Iron
trace-mineral
Key Takeaways
- Iron is a component of hemoglobin and myoglobin, critical for transporting oxygen throughout the body
- FDA daily value is 18 mg; adult men and postmenopausal women need 8 mg/day, premenopausal women need 18 mg/day
- Dietary iron exists in two forms: heme (from animal sources, better absorbed) and nonheme (from plants)
- Iron deficiency is one of the most common nutrient deficiencies worldwide, with WHO estimating approximately half of 1.62 billion anemia cases are iron-related
- Excessive iron intake can be harmful; supplementation should be guided by documented deficiency
Evidence Spectrum
14 studies reviewed →Prevention and treatment of iron deficiency anemia
Iron supplementation effectively corrects iron deficiency anemia. The WHO estimates approximately half of 1.62 billion anemia cases worldwide are due to iron deficiency. Supplementation restores hemoglobin levels and improves associated symptoms including fatigue, weakness, and impaired cognitive function.14
Cognitive function and neurological development
Iron deficiency impairs cognitive function, and iron plays essential roles in energy-yielding metabolism, DNA synthesis, oxygen transport, and neuronal functions critical for brain and muscular function. A 2014 review confirmed the importance of adequate iron for neurodevelopmental processes during pregnancy and infancy. Iron deficiency can lead to lasting cognitive deficits, particularly in early life.345
Athletic performance
Iron deficiency, with or without anemia, impairs muscle function and limits work capacity. Severe deprivation of folate and vitamin B12 combined with iron deficiency reduces endurance performance. However, supplementation in well-nourished athletes does not enhance physical performance beyond baseline.27
Thyroid and autoimmune health
Iron is among the micronutrients whose role is debated in Hashimoto's thyroiditis management. Iron deficiency may negatively affect thyroid function, but direct evidence from supplementation trials for thyroid outcomes remains limited.6
14
Studies Reviewed
0.27 mg
RDA (Infants 0-6 months (AI))
45 mg
Upper Limit
Role in the Body
Iron is an essential mineral with a central role in oxygen transport and energy metabolism. As a core component of hemoglobin in red blood cells, iron enables the transfer of oxygen from the lungs to tissues throughout the body. Iron also forms part of myoglobin, a protein that provides oxygen to muscles and supports muscle metabolism and connective tissue health. Beyond oxygen transport, iron is necessary for physical growth, neurological development, cellular functioning, and the synthesis of some hormones. Most of the 3 to 4 grams of elemental iron in adults is found in hemoglobin. The remainder is stored as ferritin or hemosiderin in the liver, spleen, and bone marrow, or located in myoglobin. Transferrin is the main blood protein that binds and transports iron. Dietary iron exists in two forms: heme iron (from meat, seafood, and poultry) and nonheme iron (from plants and fortified foods). Heme iron contributes about 10-15% of total iron intake in Western populations but is significantly better absorbed. Iron homeostasis is regulated by hepcidin, a peptide hormone that controls both iron absorption and distribution throughout the body.
- Oxygen transport via hemoglobin in red blood cells
- Oxygen storage in muscle tissue via myoglobin
- Energy-yielding metabolism and ATP production
- DNA synthesis
- Neurological development and cognitive function
- Hormone synthesis
- Cellular functioning and growth
- Immune function
Supplement Forms
Ferrous sulfate
RecommendedBioavailability: 20%
Most commonly prescribed form. Contains approximately 20% elemental iron. Inexpensive but may cause GI side effects.
Ferrous gluconate
RecommendedBioavailability: 12%
Contains approximately 12% elemental iron. Generally better tolerated than ferrous sulfate with fewer GI side effects.
Ferrous fumarate
RecommendedBioavailability: 33%
Contains approximately 33% elemental iron. Higher concentration of elemental iron per dose.
Iron bisglycinate (chelated)
RecommendedBioavailability: 20%
Chelated form with improved tolerability. May be better absorbed with fewer gastrointestinal side effects.
Heme iron polypeptide
Bioavailability: 25%
Derived from animal hemoglobin. Absorption is less affected by dietary factors but more expensive.
Food Sources
Fortified cereals (e.g., Quaker Quick Oats with Iron)
49.4 mg per 100g
Infant formula, powder, with iron
7.7-9.5 mg per 100g
Baby yogurt, fortified with iron
4.18 mg per 100g
Red meat (beef, lamb)
2-3 mg per 100g (heme iron)
Poultry (chicken, turkey)
1-2 mg per 100g (heme + nonheme)
Lentils and beans
2-3 mg per 100g (nonheme)
Spinach (cooked)
3.6 mg per 100g (nonheme)
Tofu
2-5 mg per 100g (nonheme)
Deficiency
Prevalence: One of the most common nutrient deficiencies globally. The WHO estimates approximately half of 1.62 billion anemia cases worldwide are due to iron deficiency. In the United States, iron deficiency is especially common among young children, women of reproductive age, and pregnant women.
Symptoms:
- Fatigue and weakness
- Impaired cognitive function and difficulty concentrating
- Gastrointestinal disturbances
- Impaired body temperature regulation
- Reduced immune function
- Pale skin and brittle nails
- In children: impaired growth and neurological development
Risk Factors:
- Premenopausal women (menstrual blood loss)
- Pregnant women
- Young children and infants
- Vegetarians and vegans (lower bioavailability of nonheme iron)
- Frequent blood donors
- People with malabsorptive disorders (celiac disease, IBD)
- Post-bariatric surgery patients (pmid:34302218)
- Athletes with high training loads
- People with gastrointestinal parasites (developing countries)
Safety & Interactions
Possible Side Effects:
- • Gastrointestinal distress: nausea, constipation, diarrhea, stomach pain (most common with ferrous sulfate)
- • Dark-colored stools (harmless)
- • Iron overload risk with chronic excessive supplementation
- • Acute iron poisoning (particularly dangerous in children)
Drug Interactions:
- • Vitamin C: enhances nonheme iron absorption when consumed together
- • Calcium: inhibits iron absorption when taken simultaneously; separate doses by 2+ hours
- • Tea, coffee, and phytates: reduce nonheme iron absorption
- • Antacids and proton pump inhibitors: reduce iron absorption by raising stomach pH
- • Levothyroxine: iron can reduce thyroid medication absorption; separate by 4+ hours
Contraindications:
- • Hemochromatosis (hereditary iron overload)
- • Hemosiderosis
- • Repeated blood transfusions
- • Do not supplement without confirmed deficiency or medical guidance
Frequently Asked Questions
How much iron do I need daily?
Adult men and postmenopausal women need 8 mg/day. Premenopausal women need 18 mg/day due to menstrual blood loss. The FDA daily value is 18 mg. Pregnant women require 27 mg/day.
What is the difference between heme and nonheme iron?
Heme iron comes from animal sources (meat, poultry, seafood) and is more readily absorbed. Nonheme iron comes from plants and fortified foods and its absorption is more affected by dietary factors. Consuming vitamin C with nonheme iron sources improves absorption.
Should I take an iron supplement?
Iron supplementation should generally be guided by a documented deficiency through blood tests (serum ferritin, hemoglobin). Unnecessary iron supplementation can cause side effects and, in people with hemochromatosis, can lead to dangerous iron overload.
What are the signs of iron deficiency?
Common signs include persistent fatigue, weakness, difficulty concentrating, pale skin, brittle nails, and frequent infections. Iron deficiency progresses through stages from depleted stores to iron-deficiency anemia. A blood test is needed for definitive diagnosis.
Research Sources
15 peer-reviewed studies analyzed from PubMed. 7 directly cited in this review.