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Iron

Evidence Level
Very Strong
2 Clinical Trials
4 Documented Benefits
5/5 Evidence Score

Iron is an essential mineral best known for its central role in making hemoglobin, the protein in red blood cells that carries oxygen throughout the body. Adequate iron supports healthy energy, exercise capacity, cognition, and immune function, and correcting a deficiency can relieve the fatigue, weakness, and pallor of iron-deficiency anemia. Iron is especially important for menstruating women, pregnant women, athletes, and plant-based eaters, who are at higher risk of running low. Because too much iron is harmful, supplements should generally be used to correct a confirmed deficiency, with vitamin C improving absorption and gentle forms like ferrous bisglycinate easing common digestive side effects.

Studied Dose 18 mg/day (women RDA); 27 mg/day (pregnant); 8 mg/day (men); therapeutic: 150–200 mg elemental iron/day
Active Compound Ferrous bisglycinate (best tolerated) / Ferrous sulfate / Ferrous fumarate
Deficiency information View details

Iron deficiency is the world's most common nutritional deficiency, affecting an estimated 25% of the global population. In the US, it's most common in menstruating women, pregnant women, infants, and people with GI bleeding. Iron deficiency anemia develops when stored iron runs out and red blood cell production is impaired.

Common symptoms

  • Fatigue and decreased exercise tolerance
  • Pale skin, especially inside the lower eyelid
  • Shortness of breath with exertion
  • Cold hands and feet
  • Brittle or spoon-shaped nails
  • Hair thinning or hair loss
  • Restless legs syndrome (especially at night)
  • Pica — unusual cravings for ice, dirt, or starch
  • Headaches and lightheadedness

At-risk groups

  • Menstruating women, especially those with heavy periods
  • Pregnant women (iron requirements nearly double)
  • Infants and toddlers, especially those weaned to cow's milk early
  • Adolescents during growth spurts
  • Vegetarians and vegans (plant iron is less bioavailable than heme iron)
  • Endurance athletes (foot-strike hemolysis, sweat losses)
  • People with GI conditions causing chronic blood loss (ulcers, IBD, colon cancer)
  • Frequent blood donors
  • People who've had bariatric surgery
When to see a doctor: Persistent fatigue with any combination of pale skin, shortness of breath, hair thinning, or unusual cravings warrants a CBC plus ferritin test. Important: do NOT begin iron supplementation without testing first — iron overload can cause organ damage, and iron supplements interact with many medications.
Explore related deficiencies →
Sources: NIH ODS — Iron Health Professional Fact Sheet · PMID 33549739 — Iron deficiency anemia (Lancet)

Benefits

Oxygen transport

Iron is a core component of hemoglobin in red blood cells and myoglobin in muscle tissue, enabling oxygen delivery from lungs to tissues and carbon dioxide removal.

Supported by Trial 1, Trial 2

Energy metabolism

Required for mitochondrial cytochromes involved in ATP production. Iron deficiency impairs oxidative phosphorylation even before anemia develops, causing fatigue and reduced work capacity.

Supported by Trial 1

Cognitive function

Iron is essential for dopamine and serotonin synthesis, myelin formation, and neuronal energy metabolism. Deficiency in children impairs cognitive development, attention, and learning.

Supported by Trial 2

Immune support

Required for proliferation of immune cells and production of reactive oxygen species by macrophages to kill pathogens. Both deficiency and excess impair immune function.

Supported by Trial 2, Trial 1

Mechanism of action

1

Hemoglobin and myoglobin formation

Iron binds to protoporphyrin IX to form heme, which is incorporated into globin proteins to create hemoglobin and myoglobin. Each hemoglobin molecule contains four iron atoms.

2

Electron transport chain function

Iron-sulfur clusters and heme groups in Complexes I, II, III, and IV of the mitochondrial electron transport chain shuttle electrons during oxidative phosphorylation, producing the majority of cellular ATP.

3

Neurotransmitter synthesis

Iron is a cofactor for tyrosine hydroxylase (dopamine synthesis) and tryptophan hydroxylase (serotonin synthesis). Deficiency reduces neurotransmitter production, affecting mood, attention, and cognitive performance.

Clinical trials

1
Iron for Fatigue in Non-Anemic Iron-Deficient Women — Clinical Trial

Randomized, double-blind, placebo-controlled trial of oral iron (80 mg ferrous sulfate elemental) vs placebo in 198 women aged 18-53 with unexplained fatigue and ferritin <50 µg/L (low iron stores) but no anemia (Hb >12 g/dL) for 12 weeks. (CMAJ)

198 non-anemic iron-deficient women with fatigue. 12-week intervention.

Iron significantly reduced fatigue scores vs placebo (47.7% reduction vs 28.8%). Improvement correlated with rising ferritin levels. Important finding: iron deficiency without anemia (low ferritin, normal Hb) can cause fatigue and supplementation may be beneficial. Context: chronic iron supplementation should be guided by labs (ferritin, transferrin saturation, TIBC) — not symptom-based; iron overload is harmful.

2
Iron for Cognition in Adolescent Girls — Evidence Review

Evidence review of 14 clinical trials examining iron supplementation effects on cognitive function in iron-deficient adolescent girls. (Nutr J)

Pooled across 14 clinical trials of adolescent girls.

Iron supplementation improved attention, concentration, and IQ scores in iron-deficient adolescents. Effects most pronounced in those with lowest baseline iron status. Adolescent girls (especially post-menarche) are at elevated risk for iron deficiency. Adequate iron status critical for cognitive development.

Side effects and drug interactions

Common Potential side effects

GI side effects most common: constipation, nausea, dark stools, abdominal cramping
Iron overload (hemosiderosis) with chronic excess supplementation, especially in men
Nausea reduced by taking with food, though absorption is lower with calcium-rich foods

Important Drug interactions

Calcium, antacids, and dairy products significantly reduce iron absorption — separate by 2 hours
Tetracycline and quinolone antibiotics — iron chelates drug molecules, reducing antibiotic absorption
Levothyroxine — iron reduces thyroid hormone absorption; separate by at least 4 hours
Vitamin C enhances non-heme iron absorption — take together to improve efficacy

Featured in these supplement guides

Brain Fog→Energy & Fatigue→Fertility→Hair Loss→Immune Support→Low Iron & Anemia Support→Restless Legs Syndrome→Thyroid Support→Vertigo & Dizziness→

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Frequently asked questions about Iron

How much iron should I take?

It depends on need. The RDA is 8 mg for men and 18 mg for premenopausal women (27 mg in pregnancy). Supplements for deficiency often provide 18 to 65 mg of elemental iron. Do not take high-dose iron without a blood test confirming you need it, since excess iron is harmful.

What is the best way to absorb iron?

Take iron with a source of vitamin C, such as orange juice, to boost absorption, and on an empty stomach if you tolerate it. Avoid taking it with coffee, tea, dairy, or calcium supplements, which sharply reduce absorption.

Why does iron upset my stomach?

Iron commonly causes constipation, nausea, or cramping, especially the ferrous sulfate form. Taking it with a little food, using a gentler form like ferrous bisglycinate, or dosing every other day can help. Dark stools are a normal, harmless effect.

Should I take iron every day or every other day?

Recent research suggests that for many people, every-other-day dosing absorbs as well as or better than daily dosing, because a daily dose temporarily raises a hormone (hepcidin) that blocks further absorption. Every-other-day dosing also tends to cause fewer side effects. Follow your doctor's guidance when treating a diagnosed deficiency.

What is Iron?

Iron is an essential mineral best known for its central role in making hemoglobin, the protein in red blood cells that carries oxygen throughout the body. Adequate iron supports healthy energy, exercise capacity, cognition, and immune function, and correcting a deficiency can relieve the fatigue, weakness, and pallor o…

What is Iron used for?

Iron is researched primarily for Energy and Athletic Performance. Iron is a core component of hemoglobin in red blood cells and myoglobin in muscle tissue, enabling oxygen delivery from lungs to tissues and carbon dioxide removal.

What are the signs of Iron deficiency?

Iron deficiency is the world's most common nutritional deficiency, affecting an estimated 25% of the global population. In the US, it's most common in menstruating women, pregnant women, infants, and people with GI bleeding.

What is the recommended dosage of Iron?

The clinically studied dose is 18 mg/day (women RDA); 27 mg/day (pregnant); 8 mg/day (men); therapeutic: 150–200 mg elemental iron/day Always follow the product label and check with a healthcare provider for personal advice.

Is Iron safe, and does it have side effects?

For most healthy adults, Iron is well tolerated at studied doses. Reported effects can include: GI side effects most common: constipation, nausea, dark stools, abdominal cramping Iron overload (hemosiderosis) with chronic excess supplementation, especially in men It may also interact with some medications. Iron is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Iron interact with any medications?

Possible interactions include: Calcium, antacids, and dairy products significantly reduce iron absorption — separate by 2 hours Tetracycline and quinolone antibiotics — iron chelates drug molecules, reducing antibiotic absorption If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Iron?

NutraSmarts rates the evidence for Iron as Very Strong (5 out of 5). It is backed by 2 clinical trials and 8 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(8 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012;184(11):1247-54. doi: 10.1503/cmaj.110950.PubMedUsed to support: RCT in 198 premenopausal women with fatigue and low ferritin (<50 μg/L) but not anemic: 12 weeks of oral iron significantly reduced fatigue scores (47.7% reduction vs 28.8% placebo). Backs the page's claim that iron deficiency without anemia can cause fatigue and benefits from supplementation.
  2. Allen RP, Picchietti DL, Auerbach M, Cho YW, Connor JR, Earley CJ, Garcia-Borreguero D, Kotagal S, Manconi M, Ondo W, Ulfberg J, Winkelman JW. Evidence-based and consensus clinical practice guidelines for the iron treatment of restless legs syndrome/Willis-Ekbom disease in adults and children: an IRLSSG task force report. Sleep Med. 2018;41:27-44. doi: 10.1016/j.sleep.2017.11.1126.PubMedUsed to support: IRLSSG task force consensus guidelines on iron treatment of restless legs syndrome in adults and children. Establishes iron repletion (oral or IV based on ferritin thresholds) as first-line therapy when ferritin <75 μg/L. Supports the role of iron status in RLS pathophysiology.
  3. Peña-Rosas JP, De-Regil LM, Garcia-Casal MN, Dowswell T. Daily oral iron supplementation during pregnancy. Cochrane Database Syst Rev. 2015;2015(7):CD004736. doi: 10.1002/14651858.CD004736.pub5.PubMedUsed to support: Cochrane review of 61 trials: daily oral iron in pregnancy reduced maternal anemia at term by 70% and iron deficiency by 57%. Effects on low birthweight and preterm birth were less clear. Backs the page's stance that iron supplementation is appropriate during pregnancy (27 mg/day RDA).
  4. Stoffel NU, Cercamondi CI, Brittenham G, Zeder C, Geurts-Moespot AJ, Swinkels DW, Moretti D, Zimmermann MB. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533. doi: 10.1016/S2352-3026(17)30182-5.PubMedUsed to support: Two open-label RCTs in iron-depleted women: daily and twice-daily iron dosing triggered higher serum hepcidin and reduced subsequent absorption. Alternate-day single-dose regimens optimized cumulative iron absorption (21.8% vs 16.3% consecutive-day). Key reference for modern supplement-timing guidance.
  5. Lozoff B, Beard J, Connor J, Felt B, Georgieff M, Schallert T. Long-lasting neural and behavioral effects of iron deficiency in infancy. Nutr Rev. 2006;64(5 Pt 2):S34-S43. doi: 10.1301/nr.2006.may.s34-s43.PubMedUsed to support: Comprehensive review of follow-up studies: iron deficiency in infancy is associated with persisting cognitive, motor, and social-emotional deficits from preschool through adolescence, with neurophysiologic differences attributed to disrupted neurometabolism, myelination, and neurotransmitter function during brain development.
  6. Falkingham M, Abdelhamid A, Curtis P, Fairweather-Tait S, Dye L, Hooper L. The effects of oral iron supplementation on cognition in older children and adults: a systematic review and meta-analysis. Nutr J. 2010;9:4. doi: 10.1186/1475-2891-9-4.PubMedUsed to support: Meta-analysis of 14 RCTs in older children, adolescents, and women: iron supplementation improved attention and concentration across participants regardless of baseline iron, and produced a modest 2.5-point IQ increase specifically in anaemic participants. No significant effects on memory, psychomotor skills, or scholastic achievement.
  7. Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832-43. doi: 10.1056/NEJMra1401038.PubMedUsed to support: NEJM comprehensive review of iron-deficiency anemia covering global epidemiology, iron homeostasis, hepcidin regulation, diagnostic workup, oral and IV treatment, and iron-resistant IDA. Backs the page's overall framing of iron as the most common nutritional deficiency worldwide.
  8. Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. Iron deficiency. Lancet. 2021;397(10270):233-248. doi: 10.1016/S0140-6736(20)32594-0.PubMedUsed to support: Lancet seminar on iron deficiency: comprehensive coverage of iron physiology, screening, oral vs parenteral treatment, and clinical management across at-risk populations (children, premenopausal women, low- and middle-income countries). Key modern reference for the page's discussion of iron deficiency epidemiology and intervention.