Iron
Iron carries oxygen in hemoglobin and myoglobin and sits at the active site of cytochromes and other redox enzymes. Iron deficiency is the most common nutritional deficiency worldwide. Oral iron works but is poorly tolerated, and the dosing schedule matters as much as the form.
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What It Does
About two-thirds of body iron is in hemoglobin; most of the rest is stored as ferritin in the liver, spleen, and marrow. The body has no regulated way to excrete iron, so absorption is the control point. The liver hormone hepcidin degrades ferroportin, the channel that exports iron from gut cells into blood, and hepcidin rises with iron intake and with inflammation.
That mechanism changed how oral iron is dosed. In iron-depleted young women, doses of 60 mg or more raised hepcidin for about 24 hours and cut absorption from the next day's dose by 35-45% (Moretti 2015). A single morning dose every other day produced more total absorption than the same dose daily, and splitting the dose twice a day gave no advantage (Stoffel 2017). The result held in women with iron deficiency anemia at 100 and 200 mg doses (Stoffel 2020). These were short isotope studies in small groups of women; whether alternate-day dosing corrects anemia faster in practice is less well established, but it absorbs at least as well with half the pills.
Heme iron from meat is absorbed through a separate pathway, about 15-35%, and diet barely affects it. Non-heme iron absorption ranges from about 2% to 20% depending on the rest of the meal. Phytate in grains and legumes inhibits it in a dose-dependent way, and ascorbic acid counteracts much of that inhibition (Hallberg 1989). Tea, coffee, and calcium also reduce it.
Ferritin is the best single test for deficiency, with one catch: it is an acute phase reactant, so infection, inflammation, liver disease, and obesity raise it independently of iron stores. WHO uses a ferritin below 15 mcg/L in apparently healthy adults and below 70 mcg/L when inflammation is present (WHO 2020). Ferritin read with transferrin saturation and CRP is far more reliable than ferritin alone.
Populations at risk include menstruating women with heavy losses, pregnancy, infants and toddlers, frequent blood donors, endurance athletes, vegetarians, and people with celiac disease, inflammatory bowel disease, or bariatric surgery. Iron deficiency in men or postmenopausal women is gastrointestinal blood loss until proven otherwise and needs evaluation, not just a supplement.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Elemental % | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|---|
| Ferrous Sulfateiron sulfate | 20% | HighFerrous (Fe2+) iron is absorbed by divalent metal transporter 1 (DMT1) in the duodenum. Absorption rises when stores are low and falls as hepcidin rises after a dose. | 40-65 mg elemental iron daily or on alternate days for deficiency | Low | 101 |
| Ferrous Gluconateiron gluconate | 12% | HighFerrous iron absorbed by DMT1, as with sulfate. | 27-38 mg elemental iron per tablet | Low | 67 |
| Ferrous Fumarateiron fumarate | 33% | HighFerrous iron absorbed by DMT1, as with sulfate. | 65-106 mg elemental iron per tablet | Low | 190 |
| Iron Bisglycinateferrous bisglycinate chelate, Ferrochel | 20% | HighIron chelated to two glycine molecules. The chelate is less affected by dietary phytates than iron salts, and absorption remains regulated by iron status. | 25-50 mg elemental iron daily | Moderate | 283 |
| Carbonyl Iron | Varies | ModerateMicroparticles of nearly pure elemental iron that must be dissolved by gastric acid before absorption, which slows and limits uptake. | 45-65 mg elemental iron daily | Low | 68 |
| Heme Iron Polypeptideheme iron, HIP | Varies | HighIron bound in the heme ring, absorbed intact through a separate heme pathway. Absorption is much less affected by phytates, calcium, and tea than non-heme iron. | Product-specific; per-tablet iron is low | High |
Labels column: number of labels named as iron products that list this form. A product can list more than one form, and some labels do not state a form at all.
Ferrous sulfate is about 20% elemental iron, gluconate 12%, and fumarate 33%. They are equally well absorbed per milligram of elemental iron; the labeled compound weight is what differs. Sulfate is the reference form in nearly every absorption study, including the hepcidin trials. GI side effects are its main problem: a meta-analysis found ferrous sulfate more than doubled the odds of GI complaints compared with placebo (Tolkien 2015).
Iron bisglycinate is a chelate that many people tolerate better at a lower elemental dose. The comparative trials are small. Carbonyl iron is nearly pure elemental iron in tiny particles that dissolve slowly, which makes accidental overdose in children less dangerous. Heme iron polypeptide uses the heme pathway, comes in small doses, and costs more.
Slow-release and enteric-coated products release iron past the duodenum, where most absorption happens. They cause fewer side effects mainly because less iron is absorbed.
Typical Dosing
| Group | RDA |
|---|---|
| Men 19+ | 8 mg |
| Women 19-50 | 18 mg |
| Women 51+ | 8 mg |
| Pregnancy | 27 mg |
| Upper limit (UL) | 45 mg elemental iron per day |
Treatment of deficiency commonly uses 40-65 mg of elemental iron once daily or on alternate days. Vegetarians are advised 1.8 times the RDA because non-heme iron is less bioavailable.
Food sources: fortified breakfast cereals, oysters, white beans, beef liver, lentils, spinach, tofu, kidney beans.
What to Look For
The panel lists elemental iron. A 325 mg ferrous sulfate tablet lists "Iron (as ferrous sulfate) 65 mg", and 65 mg is the number that counts. Gluconate tablets of the same weight carry about 38 mg.
Take iron in the morning on an empty stomach or with vitamin C or orange juice, and keep it at least two hours from coffee, tea, dairy, calcium supplements, and antacids. Separate it from levothyroxine by four hours. If GI effects are intolerable, alternate-day dosing or a lower dose taken with a small meal is a better first step than switching to an expensive form.
Do not take iron without a confirmed deficiency. Hereditary hemochromatosis is common in people of Northern European ancestry, and iron supplements accelerate organ damage in it. Iron overdose remains a leading cause of fatal poisoning in young children, so keep tablets in child-resistant packaging and out of reach.
Red flags: multivitamin-style "iron complexes" with unstated elemental amounts, slow-release products marketed for faster repletion, and iron in men's multivitamins.
Ferrous Sulfate
The reference standard: cheapest, best studied, and the form used in the hepcidin dosing trials. GI side effects are the price.
What to look for: 65 mg elemental iron (325 mg tablet), not enteric coated, taken once in the morning on alternate days.
Find on iHerb Affiliate linkIron Bisglycinate
Easier on the stomach for many people. Fewer head-to-head data than sulfate.
What to look for: Elemental iron stated per capsule, often 25-36 mg. Ferrochel is the most studied raw material.
Find on iHerb Affiliate linkProducts on Titrate
15,991 of the 214,745 labels in the NIH Dietary Supplement Label Database list iron as an active ingredient (8,640 on market). 1,146 are sold as iron products by name.
Top on-market labels, ranked by label completeness and simplicity:
- Pure Albion Gentle Iron 25 mg
- Iron Chelate 18 18 mg
- Iron Chelate 27 27 Milligrams
- Chelated Iron
- Iron Chelate
Iron does not have its own search category yet. Each product above links to its full label in the NIH database.
Related Tools
- Iron Absorption Estimator
- Supplement Timing Optimizer
- Ferrous Sulfate vs Iron Bisglycinate
- Carbonyl Iron vs Ferrous Sulfate
- Carbonyl Iron vs Iron Bisglycinate
- Ferrous Fumarate vs Ferrous Sulfate
- Ferrous Fumarate vs Iron Bisglycinate
- Ferrous Gluconate vs Ferrous Sulfate
- Ferrous Gluconate vs Iron Bisglycinate
- Ferrous Sulfate vs Heme Iron Polypeptide
- All iron form comparisons
- Best Multivitamin Without Iron
- All supplement guides
References
NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals.
Hallberg L, Brune M, Rossander L. Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate. Am J Clin Nutr. 1989;49(1):140-144.
Moretti D, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981-1989.
Tolkien Z, et al. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383.
Stoffel NU, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533.
Stoffel NU, et al. Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica. 2020;105(5):1232-1239.
World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020.
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