Iron Supplementation: When It Helps, When It Hurts, and How to Take It
Iron supplements are one of the most misused supplements. This guide explains who actually needs iron, how to minimise side effects, and why supplementing without confirmed deficiency can cause harm.
Quick Answer
Iron supplementation is appropriate for confirmed iron deficiency or iron deficiency anaemia. Supplementing without a blood test confirming deficiency is not recommended because excess iron is not excreted efficiently and can accumulate to harmful levels.
Key Takeaways
- 01---
- 02Premenopausal women with heavy periods
- 03Pregnant women (often prescribed routinely)
- 04Infants and toddlers (fast growth outpaces dietary intake)
- 05Vegans and vegetarians (non-haem iron from plants is less absorbable)
Quick Answer
Iron supplementation is appropriate for confirmed iron deficiency or iron-deficiency anaemia. Supplementing without a blood test confirming deficiency is not recommended because excess iron is not excreted efficiently and can accumulate to harmful levels. Side effects (constipation, nausea) can be substantially reduced with the right form and timing.
Who Needs Iron Supplementation
- Premenopausal women with heavy periods
- Pregnant women (often prescribed routinely)
- Infants and toddlers (fast growth outpaces dietary intake)
- Vegans and vegetarians (non-haem iron from plants is less absorbable)
- Endurance athletes (especially female runners — foot-strike haemolysis, sweat loss)
- People with coeliac disease, Crohn's, or other absorption disorders
Testing: What to Request
Serum ferritin is the most informative single marker for iron stores. Haemoglobin alone identifies anaemia but misses iron depletion before anaemia develops.
| Marker | Reflects | Notes |
|---|---|---|
| Serum ferritin | Iron storage | Low ferritin + normal Hb = depleted stores |
| Haemoglobin | Oxygen-carrying capacity | Low only in later deficiency |
| Serum iron | Transport iron | Highly variable; less useful alone |
| Transferrin saturation | % of transferrin carrying iron | Useful in combination |
Forms of Iron
| Form | Absorption | GI Tolerance | Notes |
|---|---|---|---|
| Ferrous sulfate | High | Moderate–poor | Cheapest; most common cause of GI complaints |
| Ferrous gluconate | Moderate | Better | Good balance of absorption and tolerability |
| Ferrous bisglycinate | Good | Best | Chelated form; significantly less constipation |
| Ferric compounds | Lower | Better | Often used in slow-release formulations |
| Carbonyl iron | Moderate | Good | Slow release; lower acute toxicity |
Evidence-Based Dosing Strategy
Recent research challenges the traditional "every day" dosing:
A 2017 study in Blood (Moretti et al.) found alternate-day dosing increased total iron absorption more than daily dosing because daily iron suppresses hepcidin (the iron-regulating hormone) overnight, which reduces absorption the next day.
Alternate-day dosing is now supported as equally or more effective than daily dosing and better tolerated.
| Approach | Dose | Schedule |
|---|---|---|
| Standard | 150–200 mg elemental iron/day | Daily (divided) |
| Alternate-day | 150–200 mg elemental iron | Every other day |
| Lower-dose (mild depletion) | 30–60 mg elemental iron | Every other day |
Maximising Absorption
- Take iron on an empty stomach or with vitamin C (ascorbic acid increases haem conversion).
- Avoid taking with calcium (dairy, antacids), coffee, tea, or whole grains within 2 hours — all significantly reduce absorption.
- Enteric-coated iron tablets have lower absorption than regular tablets.
Why Supplementing Without Deficiency Is Risky
Iron is not regulated by excretion but by absorption control. When absorption control is overwhelmed, iron accumulates. Excess iron:
- Acts as a pro-oxidant, generating reactive oxygen species
- May promote bacterial growth (some pathogens use iron)
- In haemochromatosis (genetic iron overload disorder, ~1 in 200 people of northern European descent), supplementation can accelerate organ damage
Safety Notes
- Iron overdose is the leading cause of accidental poisoning death in children under 6: Store safely.
- At therapeutic doses in adults, iron is safe under confirmed deficiency.
- Constipation is the most common side effect; ferrous bisglycinate and alternate-day dosing significantly reduce this.
- Monitor ferritin levels at 3-month intervals during supplementation to confirm response.
FAQ
Frequently Asked Questions
Medically reviewed
Last reviewed May 9, 2026 by Migaku Editorial Team
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