concentration of iron already present. Three
mechanisms affect how this is done: 1) the
chemistry of the different foods we eat, 2) the
ways these foods interact with iron (which are
unique to each individual), and 3) the amount
of iron already stored in the body.
Iron in our bodies
Iron exists in oxidation states
from – 2 to + 6, but it most often
combines with other elements in
a + 2 (iron(II), sometimes called
“ferrous”) or + 3 (iron(III), sometimes called “ferric”) form. An oxidation state refers to the hypothetical charge
that an atom would have if all bonds to
atoms of different elements were completely
ionic. Iron can combine with oxygen to form
compounds, such as iron(II) oxide (FeO) or
iron(III) oxide (Fe2O3), along with several
other combinations.
In the body, iron is stored in the iron(III)
oxidation state. To release the mineral when
the body needs it, most iron must be changed
from the (III) to the (II) oxidation state—the
form of iron most often required by humans.
The iron people eat exists
in two different forms, called
“heme” and “non-heme.” Heme iron
comes almost totally from animal sources,
while non-heme iron comes from plants,
animals, and occasionally from dirt (which
you sometimes find in spinach, for example).
The iron in steak (heme iron) and the iron
in spinach (mainly non-heme iron) behave
differently in your digestive system in ways
that are not yet fully understood. The word
“heme” actually refers to an iron-containing
compound present in hemoglobin, a protein in
the blood that carries oxygen (see “How Iron
Is Carried by the Bloodstream throughout the
Body,” below).
Heme iron is in an iron(II) state, while
non-heme iron comes in both iron(II) and
iron(III) forms. Heme iron is attached to protein molecules, is soluble, and is absorbed
by the body more efficiently than non-heme
iron. Heme iron is typically absorbed by the
human digestive tract—what scientists
call bioavailability—at a rate of 7% to 35%.
This simply means that heme
iron is already in a form the body
can use. Non-heme iron, often
insoluble, is absorbed at a lower
rate of 2% to 20%.
So it is up to you to decide which one to
choose. Meat, fish, poultry, and seafood all
contain both types of iron. Fruits and veg-
etables, along with nuts and seeds, contain
almost entirely non-heme iron. Mineral
sources, such as most iron pills, food addi-
tives, and even contaminants from cast-iron
skillets, contain non-heme iron. But because
of the bioavailability difference, vegans and
vegetarians need to ingest about twice as FIGUR
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How Iron Is Carried by the
Bloodstream throughout the Body
Aprotein in the body called transferrin carries ions of iron to the bone marrow and to other body cells. In the bone marrow, different cells
attach each ion to huge molecules called hemoglobin.
Hemoglobin is the main component of red blood cells,
and it gives them their rich, red color.
Hemoglobin carries oxygen and, with other related
proteins, it regulates muscle and brain activity, as well
as breathing and the immune system. It is a protein that
consists of four polypeptide chains—long strings of amino
acids, each of which carries a heme unit. The heme units,
(Fig. 1), which make up about 4% of each hemoglobin molecule, are complex organic compounds containing iron(II)
ions in a large nitrogen-containing ring system.
Hemoglobin works this way: Humans breathe in oxygen
(O2), which goes directly into the lungs. Hemoglobin molecules in red blood cells pick up the oxygen and carry it to all
types of cells in tissues throughout the body. The red blood
cells are produced in bone marrow, using the iron-rich foods
humans eat as raw materials.
Figure 1. Chemical structure
of hemoglobin, a protein in
the blood that carries oxygen.
A hemoglobin molecule,
mostly protein, contains four
separate structures called
hemes, each containing one
ferrous ion and each binding
to one molecule of oxygen.
