Iron in plant foods is overwhelmingly Fe³⁺ (ferric) — it is poorly soluble at neutral pH and is often locked up by phytate, tannins, and oxalate.Without help, plant-source iron is typically absorbed at only 2-10%.
2 · C reduces it
Vitamin C donates an electron and reduces Fe³⁺ to Fe²⁺ (ferrous).Fe²⁺ is far more soluble in the mildly acidic small-intestinal environment and is no longer easily grabbed by phytate — the door is half open.
3 · Soluble C-Fe complex
Vitamin C also forms a soluble chelate complex with Fe²⁺, escorting the iron to the enterocyte door so higher pH downstream cannot reoxidize it to Fe³⁺.Vitamin C doesn't just "give one electron and leave" — it also forms a soluble chelate complex with Fe²⁺, escorting the iron all the way to the enterocyte door so that downstream higher pH in the gut can't reoxidize it back to Fe³⁺. This is the fundamental reason "with the meal" works better than "separately supplemented."
4 · DMT1 uptake
DMT1 on the enterocyte pulls Fe²⁺ into the cell, and in the end 25-100 mg of vitamin C at the same meal boosts plant-source iron absorption 2-3 times.DMT1 (divalent metal transporter 1) on the enterocyte surface specializes in moving divalent metals — it pulls Fe²⁺ into the cell, then hands it off to ferroportin to enter the blood (regulated by hepcidin). The net result: 25-100 mg of vitamin C at the same meal boosts plant-source iron absorption by 2-3×.