Iron
Ferrous sulfate / ferrous bisglycinate / iron bisglycinate · Foundation
The one supplement on this list that can genuinely transform how someone feels — and the one most dangerous to take without a test.
What the evidence says
Correcting genuine iron-deficiency anaemia produces unambiguous improvements in fatigue, exercise capacity and cognition — few interventions in this catalog have evidence that clean. Iron deficiency WITHOUT anaemia is a real and commonly missed state too, particularly in menstruating women and endurance athletes. In people who are replete, supplementing does nothing good.
What studies used
Deficiency treatment is a clinical decision; alternate-day studies reported higher fractional absorption, without consistently proving better haemoglobin outcomes. Tea, coffee, calcium and dairy reduced absorption in studies; a 440-person randomised trial found added vitamin C did not improve haemoglobin recovery over iron alone.
Reported from published studies and product labelling — a record of what was used in research, not a recommendation. What is right for you is a conversation with a clinician.
Watch out for
- Clinicians assessed iron status before treating, and interpretation was not simple — ferritin rises with inflammation, so evaluation often required additional markers and a search for the cause. Excess iron accumulates and is toxic to the liver and heart, and hereditary haemochromatosis is common enough to matter
- Iron overdose is a leading cause of poisoning death in young children; bottles belong out of reach
- Constipation and GI upset are the usual reasons people stop; bisglycinate forms are generally better tolerated
Evidence base
established literature
Large human evidence base: several phase 3 or 4 trial reports, plus multiple meta-analyses or systematic reviews.
This letter measures how much human research exists, not whether it works for you — the tier above (Strong) is the read on quality.
Research (12)
- Iron Deficiency Anemia: Evaluation and ManagementAm Fam Physician · 2025
- Iron deficiency anemiaAm Fam Physician · 2007
- The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trialsNutr Rev · 2023
- Screening and treatment of iron deficiency anemia in pregnancy: A review and appraisal of current international guidelinesInt J Gynaecol Obstet · 2024
- Iron deficiency in sports - definition, influence on performance and therapySwiss Med Wkly · 2015
- Current understanding of iron homeostasisAm J Clin Nutr · 2017
- Diagnosis and treatment of iron-deficiency anaemia in pregnancy and postpartumArch Gynecol Obstet · 2017
- Oral iron supplementation and anaemia in children according to schedule, duration, dose and cosupplementation: a systematic review and meta-analysis of 129 randomised trialsBMJ Glob Health · 2023
- Iron and ferritin deficiency in women with hypothyroidism and chronic lymphocytic thyroiditis - systematic reviewEndokrynol Pol · 2024
- Guidelines for the management of iron deficiency anaemiaGut · 2011
- Iron-deficiency anaemiaBaillieres Clin Haematol · 1994
- Pre-operative anaemiaAnaesthesia · 2015
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