Iron: Complete Guide
Essential mineral for oxygen transport and energy. Deficiency is the most common nutrient deficiency worldwide, especially in women. But supplementing without testing can be dangerous.
Quick Summary
Essential mineral for oxygen transport and energy. Deficiency is the most common nutrient deficiency worldwide, especially in women. But supplementing without testing can be dangerous. 18mg daily for premenopausal women (RDA). Best form: Ferrous Bisglycinate.
What is Iron?
Iron is a trace mineral essential for hemoglobin (carries oxygen in blood) and myoglobin (carries oxygen in muscles). It's also critical for energy production, DNA synthesis, and immune function. Unlike most supplements, iron has a narrow therapeutic window: too little causes anemia, too much causes organ damage.
Who Should Consider Iron?
- Women with heavy menstrual periods (most common cause of deficiency)
- Pregnant women (blood volume increases 50%)
- Vegetarians and vegans (plant iron is 2-20% absorbed vs 15-35% from meat)
- Endurance athletes (foot-strike hemolysis + sweat losses)
- People with confirmed deficiency via blood test
- NOT men or postmenopausal women (rarely deficient, overload risk)
How It Works
Iron binds to hemoglobin in red blood cells, enabling them to carry oxygen from lungs to every cell in your body. It's also part of cytochrome enzymes in mitochondria (your cellular power plants). Your body has no active mechanism to excrete excess iron, which is why overload is dangerous. Absorption is tightly regulated by hepcidin, a liver hormone.
Forms Comparison
Different forms of Iron have varying absorption rates and best uses. Here's how they compare:
| Form | Bioavailability | Best For |
|---|---|---|
| Ferrous Bisglycinate | 90% | Best tolerated form. High absorption, minimal GI side effects. |
| Ferrous Sulfate | 10-15% | Cheapest. Effective but causes more GI issues. Common in prescriptions. |
| Ferrous Gluconate | 10-15% | Slightly gentler than sulfate. Lower iron per pill, may need more. |
| Carbonyl Iron | 70% | Very slow release. Fewer acute side effects. Lower overdose risk. |
Our recommendation: Ferrous Bisglycinate for most people due to superior absorption and tolerability.
Benefits & Evidence
Iron has 2 strongly-evidenced benefits and 2 moderately-evidenced benefits.
Treating iron deficiency anemia
strong evidenceFirst-line treatment. Hemoglobin improves within 2-4 weeks.
Energy and reduced fatigue
strong evidenceFatigue resolves in 80% of iron-deficient individuals within 12 weeks
Cognitive function restoration
moderate evidenceIron deficiency impairs concentration, memory, and learning
Exercise performance
moderate evidenceVO2 max improves when deficiency is corrected
Dosing & Timing
Recommended Doses
Timing
Side Effects & Interactions
Possible Side Effects
- Constipation and nausea. Common with ferrous sulfate
- Dark stools. Universal (harmless)
- Iron overload (hemochromatosis). Rare but dangerous if genetic
Interactions to Know
- Vitamin C increases iron absorption by 67% (take together)
- Calcium reduces absorption by 40-60% (separate by 2 hours)
- Coffee and tea reduce absorption (tannins and polyphenols)
- Zinc and iron compete for absorption (separate if taking both)
Frequently Asked Questions
No. Iron is one of the few supplements where too much is genuinely dangerous. Get a ferritin blood test first. Target: 50-150 ng/mL. Below 30 = likely deficient. Above 200 = possible overload.
Ingredient Analysis Pages
Full evidence-based analysis for each form and variant:
Related Topics
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About this information: Our analysis of Iron is based on peer-reviewed research from PubMed, ClinicalTrials.gov, and NIH databases. These statements have not been evaluated by the FDA. Supplements are not intended to diagnose, treat, cure, or prevent any disease.
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