Spirulina and Iron Metabolism / Hepcidin.

Hepcidin: Master Iron Regulator

Hepcidin (HAMP; 25-aa peptide; 4 disulphide bonds; liver-secreted; cationic; Kd for ferroportin ~1–10 nM) is the master regulator of systemic iron homeostasis. Hepcidin binds ferroportin (FPN1/SLC40A1; 9-TM; cellular iron exporter; expressed on enterocytes, macrophages, hepatocytes) → JAK2-mediated FPN1 Tyr302/303 phosphorylation → FPN1 internalisation and ubiquitin-dependent lysosomal degradation (hepcidin-induced FPN1 K8 K18 ubiquitination via ubiquitin E3) → iron trapped intracellularly → plasma iron ↓. Hepcidin is regulated by: (i) iron-sensing BMP6 (hepatic sinusoidal endothelium; elevated by iron) → BMPR2/ALK2/ALK3 → SMAD1/5/8 Ser463/465 phosphorylation → SMAD4 → BMP-RE1/2 in HAMP promoter ↑; haemojuvelin (HJV/RGMc; GPI-anchored BMP co-receptor) required; matriptase-2 (TMPRSS6) cleaves HJV → HAMP ↓ (iron-depleted state). (ii) Inflammation: IL-6 → JAK2 → STAT3 Tyr705 → STAT3-RE in HAMP promoter ↑ → anaemia of inflammation (AI); also IL-1β→NF-κB→HAMP ↑ (BMP-independent). (iii) Erythroid suppression: erythroferrone (ERFE/FAM132B; produced by erythroblasts under EPO stimulation) sequesters BMP6/BMP2 → HAMP ↓ (expanding erythropoiesis suppresses hepcidin to mobilise iron for Hb synthesis).

Cellular Iron Handling: Transferrin, Ferritin, IRP/IRE

Circulating iron is transported bound to transferrin (TF; MW ~79 kDa; two Fe³&sup+; binding sites; Kd ~10−²³ M at pH 7.4; Kd ~10−³ M at pH 5.5 endosomal) captured by transferrin receptor 1 (TfR1/TFRC; homodimer; required for erythropoiesis; TFRC promoter IRE IRP1 regulation). TF-Fe(III)→TfR1→clathrin endocytosis→endosomal acidification (V-ATPase; STEAP3 metalloreductase Fe³&sup+;→Fe²&sup+;)→DMT1/SLC11A2 exports Fe²&sup+; to cytoplasm. Iron storage: ferritin (FTH1 heavy chain ferroxidase; FTL light chain; 24-mer apoferritin shell; Kd Fe³&sup+; ~10−²&sup0; M; up to 4,500 Fe atoms). Iron-regulatory proteins IRP1/IRP2 sense cytoplasmic labile iron pool (LIP; ~0.1–1 μM chelatable Fe²&sup+;): low iron → IRP1 aconitase Fe-S cluster absent → IRE (iron-responsive element; 5′-CAGUGN-3′ hairloop) binding ↑ → 5′-IRE HAMP/FTH1/FTL translation ↓ (ferritin ↓); 3′-IRE TFRC mRNA stabilised (TfR1 ↑). IRP2 (IREB2) is degraded in high iron via FBXL5 (F-box Fe-S CRL1-SCF K48-Ub; FBXL5 Fe-S cluster senses iron).

Spirulina’s Mechanistic Actions

• Non-haem iron provision and phycocyanin chelation: Spirulina contains ~28–58 mg Fe/100g DW (varies with batch/source); iron is partly chelated to phycocyanin tetrapyrrole chromophore (PCB-Fe complex; proposed to enhance duodenal Fe²&sup+; uptake by DMT1 by maintaining solubility at intestinal pH 6.5–7.5 vs. rapid Fe(OH)&sub3; precipitation from inorganic Fe); absence of phytate/oxalate inhibitors (unlike plant sources) → Fe bioavailability ~20–30% (higher than most plant sources; lower than haem Fe).
• NF-κB/IL-6 ↓ → STAT3 ↓ → HAMP ↓: PCB→NF-κB↓→IL-6 ↓ 30–50%→JAK2→STAT3 Tyr705 ↓→STAT3-RE occupancy at HAMP promoter ↓→hepcidin ↓ 20–35% in chronic inflammation/AI models; FPN1 membrane expression ↑ 15–25% → enterocyte/macrophage iron export ↑ → plasma iron ↑ → erythropoiesis substrate available.
• EPO → ERFE → BMP↓ → hepcidin ↓ (indirect): Spirulina iron supports early Hb synthesis → reticulocyte expansion → EPO responsiveness ↑ (iron-replete erythroblasts respond to EPO better) → ERFE ↑ → BMP6 sequestration ↓ → SMAD1/5/8↓→HAMP ↓ (erythropoietic suppression of hepcidin amplifies iron mobilisation).
• Nrf2 → HO-1 → haem iron recycling: HO-1 (HMOX1) degrades haem → Fe²&sup+; + biliverdin + CO; Nrf2→HO-1 ↑ → macrophage haem catabolism ↑ → Fe²&sup+; recycled to serum (FPN1-dependent) → iron reutilisation ↑ in AI; biliverdin → bilirubin (antioxidant).
• Vitamin B12/folate co-factors for erythropoiesis: Spirulina methylcobalamin (~7–10 μg/10g)→MTR→methionine cycle→dTMP synthesis (thymidylate synthase; TYMS; requires 5,10-CH2-THF)→DNA replication in erythroblasts supported; RBC macrocytosis prevented; iron and B12 co-supplementation effect on Hb ↑ 10–20 g/L in IDA-plus-B12-deficiency context.

Clinical Correlates and Dosing

Human RCTs: 1–8 g/day spirulina for 4–16 weeks in IDA/AI subjects: Hb ↑ 5–15 g/L; serum ferritin ↑ 30–60%; TIBC ↓; hepcidin ↓ 15–25% in chronic inflammatory subjects (2 RCTs). In iron-sufficient healthy subjects, iron status unchanged (hepcidin homeostatically prevents iron overload from spirulina supplementation — safety confirmed). Interactions: iron supplements (FeSO4) + spirulina: complementary absorption enhancement (phycocyanin chelation may improve FeSO4 solubility); no adverse interactions reported. Tea/coffee polyphenols (iron absorption inhibitors): separate spirulina intake from tea/coffee by 1–2 hours to maximise absorption.