Full mechanism, in text

The body controls absorption by need

1 · DMT1 uptake

Fe²⁺ in the gut lumen is pulled into the enterocyte by DMT1, which recognizes only Fe²⁺, so vitamin C or gastric acid must first reduce the iron.Fe²⁺ in the gut lumen is pulled into the enterocyte by DMT1 (divalent metal transporter 1) on the apical membrane — this step is the same for everyone, with diet determining how much iron is available.

DMT1 only recognizes Fe²⁺, not Fe³⁺ — so the reducing action of vitamin C / gastric acid is a prerequisite for this step.

2 · Low iron → gate open

Ferroportin (FPN) on the cell's basolateral membrane is the only channel that allows iron to leave the cell and enter the blood.When the body's iron stores are low:
· Low + erythroferrone (the erythropoiesis-demand signal) suppress hepcidin synthesis in the liver.
· Plasma hepcidin level is low.
· → Ferroportin stays open → Fe²⁺ is pumped into the blood → systemic circulation.

This is the chemical root of "when iron is low, the body actively absorbs more" — on the same diet, an iron-deficient person can absorb 2-3× more.

3 · High iron / inflammation → close

When iron stores are full or inflammation is chronic, the liver makes more hepcidin, which binds ferroportin and has it degraded, locking Fe²⁺ inside the enterocyte.When iron stores are full or there is chronic inflammation ( signaling):
· The liver sharply upregulates hepcidin synthesis (a 25-amino-acid peptide).
· Hepcidin reaches enterocytes via blood → binds ferroportin → triggers internalization + lysosomal degradation.
· The ferroportin channel closes → Fe²⁺ is locked inside the enterocyte.

This is the mechanism of anemia of chronic disease — IL-6 keeps hepcidin elevated long-term → iron cannot be absorbed even when stores are low. Common in , RA, , malignancy, and other chronic inflammatory states.

4 · Shed → out in stool

Locked Fe²⁺ builds up inside the enterocyte, but an enterocyte lives only ~3-5 days, and the whole epithelial layer naturally sloughs off into the lumen and leaves with stool.This is why "healthy people don't absorb excess iron": hepcidin throttles automatically, and any excess iron is discarded before it ever reaches blood.

Exceptions:
· Hereditary hemochromatosis (HFE C282Y homozygotes) → the hepcidin pathway is broken → lifelong over-absorption → iron slowly deposits in liver, heart, and pancreas.
· β-thalassemia and similar conditions: erythroferrone is chronically high → hepcidin is chronically low → over-absorption.
· Massive transfusions → bypass absorption control and enter blood directly.

This system is counterintuitive but elegant: the body decides whether to absorb iron, not you. So "check iron before supplementing" is the only iron discipline.

Full mechanism, in text

1 · DMT1 uptake

2 · Low iron → gate open

3 · High iron / inflammation → close

4 · Shed → out in stool

Hepcidin gates ferroportin

Full mechanism, in text

1 · DMT1 uptake

2 · Low iron → gate open

3 · High iron / inflammation → close

4 · Shed → out in stool

Hepcidin gates ferroportin