Overview
Iron is the most abundant transition metal in the human body and is essential for oxygen transport, mitochondrial energy production, DNA synthesis, and immune function. It exists in two primary functional pools: hemoglobin iron (in red blood cells, carrying oxygen) and storage iron (ferritin and hemosiderin in the liver, spleen, and bone marrow). A smaller pool of iron is found in myoglobin (muscle oxygen storage) and in iron-containing enzymes including cytochromes, catalase, and ribonucleotide reductase.
Iron homeostasis is tightly regulated because both deficiency and excess are pathological. Unlike most nutrients, the body has no active mechanism for iron excretion — iron balance is controlled almost entirely at the level of absorption. This makes iron unique among minerals and explains why iron overload disorders are as clinically significant as iron deficiency.
Iron Absorption: The Hepcidin Axis
Iron absorption occurs in the duodenum and proximal jejunum via two pathways:
- Heme iron (from animal sources): absorbed via HCP1 (heme carrier protein 1) at ~25–35% efficiency, largely unaffected by dietary inhibitors
- Non-heme iron (from plant sources): absorbed via DMT1 (divalent metal transporter 1) at 2–20% efficiency; requires reduction from Fe³⁺ to Fe²⁺ by duodenal cytochrome B (DcytB); strongly influenced by dietary enhancers and inhibitors
The master regulator of systemic iron homeostasis is hepcidin, a hepatic peptide hormone that binds and degrades ferroportin — the only known cellular iron exporter. When iron stores are high or inflammation is present, hepcidin rises, blocking iron release from enterocytes, macrophages, and hepatocytes. When iron stores are low or erythropoietic demand increases, hepcidin falls, allowing increased iron absorption and mobilization.
Root Causes of Iron Deficiency
1. Inadequate Dietary Intake
The RDA for iron is 8 mg/day for adult men and postmenopausal women, and 18 mg/day for premenopausal women. Vegetarian and vegan diets rely exclusively on non-heme iron, which has significantly lower bioavailability. The WHO recommends vegetarians consume 1.8x the standard RDA to compensate for reduced absorption efficiency.
2. Increased Blood Loss
Blood loss is the most common cause of iron deficiency in adults. Sources include: heavy menstrual bleeding (menorrhagia), gastrointestinal bleeding (peptic ulcers, colorectal cancer, IBD, NSAID use, hookworm infection), frequent blood donation, and surgical blood loss. Even small, chronic GI bleeds — often asymptomatic — can deplete iron stores over months to years.
3. Increased Physiological Demand
Pregnancy dramatically increases iron requirements (27 mg/day) due to fetal development, placental growth, and expanded maternal red cell mass. Infancy and adolescence are also high-demand periods. Endurance athletes experience "sports anemia" from increased red cell turnover, foot-strike hemolysis, GI microbleeding, and sweat losses.
4. Malabsorption
Conditions impairing duodenal iron absorption include celiac disease, Helicobacter pylori infection (which sequesters iron and causes gastric atrophy), atrophic gastritis, post-gastrectomy states, and inflammatory bowel disease. Proton pump inhibitor use reduces gastric acid, impairing the reduction of Fe³⁺ to the absorbable Fe²⁺ form.
5. Anemia of Chronic Inflammation
In chronic inflammatory states (autoimmune disease, chronic infection, malignancy, obesity), elevated IL-6 drives hepcidin upregulation, blocking iron release from macrophages and reducing intestinal absorption. This produces functional iron deficiency — iron is present in stores but unavailable for erythropoiesis. This is the most common cause of anemia in hospitalized patients.
6. Dietary Inhibitors
Phytates (grains, legumes), polyphenols (tea, coffee, red wine), calcium, and oxalates all inhibit non-heme iron absorption. Consuming tea or coffee with meals can reduce iron absorption by 60–80%. These inhibitors are clinically significant in populations with marginal iron intake.
Root Causes of Iron Overload
1. Hereditary Hemochromatosis
The most common genetic disorder in populations of Northern European descent, hereditary hemochromatosis (HH) is caused primarily by mutations in the HFE gene (C282Y homozygosity in ~85% of cases). HFE mutations impair hepcidin upregulation, leading to unregulated iron absorption and progressive iron deposition in the liver, heart, pancreas, joints, and pituitary gland. Clinical manifestations include cirrhosis, hepatocellular carcinoma, cardiomyopathy, diabetes, arthropathy, and hypogonadism.
2. Transfusional Iron Overload
Patients with thalassemia major, sickle cell disease, myelodysplastic syndrome, and aplastic anemia who require chronic red blood cell transfusions accumulate iron at approximately 200–250 mg per unit transfused, with no physiological mechanism for excretion.
3. Dietary Iron Overload
Rare in developed countries but documented in sub-Saharan Africa ("African iron overload") associated with consumption of home-brewed beer fermented in iron pots. Chronic excessive supplementation can also contribute to iron accumulation, particularly in individuals with subclinical HFE mutations.
Mechanisms of Iron Deficiency
Impaired Oxygen Transport
Iron deficiency reduces hemoglobin synthesis, producing microcytic hypochromic anemia. Reduced oxygen-carrying capacity causes fatigue, dyspnea on exertion, pallor, and reduced exercise tolerance. Even pre-anemic iron deficiency (low ferritin with normal hemoglobin) impairs mitochondrial function and cognitive performance.
Mitochondrial Dysfunction
Iron is a core component of the electron transport chain (complexes I, II, and III) and is required for ATP synthesis via oxidative phosphorylation. Iron-sulfur clusters in mitochondrial enzymes are among the first casualties of iron deficiency, impairing energy production before anemia develops.
Immune Impairment
Iron is required for the proliferation and differentiation of lymphocytes and for the oxidative burst of neutrophils and macrophages. Iron deficiency impairs both innate and adaptive immunity. Paradoxically, iron is also required by pathogens — the acute-phase hepcidin response sequesters iron as an antimicrobial defense mechanism.
Neurological Effects
Iron is essential for myelin synthesis, dopamine receptor function, and monoamine oxidase activity. Iron deficiency in early childhood causes irreversible cognitive deficits, impaired attention, and behavioral problems. In adults, iron deficiency is associated with restless legs syndrome (RLS), fatigue, depression, and impaired concentration.
Mechanisms of Iron Overload Toxicity
Excess free iron participates in the Fenton reaction, generating hydroxyl radicals — the most reactive and damaging of all reactive oxygen species. This drives lipid peroxidation, DNA damage, and protein oxidation in iron-accumulating tissues. Hepatic iron overload progresses from steatosis to fibrosis to cirrhosis. Cardiac iron deposition causes restrictive cardiomyopathy and arrhythmias. Pancreatic iron causes beta-cell destruction and "bronze diabetes."
Assessment
A complete iron panel is essential for accurate diagnosis:
- Serum ferritin: Best single marker of iron stores; <30 ng/mL indicates deficiency; >300 ng/mL (men) or >200 ng/mL (women) suggests overload. Note: ferritin is an acute-phase reactant — it can be falsely elevated in inflammation.
- Serum iron: Reflects circulating iron; low in deficiency, high in overload
- TIBC (total iron-binding capacity): Elevated in deficiency (more binding sites available), reduced in overload
- Transferrin saturation: Serum iron ÷ TIBC × 100; <16% indicates deficiency; >45% suggests overload and warrants HFE gene testing
- Soluble transferrin receptor (sTfR): Elevated in true iron deficiency; not affected by inflammation — useful for distinguishing iron deficiency from anemia of chronic disease
- Reticulocyte hemoglobin content (CHr/Ret-He): Functional marker of iron availability for erythropoiesis
Integrative Protocols
Iron Deficiency — Dietary Optimization
Prioritize heme iron sources: red meat (beef, lamb), organ meats (liver is the richest source at ~5 mg/100g), poultry dark meat, and shellfish (clams, oysters). Enhance non-heme iron absorption by consuming vitamin C-rich foods simultaneously (citrus, bell peppers, strawberries) — vitamin C reduces Fe³⁺ to Fe²⁺ and chelates iron to maintain solubility. Avoid tea, coffee, and calcium-rich foods within 1–2 hours of iron-rich meals.
Iron Deficiency — Supplementation
- Ferrous bisglycinate (iron bisglycinate chelate): Highest bioavailability, best GI tolerability — preferred form for repletion
- Ferrous sulfate: Standard clinical form; effective but frequently causes GI side effects (constipation, nausea)
- Ferrous gluconate: Moderate bioavailability, better tolerated than sulfate
- Ferric forms (ferric citrate, ferric maltol): Lower GI side effects; used in CKD-associated iron deficiency
- Liposomal iron: Emerging form with high bioavailability and minimal GI side effects
Dosing: 15–200 mg elemental iron per day depending on severity. Alternate-day dosing (every other day) has been shown in clinical trials to achieve equivalent or superior absorption compared to daily dosing, with fewer side effects, due to hepcidin cycling. Take on an empty stomach with vitamin C for maximum absorption; if GI intolerance occurs, take with a small meal.
Iron Overload — Management
- Therapeutic phlebotomy: First-line treatment for hereditary hemochromatosis; removes 200–250 mg iron per unit of blood. Initial phase: weekly phlebotomy until ferritin <50 ng/mL. Maintenance: 2–4 phlebotomies per year.
- Dietary modification: Avoid vitamin C supplements with meals (enhances iron absorption), limit red meat, avoid alcohol (increases iron absorption and hepatotoxicity), avoid raw shellfish (infection risk with iron-loaded macrophages).
- Iron chelation therapy: Deferoxamine, deferasirox, or deferiprone for transfusional overload or when phlebotomy is contraindicated.
- IP6 (inositol hexaphosphate): Natural iron chelator; may reduce iron absorption; used adjunctively in mild overload states.
Key Takeaways
- Iron homeostasis is regulated by hepcidin — the master iron hormone; both deficiency and overload are pathological.
- Heme iron (animal sources) absorbs at 25–35%; non-heme iron (plant sources) at 2–20% — form and dietary context matter enormously.
- The most common causes of deficiency are blood loss, inadequate intake, malabsorption, and anemia of chronic inflammation (hepcidin-driven).
- Hereditary hemochromatosis (HFE C282Y) is the most common genetic iron overload disorder; transferrin saturation >45% warrants genetic testing.
- Assessment requires a full iron panel: ferritin, serum iron, TIBC, transferrin saturation, and sTfR for inflammation-confounded cases.
- Preferred repletion form: ferrous bisglycinate; alternate-day dosing optimizes absorption via hepcidin cycling.
- Iron overload is treated with therapeutic phlebotomy (HH) or chelation therapy (transfusional overload).
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