Purpose
Most iron in the body is used to transport oxygen.
Oxygen is carried in red blood cells through the circu-
latory system to all cells in the body. Hemoglobin is
theprotein within red blood cells that makes this
possible, and iron is at the center of the hemoglobin
molecule. An average-size adult man has about 4
grams of iron in his body, and an adult woman has
about 3.5 grams. Approximately two-thirds of this
iron is in hemoglobin. Myoglobin, a protein in muscle,
also contains iron. Myoglobin provides short-term
storage for oxygen. When muscles do work, this oxy-
gen is released to meet the increased metabolic needs
of muscle cells.
Iron is found in every cell in the body, including
brain cells. It is needed to synthesize adenosine tri-
phosphate (ATP), the compound that supplies most
of the energy to drive cellularmetabolism. Iron is also
used in enzyme reactions that create new DNA, and in
this way it affects cell division and differentiation. Iron
is also essential to other enzyme reactions that break
down potentially harmful molecules formed when
immune system cells attack bacteria.Description
Plants absorb iron from the earth, and humans
acquire iron through eating both plants and animals.
In the stomach, acid in gastric juice acts on iron and
changes it into a form that the body can absorb.
Absorption takes place mainly in the first part of the
small intestine (the duodenum). Once iron is absorbed
into the bloodstream, it binds to a protein called trans-
ferrin and is carried to all parts of the body, including
the bone marrow where new red blood cells are made.
Once in the cells, some iron is transferred to ferritin, a
protein that holds the iron in reserve. When too much
iron is absorbed, there is not enough transferrin to
bind all of it. Free iron can build up in cells and trigger
activities that cause damage and create health prob-
lems. Too little iron interferes with the body’s ability
to get enough oxygen.Sources of iron
The body has complex mechanisms to achieve
iron balance by regulating iron absorption, reuse,
and storage processes. Red blood cells live about 120
days. When they die, most of the iron in hemoglobin is
recycled in the liver and sent to the bone marrow
where it reused in new red blood cells. As a result,
humans lose only a small amount of iron daily.
Only about 10–20% of the iron in food, or 1–2 mg
for every 10 mg eaten, is absorbed into the blood-
stream. Under normal conditions, when iron stores
in the body are low, more iron is automatically
absorbed. When they are high, less is absorbed. Iron
that is not absorbed enters cells that line the intestine.
As these cells fill up with iron, they fall into the intes-
tine and leave the body in waste.
Both plant and animal foods provide humans with
iron, but that iron comes in two forms, heme and
nonheme, that are not equally available to the body.
Heme iron comes from hemoglobin. It is found mainly
in animal tissue. Red meat is an especially rich source
of heme iron. Only trace amounts of heme iron are
found in plants. Heme iron is in a form that is easier
for humans to use. It is absorbed at a higher rate thanIronRecommended Tolerable upper
dietary allowance intake level
Age (mg) (mg)
Children 0–6 mos. 0.27 Not established
Children 7–12 mos. 11 Not established
Children 1–3 yrs. 7 40
Children 4–8 yrs. 10 40
Children 9–13 yrs. 9 40
Boys 14–18 yrs. 11 45
Girls 14–18 yrs. 15 45
Men 19–50 yrs. 8 45
Women 19–50 yrs. 18 45
Adults 51 yrs. 8 45
Pregnant women 27 45
Breastfeeding women 18 yrs. 10 45
Breastfeeding women 19 yrs. 9 45
Food Heme Iron (mg)
Chicken liver, cooked, 3 oz. 12.8
Oysters, 6 med. 5.04
Beef, cooked, 3 oz. 3.2
Turkey, light meat, cooked, 3 oz. 2.3
Shrimp, cooked, 8 large 1.36
Tuna, light, canned, 3 oz. 1.3
Chicken, dark meat, cooked, 3 oz. 1.13
Halibut, cooked, 3 oz. 0.9
Crab, cooked, 3 oz. 0.8
Pork loin, cooked, 3 oz. 0.8
Food Nonheme Iron (mg)
Cereal, 100% iron fortified, 1 cup 18
Soybeans, boiled, 1 cup 8.8
Tofu, firm, ½ cup 6.22
Beans, kidney, cooked, 1 cup 5.2
Beans, lima, cooked, 1 cup 4.5
Beans, pinto, cooked, 1 cup 3.6
Blackstrap molasses, 1 tbsp. 3.5
Potato, med. with skin 2.75
Cashew nuts, 1 oz. 1.70
Bread, whole wheat, 1 slice 0.9
Raisins, small box, 1.5 oz. 0.89
mg milligram(Illustration by GGS Information Services/Thomson Gale.)
Iron