@andrewt fortunately it doesn't work this way: iron atoms are NOT ferromagnetic, this happens only in the bigger christalline structure. Otherwise MRI wouldn't be possible for humans, because hemoglobin is ~3% iron and your blood would pull at you very strongly...
@andrewt they are cheating: metallic iron is cheaper to add and they can write x% iron content on the package, but your body can't absorb it as well as molecular iron