The door only takes ferrous; vitamin C has to share the pot with plant iron.
Full mechanism, in text
Vitamin C helps plant iron absorb
1 · Most plant iron is ferric, and barely dissolves
Non-heme iron in plants is mostly ferric, barely dissolves and readily precipitates in the small intestine before it can reach the wall.Non-heme iron in plants mostly exists as ferric: more stable, at the cost of barely dissolving. In the near-neutral small intestine it readily precipitates; once it sinks, it never reaches the wall.
2 · The porter at the door only takes ferrous
DMT1 on the duodenal wall only takes ferrous iron while food supplies ferric, so a conversion step is missing in between.Iron is mainly taken up in the duodenum. DMT1 on the apical face only recognizes ferrous iron. Food supplies ferric, the door takes ferrous, and the conversion in between is missing.
3 · Vitamin C reduces and escorts in the same meal
In the lumen, vitamin C reduces ferric iron to ferrous and holds it in a soluble complex, escorting it through the door.Vitamin C hands an electron to ferric iron, turning it ferrous; it also holds the iron in a complex that can dissolve. Reduce plus escort is the full meaning of a more absorbable form. The whole sequence happens in the lumen, while this batch of chyme passes the duodenum.
4 · Later does not help; the same door is where antagonism lands
Vitamin C only helps on the spot within the same meal, and a tablet taken hours later does nothing for the iron in that meal.Vitamin C does not build an iron-absorbing constitution. It works on the spot. A tablet a few hours later does not help this meal. Tannins, oxalate, and phytate grab the iron before it reaches the door. Lowest-effort memory: keep vitamin C and plant iron in the same pot.