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Iron

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

Iron is an essential trace mineral and a central component of hemoglobin, enabling red blood cells to carry oxygen throughout the body. It is the most common nutritional deficiency worldwide, disproportionately affecting women of reproductive age, children, and athletes.

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 — RCT
PubMed

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. (Vaucher et al. 2012, 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 — Systematic Review
PubMed

Systematic review of 14 RCTs examining iron supplementation effects on cognitive function in iron-deficient adolescent girls. (Falkingham et al. 2010, Nutr J)

Pooled across 14 RCTs 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
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Frequently asked questions about Iron

What is Iron?

Iron is an essential trace mineral and a central component of hemoglobin, enabling red blood cells to carry oxygen throughout the body.

What does Iron do?

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. In clinical research, Iron has been studied for oxygen transport, energy metabolism, cognitive function.

Who should take Iron?

Iron may be most beneficial for: 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. As with any supplement, consult your healthcare provider before starting, especially if you have medical conditions or take prescription medications.

How long does Iron take to work?

Most clinical trial effects appear over weeks of consistent use; individual response varies. Acute or same-day effects (where applicable) typically appear within hours, but most cumulative benefits — particularly those affecting biomarkers, mood, sleep quality, or chronic symptoms — require 4-12 weeks of regular use to fully assess. If you don't notice benefit after 12 weeks at the appropriate dose, it may not be your responder.

When is the best time to take Iron?

For performance or energy goals, Iron is typically taken 30-60 minutes before exercise or in the morning. Some people take it with food to reduce GI sensitivity; others prefer empty-stomach timing for faster absorption. Always check product labeling and follow personalized guidance from your healthcare provider.

Is Iron worth taking?

Iron has strong clinical evidence (Evidence Level 5/5 on NutraSmarts) for its primary uses, with multiple randomized controlled trials and meta-analyses supporting its benefits. Whether it's worth taking depends on your specific goals, what you've already tried, your budget, and your overall supplement strategy. The honest framing: no supplement is essential for most people, and lifestyle factors (sleep, exercise, diet, stress management) typically produce larger effects than any single supplement. Iron is most worth trying if its evidence-supported uses align with your specific goals.

What is the recommended dosage of Iron?

The clinically studied dose for Iron is 18 mg/day (women RDA); 27 mg/day (pregnant); 8 mg/day (men); therapeutic: 150–200 mg elemental iron/day. Always follow product labeling and consult a healthcare provider for personalized dosing recommendations.

What is Iron used for?

Iron is studied for oxygen transport, energy metabolism, cognitive function. 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.