Iron Information
Iron: What is it?
What foods provide iron?
What affects iron absorption?
What is the Recommended Dietary
Allowance for iron for infants, children, and adults?
When can iron deficiency occur?
Who may need extra iron to prevent
a deficiency?
Some facts about iron supplements
Who should be cautious about
taking iron supplements?
What are some current issues
and controversies about iron?
Iron and Heart Disease
Iron and Cancer
Iron and Intense Exercise
Iron fortification and absorption
of other nutrients
What is the health risk of too
much iron?
Table of selected food sources
of heme iron
Table of selected food sources
of nonheme iron
Iron:
What is it?
Iron is an essential mineral and an important
component of proteins involved in oxygen transport and metabolism (1,2). Almost
two-thirds of the iron in your body is found in hemoglobin, the protein in red
blood cells that carries oxygen to your body’s tissues. Smaller amounts
of iron are found in myoglobin, a protein that helps supply oxygen to muscle,
and in enzymes that assist biochemical reactions in cells. About 15 percent
of your body’s iron is stored for future needs and mobilized when dietary
intake is inadequate. The remainder is in your body’s tissues as part of
proteins that help your body function. Adult men and post-menopausal women lose
very little iron except through bleeding. Women with heavy monthly periods can
lose a significant amount of iron. Your body usually maintains normal iron status
by controlling the amount of iron absorbed from food (1,3).
What
foods provide iron?
There are two forms of dietary iron: heme and
nonheme. Iron in meat, fish, and poultry is found in a chemical structure known
as heme. Heme iron is absorbed very efficiently by your body (1,3-4). Iron in
plants such as lentils and beans is arranged in a different chemical structure
called nonheme iron. Nonheme iron is not as well absorbed as heme iron (1,3-4).
Flours, cereals, and grain products that are enriched or fortified with iron
are good dietary sources of nonheme iron (5). The addition of iron to infant
formulas, cereals, and grain products has been credited with improving the iron
status of millions of infants, children, and women. The tables of selected food
sources of heme and nonheme iron suggest many dietary sources of iron.
What affects iron
absorption?
Iron absorption refers to the amount of dietary
iron that your body obtains from food. Healthy adults absorb about 15% of the
iron in their diet, but your actual absorption is influenced by your body’s
iron stores, the type of iron in the diet, and by other dietary factors that
either help or hinder iron absorption (1,3,6-9).
The greatest influence on iron absorption is the amount
stored in your body. Iron absorption significantly increases when body stores
are low. When iron stores are high, absorption decreases to help protect against
iron overload (1,3).
Absorption of heme iron is very efficient and not significantly
affected by the composition of your diet (1). Only 1% to 7% of the nonheme iron
in vegetable staples such as rice, maize, black beans, soybeans and wheat is
absorbed when consumed as a single food (3). However, dietary factors can significantly
improve nonheme iron absorption (1,3, 6-9). Meat proteins and vitamin C will
improve the absorption of nonheme iron (1,10). Diets that include a minimum
of 5 servings of fruits and vegetables daily, as recommended by the Food Guide
Pyramid, should provide plenty of vitamin C to boost nonheme iron absorption.
Calcium, polyphenols and tannins found in tea, and phytates, which are a component
of plant foods such as legumes, rice and grains, can decrease the absorption
of nonheme iron (1,11-15). Some proteins found in soybeans also inhibit nonheme
iron absorption (1,16). Most healthy individuals can maintain normal iron sores
when the diet provides a wide variety of foods as suggested by the Food Guide
Pyramid. It is most important to include foods that enhance nonheme iron absorption
when total daily iron intake does not meet the RDA, when iron losses are exceptionally
high, or when no heme iron is usually consumed.
What
is the Recommended Dietary Allowance for Iron
The Recommended Dietary Allowance (RDA) is the daily
dietary intake level that is sufficient to meet the nutrient requirements of
nearly all (97-98%) healthy individuals in each life-stage and gender group
(1). The 2001 RDAs for iron (in milligrams) for infants ages 7 to 12 months,
children and adults (1) are:
| Age |
Infants,
Children |
Males |
Females |
Pregnancy |
Lactation |
| 7
to 12 months |
11 mg |
|
|
|
|
| 1 to 3
years |
7 mg |
|
|
|
|
| 4 to 8 years |
1o mg |
|
|
|
|
| 9 to 13 years |
|
8 mg |
8 mg |
|
|
| 14-18 years |
|
11 mg |
15 mg |
27 mg |
10 mg |
| 19-50 years |
|
8 mg |
18 mg |
27 mg |
9 mg |
| 51+ years |
|
8 mg |
8 mg |
|
|
Normal full term infants are born with a supply
of iron that lasts for 4 to 6 months. Evidence is not available to establish
a RDA for iron for infants from birth through 6 months of age. Recommended iron
intake for infants from 0 to 6 months is based on an Adequate Intake (AI) of
0.27 milligrams (mg) per day that reflects the average iron intake of breastfed
infants (1). Iron in human milk (breast milk) is well absorbed by infants. It
is estimated that infants can use greater than 50% of the iron in breast milk
as compared to typically less than 12% of the iron in infant formula (1). Cow
milk is not only low in iron and poorly absorbed by infants, its use in infancy
can cause gastrointestinal bleeding and iron loss from the body. For these reasons,
cow milk should not be fed to infants until after age 1 (1). The American Academy
of Pediatrics recommends that infants who are not breastfed or who are partially
breastfed should receive an iron-fortified formula from birth to 12 months (1,
17). Formulas that contain between 4.0 to 12 milligrams of iron per liter of
formula are considered iron-fortified (17)
Results of two national surveys, the National Health
and Nutrition Examination Survey (NHANES III-1988-91) and the Continuing Survey
of Food Intakes by Individuals (1994-96 CSFII) indicate that diets of most adult
men and post-menopausal women provide recommended amounts of iron (18-19). Diets
of females of childbearing age, pregnant women, and women who breast-feed generally
do not provide recommended amounts of iron.
When can iron deficiency
occur?
The World Health Organization considers iron
deficiency the number one nutritional disorder in the world (20). It affects
more than 30% of the world’s population (21-22).
When your need for iron increases or a loss of iron from
bleeding exceeds your dietary iron intake, a negative iron balance may occur.
Initially this results in iron depletion, in which the storage form of iron
is decreased while blood hemoglobin level remains normal. Iron deficiency occurs
when blood and storage levels of iron are low, and the blood hemoglobin level
falls below normal (1).
Iron deficiency anemia may result from a low dietary intake,
inadequate intestinal absorption, excessive blood loss, and/or increased needs
(23). Women of childbearing age, pregnant women, older infants and toddlers,
and teenage girls are at greatest risk of developing iron deficiency anemia
because they have the greatest needs (20).
Individuals with renal failure, especially those receiving
dialysis, are at high risk for developing iron deficiency anemia. This is because
their kidneys cannot create enough erythropoietin, a hormone needed to make
red blood cells. Iron and erythropoietin can also be lost with blood during
dialysis, which can result in an iron deficiency. Extra iron and erythropoietin
are usually needed to help prevent iron deficiency in these individuals (24
- 26).
Iron deficiency could also be caused by low vitamin A
status. Vitamin A helps to mobilize iron from its storage sites, so a deficiency
of vitamin A limits the body’s ability to use stored iron. This results
in an “apparent” iron deficiency because hemoglobin levels are low,
even though the body can maintain normal amounts of stored iron (27,28). While
uncommon in the U.S., this problem is seen in developing countries where vitamin
A deficiency often occurs.
The anemia that may occur with inflammatory disease differs
from iron deficiency anemia. It occurs in people who have chronic infectious,
inflammatory, or malignant disorders (29,30). It is not associated with a shortage
of dietary iron, and may not respond to iron supplementation (30,31). A physician
should manage anemia associated with an inflammatory disorder.
Signs of iron deficiency anemia include feeling
tired and weak, decreased work and school performance, slow cognitive and social
development during childhood, difficulty maintaining body temperature, and decreased
immune function, which may decrease resistance to infection (1,32-35). During
pregnancy, iron deficiency is associated with increased risk of premature deliveries,
giving birth to infants with low birth weight, (36,37) and maternal complications
(1,37).
Who may need extra iron
to prevent a deficiency?
Iron deficiency and iron deficiency anemia are
relatively common in women of childbearing age, older infants and toddlers,
and teenage girls (38), so they should periodically be screened for iron deficiency.
Within these groups, iron deficiency is more common among women with heavy menstrual
losses, women belonging to minority and low-income groups, and women who have
had more than one child (38). Women taking oral contraceptives may experience
less bleeding during their periods and have a lower risk of developing an iron
deficiency while women using an intrauterine device (IUD) may experience more
bleeding and have a greater risk of developing an iron deficiency. If laboratory
tests indicate iron deficiency, iron supplements may be recommended. Many physicians
routinely prescribe iron supplements during pregnancy because of the high incidence
of iron deficiency anemia in pregnant women and the potential benefits for the
mother and the fetus. Pregnancy increases a woman’s need for iron due to
increased blood volume, increased needs of the fetus, and blood losses that
occur during delivery (1, 39).
Excluding all meat and meat products, poultry, and fish
from your diet may reduce your total iron intake and will decrease your intake
of heme iron, which is easily absorbed by your body. It will also influence
your iron status because animal proteins can improve the absorption of nonheme
iron found in plant foods. Vegetarians who exclude all animal products from
their diet may need twice as much dietary iron because the intestinal absorption
of nonheme iron is lower in plant foods (1). Vegetarians should also consider
consuming nonheme iron sources together with a good source of vitamin C, such
as citrus fruits or certain vegetables, to enhance absorption of nonheme iron.
Some facts about iron supplements
Iron supplementation is indicated when an iron
deficiency is diagnosed and diet alone cannot restore bodily iron content to
normal levels within an acceptable timeframe. Iron in supplements comes in two
forms: ferrous and ferric. The ferrous form is better absorbed and is usually
the preferred form when iron deficiency has been diagnosed (40-42).
Supplemental iron may cause gastrointestinal side effects
such as nausea, vomiting, constipation, diarrhea, dark colored stools, and/or
abdominal distress (43). To minimize these side effects, start with half the
recommended dose, gradually increasing to the full dose. Taking the supplement
in divided doses and with food also may help limit these symptoms (44).
Who should be cautious
about taking iron supplements?
Iron deficiency is uncommon among adult men
and postmenopausal women. These individuals should only take iron supplements
when prescribed by their qualified health care provider because of the risk
of iron overload. Iron overload is a condition in which excess iron is found
in the blood and stored in organs such as the liver and heart. Iron overload
is associated with several genetic diseases including hemochromatosis, which
affects approximately 1 in 250 individuals of northern European descent (45).
Individuals with hemochromatosis absorb iron very efficiently, which can result
in a build up of excess iron in organs and can cause organ damage such as cirrhosis
of the liver and heart failure (1,3,46-48). This condition often is not diagnosed
until the excess iron stores have damaged an organ. Iron supplementation may
accelerate the effects of hemochromatosis, an important reason why adult men
and postmenopausal women who are not iron deficient should not take iron supplements.
Individuals with blood disorders who require frequent blood transfusions are
also at risk of iron overload and should not take iron supplements.
What are some current
issues and controversies about iron?
Iron and Heart Disease
Several observations have led researchers
to examine the association between high iron stores and coronary heart disease.
It appears that rates of heart disease among women increase when monthly periods
stop, a time when levels of stored iron increase. Also, some researchers have
suggested that lower rates of heart disease among people living in developing
countries may be due to low meat (and iron) intake, high fiber diets that inhibit
iron absorption, and gastrointestinal (GI) parasite concentrations that result
in gastrointestinal blood (and iron) loss, all of which contribute to low iron
stores in this population (49-53). In addition, a 1980s study of Finnish men
linked high iron stores with increased risk of heart attacks (54). However,
not all studies have supported this relationship (1, 55), including a 1999 review
of 12 studies that failed to show a strong association (56). It is also true
that older women have a greater prevalence of traditional cardiovascular disease
risk factors such as high blood pressure and elevated blood cholesterol. Currently,
available data do not provide convincing support for an association between
high body iron stores and increased risk for coronary heart disease (1).
Iron and Cancer
Individuals with hereditary hemochromatosis
are at increased risk for liver cancer (1). This increased risk is associated
with an accumulation of iron in the liver, which can result in increased production
of free radicals. Free radicals are by-products of normal metabolism that can
damage your body’s cells. There is inconclusive evidence that iron status is
associated with the incidence of cancer in those who do not have hereditary
hemochromatosis.
Iron and Intense Exercise
Many men and women who engage in regular
intense exercise have marginal or inadequate iron status (1,57-60). Researchers
have estimated that daily iron loss increases in those who engage in regular
exercise. Research also indicates that iron has a shorter biologic half-life
in highly trained runners. For these reasons, the need for iron may be 30% greater
in those who engage in regular intense exercise (61).
Iron fortification and absorption
of other nutrients
Some researchers have raised concerns about
the effects of iron fortification and supplementation on the absorption of other
nutrients such as zinc, calcium, and copper. Research studies have shown that
supplemental iron may decrease the absorption of these nutrients, but generally
only when the supplement is taken on an empty stomach. Absorption of these nutrients
is generally not affected when supplementary iron is taken with food(1,62,63).
What is the health risk
of too much iron?
Iron has a moderate to high potential for toxicity
because very little iron is excreted from the body. Thus, iron can accumulate
in body tissues and organs when normal storage sites are full.
In children, acute toxicity can occur from overdoses of
medicinal iron. Ingestion of as few as five or six high-potency tablets can
provide amounts of iron that can be fatal to a child of 22 pounds. Consuming
1 to 3 grams of iron can be fatal to children under six and lower doses can
cause severe symptoms such as vomiting and diarrhea (64). It is important
to keep iron supplements tightly capped and away from children’s reach.
Any time excessive iron intake is suspected, immediately call your physician
or Poison Control Center, or visit your local emergency room. In adults high
intakes of iron supplements are associated with constipation, nausea, vomiting,
and diarrhea, especially when the supplements are taken on an empty stomach
(1).
In 2001, the Institute of Medicine set a tolerable upper
intake level (UL) of 40 mg per day for infants and children through age 13 and
45 mg per day for adolescents ages 14 to 18 years and adults 19 years of age
and older (1). The upper limit does not apply to individuals who receive iron
under medical supervision. There may be times when a medical doctor prescribes
an intake higher than the upper limit, such as when individuals with iron deficiency
anemia need higher doses of iron until their iron stores return to normal.
Selected
Food Sources of Iron
As the 2000 Dietary Guidelines for Americans
state, “Different foods contain different nutrients and other healthful
substances. No single food can supply all the nutrients in the amounts you need”
(65). The following tables suggest dietary sources of heme and nonheme iron.
As the table indicates, meat, poultry, fish and seafood are good sources of
heme iron and beans are good sources of nonheme iron. In addition, many foods
are fortified with iron. Some foods, such as cereals, may be fortified with
100% of the Daily Value (DV)* for iron. It is important for anyone who is considering
taking an iron supplement to first consider whether their needs are being met
by natural dietary sources of heme and nonheme iron and foods fortified with
iron. If you want more information about building a healthful diet, refer to
the Dietary
Guidelines for Americans and the Food
Guide Pyramid.
Table
of Food Sources of Heme Iron (4, 66)
| Food |
Milligrams
|
%DV* |
| Chicken liver, cooked, 3 ounces |
7.0 |
40 |
| Oysters, breaded and fried, 6 |
4.5 |
25 |
| Beef, chuck, braised, 3 ounces |
3.2 |
20 |
| Clams, breaded, fried, 3/4 cup |
3.0 |
15 |
| Beef, tenderloin, roasted, 3 ounces |
3.0 |
15 |
| Turkey, dark meat, roasted, 3 ounces |
2.0 |
10 |
| Beef, eye of round, roasted, 3 ounces |
1.7 |
10 |
| Turkey, light meat, roasted, 3 ounces |
1.2 |
6 |
| Tuna, fresh bluefin, cooked, dry heat,
3 ounces |
1.1 |
6 |
| Chicken, leg, meat only, roasted, 3 ounces
|
1.1 |
6 |
| Crab, blue crab, flaked & pieces, cooked,
moist heat, 1 cup |
1.1 |
6 |
| Chicken, breast, roasted, 3 ounces |
1.0 |
5 |
| Halibut, cooked, dry heat, 3 ounces |
0.9 |
5 |
| Pork, loin, meat only, broiled, 3 ounces
|
0.8 |
4 |
| Tuna, white, canned in
water, 3 ounces |
0.8 |
4 |
| Crab, blue crab, cooked, moist heat, 3
ounces |
0.8 |
4 |
| Shrimp, mixed species, cooked, moist heat,
4 large |
0.7 |
4 |
| * DV = Daily Value. DVs are reference numbers
based on the Recommended Dietary Allowance (RDA). They were developed to
help consumers determine if a food contains a lot or a little of a specific
nutrient. The DV for iron is 18 milligrams (mg). The percent DV (%DV) listed
on the nutrition facts panel of food labels tells adults what percentage
of the DV is provided in one serving. Percent DVs are based on a 2,000 calorie
diet. Your Daily Values may be higher or lower depending on your calorie
needs. Foods that provide lower percentages of the DV also contribute to
a healthful diet. |
Table
of Food Sources of Nonheme Iron (4, 62)
| Food |
Milligrams
|
%DV* |
| Ready-to-eat cereal, 100% fortified,
3/4 cup |
18.0 |
100 |
| Ready-to-eat cereal, 50% fortified, 3/4
cup |
9.0 |
50 |
| Soybeans, mature, cooked, boiled, 1 cup |
8 |
50 |
| Lentils, cooked, boiled, 1 cup. |
6 |
35 |
| Grits, white, enriched, instant, 1 packet
prepared |
5.4 |
30 |
| Oatmeal, instant, fortified, 1/2 cup |
4.1 |
25 |
| Kidney beans, cooked, boiled, 1 cup |
5.2 |
25 |
| Pinto beans, cooked, boiled, 1 cup |
4.6 |
25 |
| Lima beans, cooked, boiled, 1 cup |
4.2 |
25 |
| Navy beans, cooked, boiled, 1 cup |
3.8 |
20 |
| Black beans, cooked, boiled, 1 cup |
3.6 |
20 |
| Spinach, cooked, boiled, drained, 1/2
cup |
3.2 |
20 |
| Spinach, canned, drained solids 1/2 cup |
2.5 |
10 |
| Tofu, firm, 1/2 cup |
1.8 |
10 |
| Black-eyed-peas, cooked, boiled, 1 cup
|
1.8 |
10 |
| Spinach, frozen, cooked, boiled 1/2 cup |
1.4 |
8 |
| Whole wheat bread,
1 slice |
0.9 |
5 |
| Molasses, 1 Tablespoon |
0.9 |
5 |
| White bread, enriched, 1 slice |
0.8 |
4 |
| Raisins, seedless, 50 |
0.5 |
2 |
| * DV = Daily Value. DVs are reference
numbers based on the Recommended Dietary Allowance (RDA). They were developed
to help consumers determine if a food contains a lot or a little of a
specific nutrient. The DV for iron is 18 milligrams (mg). The percent
DV (%DV) listed on the nutrition facts panel of food labels tells adults
what percentage of the DV is provided in one serving. Percent DVs are
based on a 2,000 calorie diet. Your Daily Values may be higher or lower
depending on your calorie needs. Foods that provide lower percentages
of the DV also contribute to a healthful diet. |
This Fact Sheet
was developed by the Clinical Nutrition Service, Warren Grant Magnuson Clinical
Center, National Institutes of Health (NIH), Bethesda, MD, in conjunction
with the Office of Dietary Supplements (ODS) in the Office of the Director
of NIH. The mission of ODS is to strengthen knowledge and understanding
of dietary supplements by evaluating scientific information, stimulating
and supporting research, disseminating research results, and educating the
public to foster an enhanced quality of life and health for the US population.
The Clinical Nutrition Service and the ODS would like to thank the expert
scientific reviewers for their role in ensuring the scientific accuracy
of the information discussed in this fact sheet.
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