The Nuts and Bolts of Dietary Iron

Despite the many jokes about doctors and their love of golf, golf clubs aren’t the only “irons” physicians talk about. Although it’s in the headlines less often than protein, trans fat, or calcium, iron is more than just the metal in a golf club – it’s also an important part of your diet, and a lack of iron can cause some serious problems.

Iron is an essential mineral that is needed for the formation of red blood cells, hemoglobin (the protein in red blood cells that carries oxygen), and myoglobin (the protein that carries oxygen in muscle). In addition to carrying oxygen, iron is involved in energy metabolism, collagen formation, immune system function, and the production of neurotransmitters such as serotonin and dopamine. Approximately two-thirds of the iron in our bodies is found in hemoglobin, with smaller amounts found in myoglobin, some catalytic molecules (enzymes), and in storage molecules such as ferritin (a protein that stores iron and releases it when the blood has too little of it).


Despite iron’s many roles, the total amount of iron in the body only adds up to roughly one teaspoon. About 15% of our body’s iron is stored for future needs or as a backup for when dietary intake is insufficient. Our bodies obtain iron from the food that we eat (or supplements), and on average, we lose about 1 milligram of iron each day (a very small amount) through intestinal blood loss (and some small amounts lost through sweat and urine), although premenopausal women lose more iron than others from blood lost through menstruation.

Food sources of iron

The iron we consume from foods comes in two forms, heme and nonheme. Heme iron is found in meat, poultry, fish, and eggs. Nonheme iron is found in foods of plant origin, including dark green, leafy vegetables, nuts, seeds, legumes, iron-enriched grains, and dried fruits. Certain grain foods such as pasta, white rice, and many types of bread are enriched with iron by food manufacturers, because naturally occurring iron is often lost during food processing. And many cereals are fortified with 100% of the Recommended Dietary Allowance (RDA) for iron. (Click here for a list of dietary sources of heme and nonheme iron.)

The body absorbs anywhere from 15% to 35% of heme iron, compared to 1% to 7% of nonheme iron. However, an individual’s absorption of dietary iron depends on several factors. If iron stores are low, for example, the body compensates by increasing iron absorption from food. Likewise, if iron stores are high, absorption will normally decrease. Iron absorption is increased during a child’s growth spurt, and also during pregnancy, due to the extra demand for this mineral.

Absorption of heme iron is usually unaffected by the rest of your diet. However, some food ingredients can hinder iron absorption from foods containing nonheme iron, including caffeine and calcium. Tannins and polyphenols, which are found in tea, and phytates, found in many plant foods such as legumes and grains, also decrease the absorption of nonheme iron.

Meat proteins and vitamin C, on the other hand, increase iron absorption from foods with nonheme iron. Eating a food rich in vitamin C, such as an orange or a tomato, with a meal can increase absorption of nonheme iron sixfold. Following the Food Guide Pyramid’s recommendation of five servings of fruits and vegetables a day can help to maximize iron absorption.

Soy foods can also be a source of iron; however, like other legumes, soybeans naturally contain phytates, which reduce iron absorption. Fermentation is thought to break down phytates, so fermented soy products such as miso or tempeh provide more iron than nonfermented soy products such as tofu or soy flour.

Another way to increase iron in your diet is to cook foods in iron pots and pans. Small amounts of iron from the cookware leach into foods cooked in them.

How much do you need?

Iron requirements depend on several factors, including sex and age. For example, adult men and postmenopausal women need less iron compared to children, adolescents, and women of childbearing age. Infants need very little iron while pregnant women need substantially more than others.

Information gathered from two major food surveys, the National Health and Nutrition Examination Survey (NHANES III 1988—1991), and the Continuing Survey of Food Intakes by Individuals (CSFII 1994—1996), yielded a lot of information about the nutrition status of the American population. As far as iron goes, adult men and postmenopausal women tend to consume the recommended dietary intake for iron. However, women of childbearing age and pregnant and breast-feeding women often do not consume enough iron in their diets.

The RDA is the daily intake of a nutrient that is sufficient to meet the requirements of approximately 98% of healthy individuals in a certain population. This table lists the RDAs for iron based on age and sex.

Healthy infants are born with iron stores that typically last them for four to six months. In addition to providing infection-fighting antibodies and important fatty acids, breast milk is also a good source of a well absorbed form of iron. The American Academy of Pediatrics recommends that infants who are not breast-fed receive an iron-fortified formula from birth until 12 months. Cow’s milk should not be fed to babies less than 12 months old because it is low in iron (a form that is also not well absorbed) and can cause infants to experience gastrointestinal bleeding and loss of iron. Once a baby starts to eat solid food, it’s important to give him foods high in iron to prevent iron deficiency.

Iron-deficiency anemia

Iron-deficiency anemia can occur when there is not enough hemoglobin in the blood. The World Health Organization reports that iron deficiency is the most common nutrition disorder in the world, and it’s estimated that more than 30% of the world’s population is affected. In the United States, approximately 5% of women and 2% of men have iron-deficiency anemia.

Insufficient iron in the body will eventually lead to iron-deficiency anemia. Here’s what happens: A lack of iron in the blood due to, say, a low dietary intake of iron, signals the body to draw from its iron reserves in the liver, spleen, and bone marrow. However, when iron stores in the bone marrow also become low, the marrow then cannot make enough hemoglobin for the red blood cells. The resulting red blood cells are small and carry too little oxygen for organs and tissues throughout the body, causing anemia.

Causes. Because anemia results from low stores of iron in the body, a health-care provider must first look for reasons why iron stores are low. Typically, low iron levels are the result of an inadequate intake of dietary iron, poor absorption of iron in the body, internal bleeding due to ulcers or cancer, heavy menstrual periods, or an increased need for iron because of the needs of a fetus during pregnancy, the demands of the growing bodies of children and adolescents, or an increased loss of iron during endurance sports.

People who are on kidney dialysis often become anemic. Diseased or malfunctioning kidneys can’t make enough of a hormone called erythropoietin, which is needed to make red blood cells. Erythropoietin and iron are also lost during the dialysis process. Therefore, people on dialysis typically need supplements of iron and erythropoietin to prevent anemia.

Iron-deficiency anemia can occur also due to a deficiency of vitamin A. Vitamin A helps the body use its stored iron. So, even if body stores of iron are normal, a lack of vitamin A prevents the body from accessing those stores to create hemoglobin. Vitamin A deficiency is rarely seen in the United States but is quite common in developing countries.

Symptoms. Anemia may not always be quickly diagnosed, since the signs and symptoms are similar to other health conditions. However, contact your health-care provider if you experience any of the following, because they can be signs that you are anemic:

  • Fatigue
  • Weakness
  • Lightheadedness or dizziness
  • Ringing in the ears
  • Rapid heartbeat
  • Shortness of breath
  • Headaches
  • Irritability
  • Paleness
  • Difficulty concentrating
  • Brittle nails
  • Cracked lips
  • Smooth, sore tongue

Children with anemia may experience a decrease in their performance in school and slow cognitive and social development. Pregnant women who are anemic may exhibit unusual conditions called pica and pacophagia. Pica is the consumption of nonedible items such as dirt, clay, or paper. In pacophagia, a person constantly craves ice. Iron-deficiency anemia during pregnancy is a cause for concern because there is an increased risk for premature delivery of the baby, a low birth-weight baby, and complications that occur in the mother.

Diagnosing anemia. If your health-care provider suspects that you have iron-deficiency anemia, you will likely have a physical exam, a review of your symptoms, and special blood tests. These include a complete blood count, which can give an idea of the condition of your blood cells; iron tests, which can determine the type and severity of anemia; reticulocyte count, a count of immature red blood cells; and a ferritin level test to determine the level of iron stores in the body. Sometimes a portion of the bone marrow is also obtained to better determine the cause of anemia. If your physician suspects bleeding in the digestive tract, he’ll also order a fecal occult blood test, a colonoscopy, an endoscopy, and/ or an upper GI series (x-rays of the upper gastrointestinal tract).

People at risk. Because older infants, toddlers, teenage girls and boys, women of childbearing age, and pregnant women have higher iron needs, they are more likely to not get enough iron, putting them at risk for a deficiency. Women who have heavy menstrual periods also often need more iron to prevent deficiencies. Women in minority groups and women from a low-income background are more likely to not be meeting the RDA for iron because of inadequate diets.

Other people may be at risk for anemia. Endurance athletes, such as long-distance runners, can be at increased risk if they do not increase their intake of iron to compensate for their increased losses of iron (from sweat and a faster cycle of death and production of their red blood cells). Strict vegetarians are also at risk for anemia because they avoid the animal foods that are the richest sources of absorbable dietary iron. However, anemia can be easily prevented in vegetarians with an eating plan rich in fruits and vegetables high in vitamin C. And there’s evidence that vegetarians actually adapt to a lower iron intake by increasing iron absorption and decreasing iron losses. Anyone who has problems with malabsorption of food in the digestive tract because of celiac disease, Crohn disease, or ulcerative colitis, for example, or who has had a portion of his intestines removed is at high risk for developing anemia.

Iron supplements

Given the ready availability of iron supplements and the small but real possibility of developing iron-deficiency anemia, it may seem logical to take a supplement just in case. However, chronic high intakes of iron can lead to iron overload, a condition in which an excess amount of iron is found in the blood and is stored in the heart and liver, sometimes damaging those organs, so single-nutrient supplements of iron (those that provide from about 18 to 65 milligrams of iron) should only be taken under the guidance of your health-care provider. Taking an iron supplement on your own to self-treat what you suspect is anemia may also delay the diagnosis of a more serious problem, such as a bleeding ulcer or colon cancer.

In most cases, healthy people do not need to take an iron supplement to ensure an adequate intake of iron, but certain groups of people are more likely to need one. Most pregnant women are prescribed an iron supplement in the amount of 30 milligrams (mg) each day until the birth of the baby. Anyone with one or more of the following conditions may also need an iron supplement:

  • Women with heavy menstrual periods
  • Bleeding problems
  • Burns
  • Intestinal diseases
  • Stomach problems
  • Removal of stomach or intestines
  • Hemodialysis

If your health-care provider recommends an iron supplement for you, chances are it will be in the form of ferrous sulfate, ferrous gluconate, or ferrous fumarate. Ferrous sulfate is more readily absorbed but is more likely to cause gastrointestinal distress and constipation. In that case, you may be advised to switch to ferrous gluconate, which is less irritating to the digestive tract (although it is not as readily absorbed by the body).

Iron supplements are generally sold over the counter at drugstores and come in several forms, including capsules, tablets, and liquids. Iron is also available by injection (given by a health-care provider). People who have difficulty absorbing iron from food or supplements or who have stomach problems may need iron injections.

Possible side effects from iron supplements include the following:

  • Upset stomach
  • Heartburn
  • Nausea
  • Vomiting
  • Constipation
  • Diarrhea
  • Dark-colored stools
  • Dark-colored urine
  • Leg cramps
  • Stained teeth (from liquid supplements)

Some of these side effects, such as upset stomach and constipation, are relatively common. Taking iron with food or immediately after eating can help lessen stomach upset, and increasing fiber and fluid intake may help prevent constipation. Dark green or black stools are normal and occur due to unabsorbed iron. (However, you should call your health-care provider if your stools are dark with a sticky consistency, or if you see red streaks, and if you have any abdominal cramping or pain because these can indicate serious gastrointestinal bleeding.)

Unless you have a chronic condition that impairs iron absorption, iron supplements are not meant to be taken indefinitely, so pay attention to your health-care provider’s instructions as to how long you should take your supplement. If you are anemic, for example, you may need to take an iron supplement for up to six months until the anemia resolves.

Supplement safety

Iron supplements may interact or interfere with the absorption and action of some medicines. In addition, taking iron can worsen other medical conditions, such as kidney disease, heart disease, asthma, intestinal disorders, and stomach ulcers. Once again, it’s important to discuss taking iron supplements with your health-care provider, especially if you have any health concerns.

Certain foods decrease iron absorption when taken with an iron supplement, such as dairy foods, eggs, spinach, whole-grain products, tea, and coffee. Either take your iron one hour before eating these foods, or wait a couple hours after eating them. Don’t take your iron with calcium supplements and antacids. (Remember, to increase iron absorption from food and supplements, eat a food high in vitamin C with your meal, such as an orange, a tomato, strawberries, or green pepper.)

Store iron supplements away from heat and light, and discard any expired supplements. Always keep iron out of the reach of children, because an iron overdose can be fatal. Because of this risk, expired iron supplements should be discarded in a way that keeps them away from curious children such as flushing them down the toilet. If you suspect a child has taken too much iron, call the Poison Control Center at (800) 222-1222 or take the child to the nearest emergency room.

If you take a multivitamin supplement or a heavily fortified cereal (such as Product 19 or Total), be sure to check the label for the amount of iron it contains. A multivitamin supplement that contains 18 mg of iron (or a cereal that contains 100% of the daily value for iron, which is also 18 mg), for example, is fine for most women under 50. However, this is too much iron for men and postmenopausal women, who should choose a supplement with no more than 8 or 9 mg of iron. In fact, men, postmenopausal women, and anyone who is at risk for iron overload can switch to a multivitamin that contains no iron, such as One-A-Day Men’s High Potency Multivitamin/Multimineral Supplement.

Iron overload

As mentioned earlier, iron overload, in which excess iron is in the blood and becomes stored in vital organs, such as the liver and heart, can be caused by getting too much iron from a supplement. A common genetic condition called hemochromatosis can also cause the body to absorb too much iron. Over time, iron builds up in organs, leading to problems such as cirrhosis (damage to the liver) and heart failure. Although some studies have found no connection, others have found a link between hemochromatosis and diabetes. Hemochromatosis occurs in about 1 out of 250 people of Northern European descent and is less common in other ethnic groups. This condition is often not diagnosed until signs of organ damage occur, which is another reason why men and postmenopausal women who are not iron-deficient should not take iron supplements unless prescribed by their health-care provider.

Iron and bone health

New research shows that iron may be just as important as calcium and vitamin D in building and maintaining healthy bones. A study published in the November 2003 issue of the Journal of Nutrition looked at the effect of iron on bone mineral density in 242 postmenopausal women. Women who consumed approximately 20 mg of iron, along with an intake of 800—1200 mg of calcium per day, had the greatest bone mineral density. This study illustrates an interesting paradox because calcium reduces iron absorption. Of course, other nutrients are essential for bone health, too, such as vitamin K and phosphorus.

Iron and Type 2 diabetes

Two recently published studies have established a link between high iron levels and the development of Type 2 diabetes. One study, out of the Harvard School of Public Health, looked at data gathered from approximately 33,000 healthy women (none of whom had diabetes) participating in the ongoing Nurses’ Health Study. After 10 years of follow-up, about 700 of these women developed Type 2 diabetes. These 700 women also had significantly higher iron stores (shown by high levels of the iron-storing molecule ferritin) compared to a control group of about 700 women who did not develop diabetes. Women with the highest ferritin levels were almost three times as likely to develop diabetes than women with low ferritin levels. Although a single study isn’t conclusive, the thought is that high iron levels in the blood may increase the formation of free radicals (highly reactive by-products of normal chemical reactions in the body), causing insulin resistance and impaired glucose metabolism.

The other study used data from the Iowa Women’s Health Study, which followed about 36,000 postmenopausal women without diabetes for 11 years. Women who drank alcohol and had a greater intake of iron (from iron supplements or heme iron from meats) had an increased risk of Type 2 diabetes compared to women who did not drink alcohol. The researchers also discovered that among women who did not drink, more nonheme iron in the diet corresponded to decreased risks of developing diabetes.

The link between iron and Type 2 diabetes may apply to men as well. Researchers from the Harvard School of Public Health published a study earlier this year in the American Journal of Clinical Nutrition. They looked at iron intake and blood donation data in about 33,000 men without diabetes who participated in the Health Professionals’ Follow-up Study. Men whose iron intake came primarily from red meat were at higher risk for Type 2 diabetes compared to men who got their iron from nonred meat or nonmeat sources. Blood donations were not linked with Type 2 diabetes.

Everyday iron

It’s easy to underestimate the importance of a substance that we need so little of, but iron is important for health. In the United States, most peoples’ diets ensure that they get the right amount of iron, but if you fall into an at-risk group or if you have any questions, a registered dietitian can help make sure your eating plan is “iron clad.”