Iron

Iron is a mineral your body cannot do without. It sits at the core of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the tissues. It is also at the core of myoglobin, which supplies oxygen to muscle.

An adult holds 3 to 4 g of iron, most of it in hemoglobin, says the NIH (the US National Institutes of Health). Iron is also needed for growth, brain development and the making of some hormones.

Here is the part most people do not know: your body has no way to get rid of extra iron on purpose. Hurrell and Egli put it plainly: iron balance is controlled by absorption alone, because the body has no way to excrete iron.

The brake is a liver hormone called hepcidin. It was first described in 2000 and 2001 as a germ-killing peptide (a small protein) in blood and urine. Later work showed that it controls iron. It is why one iron pill can lower how much you absorb from the next.

Too little iron is common. The WHO (World Health Organization) estimates that iron deficiency causes about half of the 1.62 billion cases of anemia worldwide, the NIH reports. Anemia means too few healthy red blood cells. Young children, women who menstruate and pregnant women are most at risk.

Too much is a real risk as well. US labels on iron pills warn that accidental overdose is a leading cause of fatal poisoning in children under 6. About 4.4 in 1,000 White people carry two copies of the gene change behind hereditary hemochromatosis, which makes the body absorb too much iron. This page covers both sides.

Health benefits of iron

• ★★★Less anemia in women who menstruateDaily iron supplements made anemia less than half as likely (0.39 times the risk across 67 trials). Tiredness eased too.
• ★★★Less iron-deficiency anemia in pregnancyDaily iron supplements in pregnancy cut iron-deficiency anemia at term from 18.4% to 5.0% (7 trials). Low birth weight fell from 6.1% to 5.2%.
• ★★★Less anemia in young children, no development gainIn 35 trials of children aged 4 to 23 months, daily iron supplements made iron-deficiency anemia 0.14 times as likely. Mental and motor development scores did not differ.
• ★★★More constipation and other gut side effectsIn women who menstruate, daily iron supplements made constipation 2.07 times as likely. Ferrous sulfate pills raised the odds of gut side effects 2.32 times across 20 trials against placebo.
• ★★★Little or no difference in malariaIn 40 trials of children in malaria areas, iron supplements made clinical malaria 0.93 times as likely. It may slightly raise malaria where prevention and treatment were lacking, and slightly lower it where they existed.

Foods high in iron

Soybeans lead our ranking per serving, which uses USDA (US Department of Agriculture) data. One cup (172 g) gives 8.84 mg, 49% of the Daily Value. Lentils (6.59 mg per cup) and chickpeas (4.74 mg per cup) follow. Next come adzuki beans and lima beans, then oysters at 4.34 mg in 3 oz, the only animal food in the top 10.

Per 100 g, herbs and seeds lead: thyme (17.5 mg), sesame seeds (14.6 mg) and cocoa powder (13.9 mg). A real serving of those is far smaller than 100 g. See the pages for soybeans, lentils, chickpeas, oysters, sesame seeds and cocoa powder.

Beef liver and fortified cereals are not in our ranking. The NIH lists pan-fried beef liver at 5 mg in 3 oz (28% of the Daily Value). Fortified breakfast cereals give up to 18 mg a serving (100%). In the US, about half of dietary iron comes from bread, cereal and other grain products.

Heme iron, the kind in meat and seafood, is better absorbed than the non-heme iron in plants. Phytate and polyphenols (natural plant compounds) hold plant iron back. The NIH notes that spinach is a good source on paper, but its bioavailability (how much your body absorbs) is low because of these blockers. Vitamin C (see vitamin C) and meat raise absorption of plant iron.

How iron works

The basic biology of iron is well established. But it comes from reviews, cell and animal work, and absorption studies in small groups of people, not from trials of extra iron. Four ideas cover most of it.

Iron carries oxygen and does other jobs. The body controls its iron through absorption and the hormone hepcidin. What you eat with iron changes how much you take in, and the timing of a supplement matters. Here is each one, and what has been seen in people.

It carries oxygen and does other jobs

Hemoglobin and myoglobin are the best-known iron proteins. The NIH says iron also supports cell function, growth and the making of some hormones. The EU authorizes claims for iron in energy-yielding metabolism, the immune system and cell division.

Iron is stored as ferritin and moved in the blood by transferrin. Small amounts are lost in urine, feces, sweat and shed skin cells, and more in women who menstruate. Pasricha and colleagues note that iron deficiency can cause clinical and functional problems even without anemia.

The body controls iron through hepcidin

Because the body cannot excrete iron on purpose, the gate is at absorption. Hepcidin is made by the liver and binds ferroportin, the protein that lets iron out of gut cells and storage cells. The bound ferroportin is pulled into the cell and broken down, so less iron reaches the blood.

Iron stores raise hepcidin, and so does inflammation. In human volunteers and mice, the immune messenger IL-6 was necessary and sufficient to switch it on. That explains why infection lowers blood iron and why oral iron works less well during inflammation. The NIH adds that hepcidin controls both absorption and where iron sits in the body.

What you eat with iron changes how much you absorb

Plants and fortified foods hold only non-heme iron, while meat, seafood and poultry hold both heme and non-heme iron. The NIH says heme iron is more bioavailable. It puts bioavailability at 14% to 18% from mixed diets rich in meat, seafood and vitamin C, and 5% to 12% from vegetarian diets.

In single-meal studies, the blockers are strong. Phytate in grains and beans cut absorption by 64% at 25 mg and 82% at 250 mg. Black tea cut it by 79% to 94%, and 165 mg of calcium by 50% to 60%; calcium also lowered heme iron absorption. In the same bread-meal work, vitamin C counteracted phytate.

Whole diets blunt these effects. In a five-day study of complete diets, vitamin C intakes from 51 to 247 mg a day made no clear difference to iron absorption. The NIH says the enhancers and blockers have little effect on most people’s iron status in a typical Western diet. Iron status itself matters more: Hurrell and Egli report that status generally has a greater effect than diet makeup.

When you take a supplement matters

Hepcidin also explains an odd result. In 54 young women without anemia but with low ferritin, doses of 60 mg or more raised hepcidin. Then, 24 hours later, the share of iron absorbed was 35% to 45% lower. A sixfold rise in dose (40 to 240 mg) gave only a threefold rise in iron absorbed (6.7 to 18.1 mg).

In two more small trials, the same 60 mg dose was absorbed better on alternate days than on consecutive days. Splitting a dose across the day did not help. In 19 women with anemia, the first dose was absorbed 40% to 50% better than a dose on the second day in a row. The same held for a dose taken after a day off.

What has been seen in people

Where people are short of iron, the effects are clear. Supplements raise hemoglobin and prevent anemia in menstruating women, in pregnancy and in young children, as the study cards above show.

The hepcidin results have led to trials of alternate-day dosing. So far, one trial shows similar iron stores with fewer stomach side effects. A pooled analysis of 11 trials found a small edge for daily dosing that could be chance.

Beyond deficiency the pattern is mixed. Low-iron adults had less fatigue, and school-age children gained in thinking skills. Breastfed infants without anemia saw no benefit, and there were harm signals in a malaria area.

How much iron do you need?

Adults need 8 mg of iron a day, except women aged 19 to 50, who need 18 mg. These are the US Recommended Dietary Allowances (RDAs, the daily amounts that cover nearly all healthy people). EFSA (the EU’s food safety agency) sets a Population Reference Intake, its version of the RDA. It is 11 mg a day for men and postmenopausal women and 16 mg for premenopausal women.

In pregnancy, the US value rises to 27 mg a day and EFSA’s is 16 mg. While breastfeeding, the US values are 10 mg at ages 14 to 18 and 9 mg at 19 to 50, and EFSA’s is 16 mg. After 50, US women need 8 mg, the same as men, and EFSA’s 11 mg applies to men and postmenopausal women.

The NIH says the requirement is 1.8 times higher for people on vegetarian diets, because plant iron is absorbed less well. The table below shows the full US and EU values by age.

Iron deficiency: signs and who is at risk

Deficiency runs in stages, the NIH explains. Iron stores fall first. Next, the supply to red blood cell production drops while hemoglobin is still normal. Last comes iron-deficiency anemia, with low hemoglobin and small, pale red cells.

People with anemia may have weakness, fatigue, trouble concentrating, and stomach and bowel problems. In infants, it can affect thinking and behavior in ways that might be irreversible.

A ferritin level below 30 µg/L suggests iron deficiency, and below 10 µg/L suggests anemia from it. Inflammation can push ferritin up, so doctors combine tests. Hemoglobin below 11 g/dL in children under 10, or below 12 g/dL from age 10, suggests iron-deficiency anemia. But hemoglobin alone is neither sensitive nor specific: it misses some cases and flags others.

Groups at higher risk include pregnant women, infants and young children, and women with heavy periods (at least 10% of menstruating women by some estimates). Frequent blood donors are at risk too, as 25% to 35% develop iron deficiency. So are people with cancer, bowel disorders or bowel surgery, and people with heart failure (about 60% are iron deficient).

Because the cause matters, a doctor should look for it. The Lancet review says that includes screening for celiac disease and, in men and postmenopausal women, looking for bleeding in the gut. A blood test is the sensible first step before taking iron on your own.

Forms of iron: absorption and effects

Iron pills differ in how much iron they hold, how well they dissolve and how they feel in the stomach. The NIH says ferrous and ferric salts are the most common forms. It says ferrous iron is more bioavailable than ferric because it dissolves better. High doses (45 mg a day or more) may cause nausea and constipation.

Supplement Facts panels list elemental iron, so you do not need to work it out from the salt. Taking iron with food can ease side effects, the NIH adds.

The honest bottom line: ferrous sulfate is the best-studied form and the usual comparison. In head-to-head trials, other forms have matched it on hemoglobin or done worse (iron polysaccharide complex in toddlers, heme iron after gastric bypass). In a pooled analysis, bisglycinate did a little better in pregnancy. Claims of fewer side effects are mixed, and most trials measure absorption or hemoglobin over weeks, not long-term health.

The trials above used these forms. Ferrous sulfate was used in the two fatigue trials (80 mg of iron a day) and in the absorption trials. Iron polysaccharide was used in the oral heart failure trial, and micronized ferric pyrophosphate in the breastfed-infant trial. Intravenous ferric carboxymaltose was used in FAIR-HF and HEART-FID.

There are 15 comparisons below, and they are counted in this page’s total of 43 studies.

Form	Share of iron by weight	What studies show	Source
Ferrous sulfate	20%	The usual comparison. Raised the odds of stomach and bowel side effects 2.32 times against placebo. Beat iron polysaccharide complex in toddlers and heme iron polypeptide after gastric bypass.	1, 2, 7, 11
Ferrous gluconate	12%	In a trial in blood donors, 37.5 mg of iron a day restored lost hemoglobin and iron in less than half the time of no supplement. A review found side effects in 30.9% of users, but not in head-to-head trials. We found no randomized trial against ferrous sulfate.	1, 3
Ferrous fumarate	33%	Absorbed about as well as ferrous sulfate in women, infants and toddlers. Highest side-effect rate (47.0%) in the review of mixed study designs.	1, 3, 4
Ferrous bisglycinate (amino acid chelate)	Not stated	Absorbed 6.0% against 1.7% for ferrous sulfate from maize porridge. In pregnancy, higher hemoglobin and fewer stomach and bowel events in a meta-analysis; no difference in children.	1, 5, 6
Ferric salts (ferric citrate, ferric sulfate)	Not stated	The NIH says ferric iron is less bioavailable than ferrous. Ferric trisglycinate was absorbed 2.3% from maize.	1, 5
Ferric maltol (prescription)	Not stated	In inflammatory bowel disease, raised hemoglobin by 2.25 g/dL more than placebo at 12 weeks in people who had failed ferrous products. The sponsor was the maker.	12
Iron polysaccharide complex	Not stated	Raised hemoglobin 1.0 g/dL less than ferrous sulfate in toddlers, with more diarrhea. Did not help exercise capacity in heart failure.	7, 13
Iron(III) polymaltose complex	Not stated	In pregnancy, similar hemoglobin rise and fewer side effects in an open-label trial. In 118 children, ferrous sulfate gave higher hemoglobin and fewer complaints.	8, 9
Heme iron polypeptide	Not stated	No better than ferrous sulfate in Gambian infants; no response after gastric bypass in 14 women. The NIH says it might cause fewer stomach and bowel effects.	1, 10, 11
Sucrosomal iron (ferric pyrophosphate in a lipid carrier)	Not stated	Evidence is mostly small studies of this one product, and a 2023 review included authors who work for the company that developed it (Alesco) and for Pharmanutra, which markets sucrosomal iron products. A 60-patient open-label trial compared it with no iron, not with another form.	14, 15
Intravenous iron (iron sucrose, ferric carboxymaltose; prescription drugs)	Not applicable	More hemoglobin than oral iron in pregnancy, fewer stomach effects, but severe infusion reactions were more common (risk ratio 2.47) across 103 trials.	16, 17, 18

Some smaller points. In 19 women with anemia, the first dose was absorbed 40% to 50% better than a dose on the second day in a row. So was a dose taken after a day off. One 200 mg dose on alternate days gave about twice the iron of 100 mg on consecutive days.

In pregnancy, a Cochrane review of intermittent against daily iron found similar outcomes for mothers and babies, with fewer side effects (0.56 times the risk). But the evidence was of low or very low quality. In a Danish trial of 80 pregnant women, 25 mg of bisglycinate prevented deficiency about as well as 50 mg of ferrous sulfate. It also caused fewer stomach complaints.

For carbonyl iron and several forms sold as gentle, we found no head-to-head randomized trial. The NIH only says some forms “might” cause fewer stomach and bowel effects.

Is iron safe? Supplements, dose and interactions

Adults with normal bowel function have very little risk of iron overload from food, the NIH says. The US tolerable upper intake level, the most per day that is likely safe, is 45 mg for ages 14 and up. For infants and children up to 13, it is 40 mg. It was set from the stomach and bowel effects of iron salts and covers food and supplements together; doctors sometimes prescribe more for anemia.

EFSA could not set an upper level for iron, either in 2004 or in its 2024 review. Instead, in 2024 it set a safe level of intake of 40 mg a day for adults, including pregnant and breastfeeding women. For children and teenagers it rises with age, from 10 mg a day at ages 1 to 3 to 35 mg at 15 to 17. A safe level is a weaker tool than an upper level, because it does not say where the risk starts to rise.

In 2004 the panel judged the risk from food iron, including fortified foods, to be low. The exception was people with two copies of the hemochromatosis gene, up to 0.5% of the population. It warned that supplements may raise the share of men and postmenopausal women with signs of high iron stores in blood tests.

• Stomach and bowel side effects. High doses can cause nausea, constipation, stomach pain, vomiting and diarrhea, the NIH says. In the Cochrane review of menstruating women, constipation was twice as likely on iron (2.07 times the risk). Taking iron with food helps, and in a trial, alternate-day dosing caused fewer stomach side effects (1.56 times as common with daily dosing). Supplements with 25 mg of iron or more can also lower zinc absorption.
• Children and accidental overdose. The FDA requires solid iron supplements to carry a warning. It says accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Keep the product out of reach of children. In case of accidental overdose, call a doctor or poison control center immediately. At least 43 US children died from iron supplements between 1983 and 2000, the NIH says. Swallowing more than 20 mg per kg of body weight can damage the intestine. Never give a child an adult iron product.
• Hemochromatosis and iron overload. About 4.4 in 1,000 White people inherit the common gene change from both parents. The NIH says the American Association for the Study of Liver Diseases advises people with hemochromatosis to avoid iron and vitamin C supplements. In the Melbourne cohort, 28.4% of men who carry it from both parents had iron-overload disease, against 1.2% of women. The EASL guideline (European liver experts) says early diagnosis and treatment by phlebotomy (regular blood removal) can prevent cirrhosis, liver cancer, diabetes and joint damage. Higher heme-iron intake has been linked to type 2 diabetes and colorectal cancer in observational studies, which cannot show cause.
• Infants and malaria areas. A large trial in Pemba linked iron and folic acid to more hospital admissions. The Cochrane review, though, found no overall increase in clinical malaria. The WHO advises giving iron in malaria-endemic areas along with steps to prevent, diagnose and treat malaria, the NIH says.
• Medicines. Taken with ferrous sulfate, levothyroxine (a thyroid hormone pill) worked less well in 14 people: TSH rose from 1.6 to 5.4 mU/L. The NIH says some labels advise waiting 4 hours. Ferrous sulfate cut peak levodopa (a Parkinson’s drug) levels by 55% in 8 healthy volunteers. In crossover trials, it lowered absorption of the antibiotics norfloxacin by 73%, ciprofloxacin by 57% and ofloxacin by 25%. The authors advised against taking norfloxacin or ciprofloxacin together with it. Proton pump inhibitors reduce stomach acid and iron absorption: two or more years of use went with 2.49 times the odds of iron deficiency. Calcium supplements should also be taken at a different time.
• Who should ask a doctor first. Pregnant women: the ACOG (US obstetricians) advises low-dose iron. The USPSTF (a US prevention panel) finds the evidence insufficient for routine supplements. Also men and postmenopausal women, people with heart failure, bowel disease or kidney disease, anyone who takes the medicines above, and parents of young children.

How to get more iron

• Pair plant iron with vitamin C or a little meat or fish. The EU authorizes claims for both, and in single-meal studies vitamin C offset phytate. But in complete diets, vitamin C intakes of 51 to 247 mg a day made no clear difference. So a vitamin C pill is not a fix for a low-iron diet.
• Keep tea, coffee and calcium away from iron-rich meals and pills. Black tea cut absorption from a bread meal by 79% to 94%, and 165 mg of calcium by 50% to 60%. Adding milk to tea or coffee changed little. The NIH suggests taking calcium and iron supplements at different times of the day.
• Cook in cast iron and look for fortified foods. In a review of three trials, food cooked in iron pots raised hemoglobin in anemic or iron-deficient people. Results were shaped by use, age, malaria and hookworm. Wheat and other flours are fortified with iron in the US and Canada.

History of iron

For centuries, young women fell ill with a condition called chlorosis, or green sickness. It was named at the start of the 17th century. Many explanations were offered, including psychological ones, before it was recognized as hypochromic anemia (pale red cells), or iron deficiency.

In the 19th century, hydrotherapy (water cures) gave way to iron therapy, and by the early 20th century the disease had disappeared. In 1898 Catharine van Tussenbroek, the first female Dutch gynecologist, pointed to the social factors at its root.

The hormone that controls iron was found by accident. In 2000 one team reported a new germ-killing peptide from human blood. In 2001 another described a peptide from human urine and named it hepcidin, for its liver origin and its killing of microbes. In 2004 two papers showed that hepcidin binds ferroportin and shuts the iron door, and that the inflammation messenger IL-6 switches it on.

Iron’s danger to children produced a rare public health success. The US required unit-dose packaging of iron supplements in 1997. In the years after, poison-center deaths in children under 6 fell from 29 to 1, as the study cards above show. A court decision led the FDA to drop the packaging requirement in 2003, but the warning label remains.

Frequently asked questions about iron

Which foods are highest in iron?

In our USDA ranking per serving, soybeans lead with 8.84 mg in one cup (49% of the Daily Value). Lentils (6.59 mg per cup) and chickpeas (4.74 mg per cup) follow, and oysters give 4.34 mg in 3 oz. The NIH lists pan-fried beef liver at 5 mg in 3 oz and fortified breakfast cereals at up to 18 mg a serving. Heme iron from meat and seafood is better absorbed than the plant iron in beans.

How much iron do vegetarians need?

The NIH says the requirement is 1.8 times higher for people on vegetarian diets, because plant iron is absorbed less well. That works out to about 14 mg a day for men and 32 mg for women aged 19 to 50. Bioavailability is 5% to 12% on vegetarian diets against 14% to 18% on mixed diets. Ask for a ferritin test before taking a supplement.

Does coffee or tea block iron absorption?

Yes, in test meals. Black tea cut iron absorption from a bread meal by 79% to 94%, peppermint tea by 84% and cocoa by 71%. Adding milk to tea or coffee changed little, and the NIH says such effects are smaller in a typical Western diet. The simple way around it is to drink them away from iron-rich meals and iron pills.

Is it better to take iron every day or every other day?

Absorption is better on alternate days: 21.8% against 16.3% in a trial of iron-depleted women, because each dose raises hepcidin for about 24 hours. A larger trial found the same ferritin with fewer stomach side effects on alternate days. A pooled analysis of 11 trials found a small edge for daily dosing that could be chance, with very low certainty. Ask your doctor, especially if you are anemic or pregnant.

Which form of iron is best tolerated?

No form has been shown to be clearly gentler in every group. Ferrous sulfate raised stomach side effects 2.32 times against placebo, while bisglycinate caused fewer gut problems than other iron in pregnancy (0.36 times the rate). Iron polysaccharide complex caused more diarrhea than ferrous sulfate in toddlers (58% against 35%). Taking iron with food also helps, the NIH says.

Does iron help with tiredness if I am not anemic?

Possibly, if your iron stores are low. Pooled trials in adults with low ferritin but no anemia found less self-reported fatigue (standardized mean difference −0.38), but no change in exercise tests. In one trial, the gain was only in women with ferritin of 50 µg/L or less. If your iron is normal, the evidence of benefit is thin, so a blood test comes first.

Can too much iron be harmful?

Yes. The US upper level for adults is 45 mg a day, and the EU safe level is 40 mg a day. Both count food and supplements together.

Doses of 45 mg a day or more make stomach upset more likely. People with hemochromatosis can build up iron in the liver, heart and pancreas. Iron pills are also a leading cause of fatal poisoning in children under 6, which is why US labels carry a warning. Men and postmenopausal women should not take iron without a reason.

Does cooking in a cast iron pan add iron?

It can. A review of three trials found that food cooked in iron pots raised hemoglobin in anemic or iron-deficient people. Results depended on how the pots were used and on malaria and hookworm. The review’s authors call it a promising approach that needs more research; it adds to, but does not replace, a diet with enough iron.