An Element of Excess: How Iron Fortification May Be a Double-Edged Sword
Iron is a most versatile element. It is essential to many of the enzymes that are the engines for life, and in mammals is also used to carry oxygen on hemoglobin in blood. Remember Popeye and his spinach: all that iron made him strong.
But the very quality that makes iron so useful also makes it dangerous. Iron can easily lose or gain one electron going from the ferrous (Fe++) to the ferric (Fe+++) state, back and forth indefinitely. This is how it carries oxygen, for example.
It also means it can be a potent pro-oxidant – it catalyzes the production of free radicals which can destroy cells and tissue, and thereby contribute to cancer and heart disease.
Life forms like us have developed extensive defense mechanisms that allow us to use iron for life's work while keeping it away from anywhere it is not immediately needed within cells and in the body in general.