries iron into the babies blood stream) gets
supersaturated from the iron in the formula.  So effectively neither the iron
from the breastmilk nor the iron from the formula get absorbed because the
lactoferritin cannot work when it is overloaded.  This happens with the other
nutritive properties of breastmilk also that they are not as well assimilated
when a baby is supplemented with formula or solids.  This is one of the
rationale for exclusively breastfeeding for the first 6 mo without
supplements.  Of course a baby who is failure to thrive or having feeding
problems is a different situation, you have to choose the lesser of two evils.
There is also milk from human milk banks that insurance will pay for if the
prescription is written by the physician.
     I know this is a touchy subject and last time I wrote this information
people jumped on me like a duck on a june bug and stated that iron was not
everything.  Well it is to some healthcare providers.