Iron exists in the human body in two functional pools: hemoglobin iron (in red blood cells, carrying oxygen — approximately 65–70% of total body iron) and storage iron (ferritin and hemosiderin in liver, spleen, and bone marrow — approximately 25–30%). The clinical obsession with hemoglobin misses the critical insight that ferritin (storage iron) reflects iron adequacy long before hemoglobin falls. Ferritin is depleted first — for months or even years — while hemoglobin remains normal. During this period of "iron deficiency without anemia," all iron-requiring processes other than hemoglobin synthesis are impaired: mitochondrial cytochrome function (energy production), myoglobin synthesis (oxygen delivery to muscle), thyroid peroxidase activity (thyroid hormone production), dopamine synthesis (neurological function), and catalase/ribonucleotide reductase (immune function and DNA repair).
Hepcidin is the master regulator of iron homeostasis. Produced by the liver in response to iron loading, inflammation (IL-6), and hypoxia, hepcidin binds to ferroportin (the only known iron export protein) on gut enterocytes, macrophages, and hepatocytes, triggering its degradation and trapping iron inside cells where it cannot be used. This is the mechanism of "anemia of chronic disease" — in chronic inflammation, high hepcidin blocks iron from reaching the bloodstream even when iron stores are adequate; iron supplements are largely ineffective in this state. Hepcidin also explains why iron absorption is highest in the morning (hepcidin rises with activity), why iron absorption is better on alternate days (Moretti 2015: daily dosing triggers hepcidin elevation that suppresses absorption of subsequent doses; alternate-day dosing produces equivalent iron repletion with less total dose), and why oral iron is ineffective in IBD (elevated hepcidin from chronic inflammation).
| Form | Elemental Iron % | Bioavailability | GI Tolerability | Notes |
|---|---|---|---|---|
| Ferrous bisglycinate (iron bisglycinate) | ~20% | Highest of oral forms — absorbed via PepT1 peptide transporter AND DMT-1; not affected by food or calcium | Best — minimal nausea, constipation, dark stools; can be taken with food without significant absorption penalty | Preferred form for most supplementation; more expensive than sulfate but fewer side effects and better compliance; Ferrochel (Albion brand) is the most studied chelate; 25mg elemental iron provides equivalent response to 50mg ferrous sulfate |
| Ferrous sulfate | ~20% | High — DMT-1 mediated; well-absorbed on empty stomach; food reduces absorption ~40% | Poorest — nausea, constipation, abdominal cramps, dark stools commonl limit compliance; 33–45% of users report adverse effects at standard 65mg dose | Cheapest and most widely available; first-line in NHS iron deficiency protocols due to cost; dose: 65mg elemental iron (325mg ferrous sulfate) 3×/day for anemia treatment; poor compliance limits real-world efficacy; take on empty stomach with vitamin C |
| Ferrous gluconate | ~12% | Moderate — better tolerated than sulfate; absorption similar | Better than sulfate — lower elemental content means less luminal iron to cause oxidative GI damage; common in pregnancy supplements | Lower elemental iron per mg means more capsules needed for equivalent dose; good option when sulfate causes GI issues and bisglycinate is unavailable; 300mg ferrous gluconate = 36mg elemental iron |
| Ferrous fumarate | ~33% | High — similar to sulfate | Moderate — better than sulfate, worse than bisglycinate | High elemental iron per mg; common in UK iron formulations; 210mg ferrous fumarate = 69mg elemental iron; twice-daily dosing achieves equivalent repletion to three times daily sulfate |
| Carbonyl iron (elemental iron) | ~100% | Lower rate of absorption — slow dissolution reduces peak luminal iron concentration | Best safety profile for acute overdose — slower dissolution prevents toxicity spikes; better tolerated than sulfate | Used in some over-the-counter formulations; slower repletion kinetics than ionic forms; preferred in pediatric iron where accidental overdose risk is a concern |
| Liposomal iron (sucrosomial iron) | Variable | Claimed superior bioavailability (absorbed via lymphatics bypassing DMT-1); Pisani 2015: sucrosomial iron restored ferritin as effectively as IV iron in IBS/IBD patients who failed oral iron | Excellent — delivered in a phospholipid matrix that bypasses gut lumen; no oxidative GI damage | Specifically indicated for people who fail standard oral iron (IBD, post-gastric bypass, severe GI sensitivity); more expensive; evidence base smaller than bisglycinate but growing rapidly; Sideral Forte and Ferrum Vivo are studied formulations |
Moretti et al. 2015 (Blood, N=54 iron-deficient women): 40mg elemental iron every other day (total 280mg/week) absorbed more iron than 80mg/day (total 560mg/week) — despite the lower total dose. The mechanism: daily iron supplementation raises hepcidin for 24+ hours after each dose, suppressing absorption of the next day's supplement. Every-other-day dosing allows hepcidin to return to baseline between doses, restoring full absorptive capacity for each dose. The practical implication: for iron deficiency (not acute anemia), alternate-day dosing with a high-quality form (bisglycinate) in the morning, on an empty stomach, with vitamin C, produces equivalent or superior ferritin restoration with lower total iron intake and fewer GI side effects than traditional daily dosing at higher doses. For frank iron deficiency anemia requiring rapid repletion, daily dosing is still used clinically.
Test first — ferritin is the key marker: Request serum ferritin, serum iron, TIBC, and transferrin saturation (not just hemoglobin/CBC). Target ferritin levels: >30 ng/mL for symptomatic resolution; >50 ng/mL for optimal function; 70–100 ng/mL for athletes and high-performance individuals. Transferrin saturation below 20% suggests functional iron deficiency. CRP should be measured simultaneously — elevated CRP indicates inflammation that will inflate ferritin (ferritin is an acute phase reactant) and may make iron stores appear higher than they are.
Dose — calibrate to deficiency: Mild iron deficiency (ferritin 15–30 ng/mL, normal Hb): ferrous bisglycinate 25–36mg elemental iron every other day (alternate-day protocol). Moderate deficiency (ferritin <15, normal Hb): bisglycinate 36mg elemental every other day, or 65mg ferrous sulfate every other day. Iron deficiency anemia (low Hb): 100–200mg elemental iron/day in divided doses (physician supervision recommended); consider IV iron for malabsorptive conditions or non-response to oral iron after 8 weeks.
Timing and enhancers: Take iron on an empty stomach in the morning (lowest hepcidin, highest absorptive capacity). Co-administer with vitamin C (200–500mg ascorbic acid): reduces ferric iron (Fe³⁺) to ferrous (Fe²⁺), the absorbed form; also inhibits hepcidin synthesis; increases absorption 2–4x. Avoid simultaneously: calcium (>300mg blocks iron absorption significantly — do not take with dairy or calcium supplements); tea or coffee (tannins/polyphenols chelate iron — separate by 1–2 hours); antacids/PPIs (iron requires acidic environment for dissolution — particularly important for ferrous sulfate; bisglycinate is less affected); other divalent minerals (zinc, magnesium at high doses).
Monitoring and repletion timeline: Reticulocytes rise within 7–10 days of adequate iron supplementation (early sign of response). Hemoglobin rises 1–2g/dL per month with adequate supplementation. Ferritin repletion is slower — expect 3–6 months to raise ferritin from <15 to >50 ng/mL. Retest ferritin at 3 months; adjust dose. Continue at maintenance dose (25mg every other day) after target ferritin achieved for 1–3 months to consolidate stores. Women with heavy menstrual bleeding may require indefinite supplementation or referral to gynecology for menorrhagia treatment.
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