Iron

Also known as: iron bisglycinate, ferrous sulfate, ferrous bisglycinate chelate

Grade A — Strong Evidencemineralfoundational
18–45 mgEmpty stomach with vitamin C for best absorption. Every other day may improve absorption.

Iron is an essential mineral required for oxygen transport via hemoglobin, energy production, and immune function. Deficiency is the most common nutritional deficiency worldwide, particularly affecting women of reproductive age, vegans, and endurance athletes. Supplementation should only be undertaken when deficiency is confirmed by ferritin testing, as excess iron acts as a pro-oxidant. The bisglycinate chelate form offers the highest bioavailability with the fewest gastrointestinal side effects.

TL;DR: Essential for oxygen transport and energy. ONLY supplement if deficient — excess iron is a pro-oxidant. Bisglycinate form preferred.

⚡Energy
Last reviewed: 2026-03-12

Quick Facts

Evidence

Grade A

Studies

3

Interactions

5

Forms

4

Evidence Rating: Grade A — Strong Evidence

Multiple high-quality RCTs, systematic reviews, or meta-analyses with consistent findings.

Forms Comparison

Iron Bisglycinate

✓ High Absorption

Chelated form that is much gentler on the stomach with high absorption. Preferred form.

sensitive stomachspregnancygeneral deficiency

Ferrous Sulfate

~ Moderate Absorption

Most prescribed form. Effective but commonly causes GI side effects (constipation, nausea).

clinical deficiencybudget

Ferrous Fumarate

~ Moderate Absorption

Higher elemental iron content. Similar GI side effects to ferrous sulfate.

severe deficiency

Iron Polysaccharide Complex

~ Moderate Absorption

Slow-release form with fewer GI effects than sulfate, but possibly lower peak absorption.

GI-sensitive individuals

Interaction Warnings

CalciumModerate

Calcium inhibits iron absorption. Separate by 2+ hours.

Thyroid medicationsModerate

Iron reduces levothyroxine absorption. Separate by 4 hours.

Coffee/tea (tannins)Moderate

Block absorption — separate by 1 hour

ZincLow

May compete for absorption at high doses

Vitamin CLow

Doubles absorption — take together for deficiency

Important Notes

  • Only supplement if deficient — excess iron is harmful
  • Get ferritin levels tested first
  • Bisglycinate form is gentler on stomach
  • Keep away from children — overdose risk

Clinical Evidence (3 studies)

Iron deficiency without anemia — common, overlooked, treatable

Soppi ET. (2018) — Clin Case Rep

Non-anemic iron deficiency causes fatigue, impaired cognition, and reduced exercise capacity

View on PubMed

Ferrous bisglycinate significantly better tolerated

Tolkien Z et al. (2015) — PLoS ONE

Bisglycinate has fewer GI side effects than ferrous sulfate

View on PubMed

Alternate-day iron dosing improves absorption

Moretti D et al. (2015) — Blood Adv

Alternate-day dosing improves absorption vs daily

View on PubMed

Alternatives to Consider

These supplements target similar goals and may be worth considering alongside or instead of Iron.

Potassium

Grade A

Critical electrolyte for heart and muscle function. Most people need more, but get it from food — supplemental doses are limited.

⚡ Energy

Vitamin B6

Grade A

Essential coenzyme for neurotransmitter synthesis (serotonin, GABA, dopamine). Strong evidence for PMS. Deficiency common in those on OCP or with poor diet.

⚡ Energy

Iodine

Grade A

Essential trace mineral for thyroid hormone synthesis. Critical for metabolic rate and cognitive development. Most people get enough from iodized salt — test before supplementing.

⚡ Energy

Related Supplements

Iodine

Grade A — Strong Evidence

Essential trace mineral for thyroid hormone synthesis. Critical for metabolic rate and cognitive development. Most people get enough from iodized salt — test before supplementing.

B-Complex

Grade B — Moderate Evidence

Group of essential vitamins for energy metabolism, nervous system function, and mood. Methylated forms preferred for MTHFR variants.

Potassium

Grade A — Strong Evidence

Critical electrolyte for heart and muscle function. Most people need more, but get it from food — supplemental doses are limited.

Vitamin B6

Grade A — Strong Evidence

Essential coenzyme for neurotransmitter synthesis (serotonin, GABA, dopamine). Strong evidence for PMS. Deficiency common in those on OCP or with poor diet.

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⚕️ Important Disclaimer

This information is for educational purposes only. It is not medical advice. Always consult a healthcare provider before starting any supplement regimen, especially if you take medications or have health conditions.

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