Iron: Benefits, Dosage, and Safety

What iron is

Iron is an essential trace mineral naturally present in many foods, added to some fortified food products, and available as a dietary supplement. Dietary iron comes in two main forms: heme iron (from meat, seafood, and poultry) and nonheme iron (from plants and iron-fortified foods). Adults carry about 3 to 4 grams of elemental iron, most of it in hemoglobin, with the remainder stored as ferritin or hemosiderin in the liver, spleen, and bone marrow or held in muscle myoglobin.

What it does

• Serves as an essential component of hemoglobin, the red blood cell protein that carries oxygen from the lungs to the tissues
• Forms part of myoglobin, a protein that supplies oxygen to muscle and supports muscle metabolism and healthy connective tissue
• Supports physical growth and neurological development
• Contributes to normal cellular functioning
• Is needed for the synthesis of some hormones

How much iron per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Iron deficiency progresses through stages, from depletion of iron stores (falling serum ferritin) to iron-deficient erythropoiesis, and finally to iron deficiency anemia (IDA), which is marked by low hemoglobin and reduced hematocrit and mean corpuscular volume. Functional effects reported by ODS include gastrointestinal disturbances, weakness, fatigue, difficulty concentrating, and impaired cognitive function, immune function, exercise or work performance, and body temperature regulation. In infants and children, IDA can cause psychomotor and cognitive abnormalities that, without treatment, can lead to learning difficulties; some effects may be irreversible. A serum ferritin below 30 mcg/L suggests iron deficiency and below 10 mcg/L suggests IDA.

Who may need more

• Pregnant women, who have substantially increased iron requirements (RDA 27 mg)
• Premenopausal women, including those with heavy menstrual bleeding
• Infants and young children with high iron needs from rapid growth
• Frequent blood donors
• People with malabsorptive gastrointestinal conditions or after GI surgery
• People with diagnosed iron deficiency or iron deficiency anemia under medical guidance
• Vegetarians and vegans, who need about 1.8 times the RDA

Food sources

Richest heme sources are lean meat and seafood. Per ODS: fortified breakfast cereals (100% DV, 18 mg per serving), cooked eastern oysters (8 mg per 3 oz), canned white beans (8 mg per cup), pan-fried beef liver (5 mg per 3 oz), boiled lentils (3 mg per half cup), boiled spinach (3 mg per half cup), firm tofu (3 mg per half cup), dark chocolate 45-69% cacao (2 mg per oz), canned kidney beans (2 mg per half cup), and canned sardines with bone (2 mg per 3 oz). About half of dietary iron in the US comes from bread, cereal, and other grain products.

Forms and absorption

Common supplement forms are ferrous and ferric iron salts, including ferrous sulfate, ferrous gluconate, ferric citrate, and ferric sulfate. Because of its higher solubility, ferrous iron is more bioavailable than ferric iron. Forms contain different amounts of elemental iron by weight: ferrous fumarate is about 33% elemental iron, ferrous sulfate about 20%, and ferrous gluconate about 12%; elemental iron is listed on the Supplement Facts panel. Other forms such as heme iron polypeptides, carbonyl iron, iron amino-acid chelates, and polysaccharide-iron complexes may cause fewer gastrointestinal side effects. Iron-only supplements often provide about 65 mg (360% of the DV), while multivitamin/mineral products for women typically provide 18 mg.

Safety, excess and interactions

Too much

Supplements containing 25 mg iron or more can reduce zinc absorption and plasma zinc concentrations. High doses of supplemental iron (45 mg per day or more) commonly cause gastrointestinal side effects including gastric upset, constipation, nausea, abdominal pain, vomiting, and diarrhea; taking iron with food can reduce these effects. Case reports, some at doses around 130 mg, describe gastritis and gastric lesions. Acute intakes above 20 mg/kg (about 1,365 mg for a 150-lb person) from supplements or medicines can cause corrosive necrosis of the intestine, fluid and blood loss, shock, tissue damage, and organ failure; one-time ingestions around 60 mg/kg (about 4,090 mg for a 150-lb person) can lead to multisystem organ failure, coma, convulsions, and death. People with hereditary hemochromatosis can accumulate excess iron leading to organ damage.

Interactions to know

• Levodopa: iron supplements can reduce absorption of levodopa (in Sinemet and Stalevo), possibly by chelation, and may diminish its clinical effectiveness
• Levothyroxine: taking iron at the same time as levothyroxine (Synthroid, Levoxyl, Tirosint, Unithroid) can reduce levothyroxine absorption and efficacy; labels advise separating iron by at least 4 hours
• Proton pump inhibitors: drugs such as lansoprazole (Prevacid) and omeprazole (Prilosec) lower stomach acidity and can reduce nonheme iron absorption, and people with iron deficiency may respond suboptimally to iron supplements
• Calcium: calcium may reduce the bioavailability of both nonheme and heme iron, though the effect is not definitively established; some experts suggest taking calcium and iron supplements at different times
• Zinc: supplemental iron at 25 mg or more can reduce zinc absorption and plasma zinc concentrations

How status is measured

Iron status is assessed mainly with hematological indicators. Serum ferritin, a measure of body iron stores, is the most efficient and cost-effective test for diagnosing iron deficiency (below 30 mcg/L suggests deficiency, below 10 mcg/L suggests IDA), though it rises with inflammation. Hemoglobin and hematocrit are the most common screening tests but are neither sensitive nor specific; transferrin saturation and mean corpuscular volume are also used. No single indicator fully captures iron status.

Iron is regulated by the FDA as a dietary ingredient in foods and dietary supplements, not as a drug; the Daily Value (DV) used on labels is 18 mg for adults and children age 4 and older. FDA requires food labels to list iron content. Dietary supplements are not FDA-approved to diagnose, treat, cure, or prevent disease.

What the evidence supports

We grade the strength of evidence behind each common reason people take iron, mirroring how the NIH describes it.