Iron: What the Research Says

What Is Iron

Iron is an essential mineral required for oxygen transport (as a component of hemoglobin), energy metabolism, DNA synthesis, and immune function. The body has no regulated mechanism for excreting excess iron — it can only lose iron through bleeding, skin cell shedding, and minor GI losses. This means iron accumulates if intake exceeds need, making it fundamentally different from water-soluble nutrients the body can simply excrete.

Because iron deficiency and iron overload are both dangerous, supplementation decisions must be guided by laboratory testing — specifically serum ferritin, which reflects total body iron stores. Hemoglobin alone is insufficient because it drops only after stores are already depleted (late-stage deficiency).

What the Evidence Shows

Condition	Evidence	Key Finding
Iron-deficiency anemia	Strong	First-line treatment; IV ferric carboxymaltose superior for IBD-related anemia[1]
Children's cognition	Strong	Intelligence SMD 0.46, attention SMD 0.44, memory SMD 0.44 in school-age children; stronger in anemic (SMD 0.79)[2]
Pregnancy outcomes	Strong	Iron-folic acid reduces maternal anemia and improves birth outcomes[3]
Hair loss	Moderate	Ferritin -18.51 ng/dL lower in women with hair loss; 21% severely depleted (10,029 participants)[4]
Fatigue (non-anemic)	Moderate	d = 0.34-1.01 fatigue reduction even in non-anemic iron-deficient adults (1,408 participants)[5]
Athletic performance	Limited	Improved endurance in iron-depleted women[6]

A critical pattern across all conditions: iron supplementation only helps people who are actually deficient. In iron-replete individuals, supplementation provides no benefit and introduces risk of harm. This is why testing is non-negotiable.

Deep Dives

Who Should Be Tested

The following groups are at higher risk of iron deficiency and should discuss testing with their provider:

• Premenopausal women — menstrual blood loss is the leading cause in developed countries
• Pregnant women — blood volume expansion increases requirements to 27 mg/day
• Vegetarians and vegans — plant-based (non-heme) iron is less bioavailable; requirements may be 1.8x higher
• Endurance athletes — foot-strike hemolysis, sweat losses, and GI microbleeding
• Frequent blood donors — each donation removes ~250 mg of iron
• People with GI conditions — celiac disease, IBD, or chronic NSAID use impair absorption or cause occult bleeding
• Children and adolescents — rapid growth increases iron demand

Dosing

Indication	Dose (elemental iron)	Notes
Iron-deficiency anemia	100-200 mg/day	Split into 2-3 doses; take on empty stomach with vitamin C
Pregnancy (prevention)	27-60 mg/day	WHO recommends 30-60 mg/day where anemia prevalence is high
Children (if deficient)	30-60 mg/day	Only if blood testing confirms deficiency
Fatigue (non-anemic deficient)	50-100 mg/day	Target ferritin >30 ng/mL; retest after 8-12 weeks
Hair loss (low ferritin)	Correct to >40-70 ng/mL	Dose based on severity of depletion
Athletic performance	100 mg/day if depleted	Only for confirmed iron depletion (ferritin <20 ng/mL)

Absorption tips: Research shows absorption is enhanced when taken on an empty stomach with vitamin C (e.g., a glass of orange juice) to enhance absorption. Calcium, tea, coffee, dairy, and high-fiber foods are known to inhibit absorption. Separate calcium, tea, coffee, dairy, or high-fiber foods, which inhibit absorption. If GI side effects are intolerable, take with a small amount of food — absorption decreases but compliance improves.

Forms Compared

Form	Elemental Iron	Verdict
Ferrous bisglycinate	~20%	Best tolerated, highest bioavailability, fewest GI side effects. Preferred form.
Ferrous sulfate	~20%	Cheapest, most studied. Effective but higher rates of constipation and nausea.
Ferrous fumarate	~33%	High elemental iron content. Similar GI profile to sulfate.
Ferrous gluconate	~12%	Gentler on the stomach. Lower elemental iron per dose.
Heme iron polypeptide	Varies	Derived from animal sources. Better absorbed than non-heme but more expensive.

Safety: Iron Overload Is Dangerous

Unlike most supplements where the downside is "you wasted money," excess iron causes organ damage. This section is not a formality — it is the most important part of this page.

• Organ toxicity: Excess iron deposits in the liver (cirrhosis), heart (cardiomyopathy), and pancreas (diabetes). Damage can be irreversible.
• Hemochromatosis: Approximately 1 in 200 Northern Europeans carry genes for hereditary hemochromatosis, a condition causing dangerous iron accumulation. These individuals must never take iron supplements unless directed by a hematologist.
• Acute poisoning in children: Iron supplements are a leading cause of pediatric poisoning deaths. Keep all iron supplements out of reach of children.
• Upper Limit: 45 mg/day for adults. Therapeutic doses for anemia (100-200 mg) intentionally exceed this under medical supervision — they are not meant for self-prescribing.

Common Side Effects

• Constipation — the most common complaint; ferrous bisglycinate causes less
• Nausea and stomach pain — worse on an empty stomach; dose-dependent
• Dark/black stools — harmless but expected; not a sign of GI bleeding (though cannot rule it out)

Drug Interactions

Iron interacts with numerous medications. Separate iron from these drugs by at least 2 hours:

• Levothyroxine — iron reduces thyroid hormone absorption significantly
• Antibiotics (tetracyclines, quinolones) — mutual absorption reduction
• Bisphosphonates (alendronate) — iron impairs absorption
• Levodopa — iron chelates levodopa, reducing its efficacy
• Antacids and PPIs — reduce iron absorption by raising stomach pH
• Calcium supplements — inhibit iron absorption; separate by 2 hours

Dietary inhibitors: Tea and coffee reduce iron absorption by 60-70% due to polyphenol binding. Avoid consuming within 1 hour of iron supplements.

References

1. IV ferric carboxymaltose for IBD-related iron-deficiency anemia. 2025. PubMed
2. Iron supplementation and cognition in school-age children: intelligence SMD 0.46, attention 0.44, memory 0.44. 2023. PubMed
3. Iron-folic acid supplementation in pregnancy: maternal anemia and birth outcomes. 2025. PubMed
4. Serum ferritin and hair loss in women: ferritin -18.51 ng/dL, 21% severely depleted. 10,029 participants. 2022. PubMed
5. Iron supplementation for psychiatric, cognitive, and fatigue outcomes in iron-deficient individuals: d = 0.34-1.01. 1,408 participants. 2025. PubMed
6. Iron supplementation and endurance in iron-depleted female athletes. 2019. PubMed
7. World Health Organization. Daily iron supplementation in adult women and adolescent girls. Geneva: WHO; 2016.
8. Hereditary hemochromatosis: prevalence, screening, and management. Am Fam Physician. 2019.

Related Guides