Iron deficiency is the most prevalent nutritional deficiency in the world — affecting approximately 1.6 billion people globally by World Health Organization estimates, across every income level and geography. Despite its prevalence, it is routinely under-diagnosed in clinical practice because the standard diagnostic threshold (iron-deficiency anemia defined by hemoglobin below 12 g/dL in women) misses the substantial population of patients with iron deficiency without anemia (IDWA) — people whose hemoglobin is normal but whose ferritin is low and who are genuinely symptomatic.
When iron stores are depleted, the body prioritizes what little iron it has for hemoglobin synthesis (to maintain oxygen delivery) at the expense of other iron-dependent functions: mitochondrial energy production, neurotransmitter synthesis (dopamine and serotonin both require iron-dependent enzymes), hair follicle cycling, thyroid hormone metabolism, and immune function. The result is the classic IDWA symptom cluster — fatigue disproportionate to anemia, brain fog, hair loss (diffuse, telogen effluvium), cold intolerance, restless legs syndrome, and impaired exercise tolerance — in a person whose complete blood count comes back "normal."
| Form | Elemental Iron % | Relative Bioavailability | GI Tolerability | Notes |
|---|---|---|---|---|
| Ferrous sulfate | 20% | Standard reference (1.0×) | Poor — 50–70% GI side effects | Cheapest; most prescribed; problematic tolerability; if GI side effects occur, switch to bisglycinate rather than persisting |
| Ferrous gluconate | 12% | ~0.9× sulfate | Moderate — fewer side effects than sulfate at equivalent elemental dose | Lower elemental iron content means larger tablets for equivalent dose; better tolerated but less common |
| Ferrous bisglycinate (chelated) | 20% | ~1.5–1.8× sulfate | Good — significantly fewer GI side effects | Best overall choice for most patients; Albion Labs patent; higher cost but superior compliance; equivalent efficacy at lower elemental iron dose due to better absorption |
| Ferric maltol (Accrufer) | 30% | Moderate | Good — ferric forms cause fewer GI symptoms | Prescription in many countries; studied specifically in IBD patients with iron intolerance; good option when ferrous forms fail |
| Carbonyl iron | 98% | ~0.7× sulfate (slower release) | Good — slow absorption reduces GI exposure | Safe at higher doses; slower absorption acts as a safety buffer; used in children's supplements |
| Heme iron (from animal sources) | Variable | 2–3× non-heme iron | Excellent | Absorbed via different pathway (heme transporter, not DMT1); not inhibited by phytates, calcium, or tannins; dietary heme iron (red meat, organ meat) is the most bioavailable source; heme iron supplements available but expensive |
Hepcidin is a peptide hormone produced by the liver that is the master regulator of systemic iron homeostasis. It works by binding to ferroportin (the iron export channel on intestinal cells, macrophages, and hepatocytes) and causing its internalization and degradation — effectively closing the door on iron export into the bloodstream.
When you take an iron supplement, iron absorption occurs over 2–4 hours in the duodenum and proximal jejunum. The iron that enters the bloodstream signals the liver to produce hepcidin, which peaks approximately 6–24 hours after the dose. The next day, when you take your second daily dose, hepcidin is still elevated — the intestinal iron channel is suppressed — and significantly less iron is absorbed.
By taking iron every other day, you allow hepcidin to decline back to baseline before the next dose. The result, as Stoffel 2017 demonstrated, is 40% higher fractional iron absorption over the same time period. The same total amount of elemental iron administered every other day produces faster iron repletion than daily dosing. This is now the standard recommendation from hematologists and supported by European iron deficiency treatment guidelines.
Practical protocol: Take iron every other day (Monday, Wednesday, Friday, or similar), in the morning on an empty stomach (maximum absorption) or with a small amount of food if GI intolerance occurs; always co-administer with 200mg vitamin C; avoid within 2 hours of calcium-containing foods (dairy, calcium supplements), coffee, tea (tannins), or antacids; morning dosing on an empty stomach is 2–3× more effective than evening with food.
Form: Ferrous bisglycinate 25–30mg elemental iron (equivalent to approximately 125–150mg ferrous bisglycinate salt) every other day; if bisglycinate is unavailable or cost-prohibitive, ferrous sulfate 325mg (65mg elemental iron) every other day; if GI side effects with sulfate, switch to bisglycinate rather than reducing dose.
Co-administration: 200mg vitamin C (ascorbate) taken at the same time as iron; do NOT take with dairy, calcium supplements, coffee, tea, or antacids (wait 2 hours); morning fasted or with a small non-calcium-containing snack is optimal.
Duration: Iron repletion from depleted stores (ferritin <15 ng/mL) to optimal (ferritin >50 ng/mL) typically takes 3–6 months; recheck ferritin and CBC at 3 months; hemoglobin typically normalizes within 4–8 weeks; ferritin takes longer to replete; continue supplementation for 3 months after hemoglobin normalization to fully replete iron stores.
Monitoring: Ferritin, serum iron, TIBC, hemoglobin, MCV at baseline and 3 months; if ferritin is not rising despite supplementation, investigate: malabsorption (coeliac disease), ongoing blood loss (heavy menstruation, GI bleeding), H. pylori infection (reduces iron absorption), inflammatory state (CRP elevation falsely elevates ferritin making it unreliable as a deficiency marker in acute illness).
Dietary iron optimization: Heme iron (red meat, liver, dark poultry) is 2–3× more bioavailable than non-heme iron (legumes, leafy greens, fortified foods); eat non-heme iron foods with vitamin C sources; liver (beef or chicken) is the single richest food source of highly bioavailable iron (6–7mg elemental iron per 100g, heme form); avoid tea/coffee within 1 hour of iron-rich meals.
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