Tannin, oxalate, and phytate grab before the door; calcium messes with the step of going in.
Full mechanism, in text
Calcium and phytate · also drag iron
1 · Iron enters through only a very narrow door
DMT1 in the duodenum only takes ferrous iron, so the ferric iron in food must first be reduced in the lumen before it can get in.Iron is mainly taken up in the duodenum. DMT1 on the food-facing side only recognizes ferrous iron. Most iron in food is ferric and has to be reduced in the lumen first. Inside the cell, another door still has to send it into blood.
2 · Class one: grab the iron before the door
Tannins and phytate wrap iron into insoluble clumps in the gut fluid, intercepting it before it reaches the cell.Tannins and phytate carry sites that grab metals and wrap iron into an insoluble clump in the gut fluid. Locked inside, it can neither be reduced to ferrous nor squeeze into the porter. The intercept happens before iron meets the cell. Oxalate, often named alongside them, did not lower iron absorption when added to a meal in a human trial; what it mainly binds is calcium.
3 · Class two: iron has reached the door, calcium messes with that step
Calcium does not clump iron but interferes with the transport itself, so a lot of calcium in the same meal lets less iron from that meal in.Calcium is not the kind of molecule that wraps iron into a clump. It interferes with this act of transport, after the iron already qualifies to go in; exactly which step it blocks is still unsettled. A large amount of calcium in the same meal and that meal's iron gets in less. The next meal without calcium lets iron in as usual. It differs from class one in one more way: it may lower the absorption of heme iron from meat a little too.
4 · The fixes differ because the blocks sit in different places
Class one is solved within the same meal by vitamin C, soaking or fermenting; for calcium, the most reliable fix is a different time from the iron-rich meal.For class one you do not have to space them in time: vitamin C, soaking, fermenting all solve it in the same meal. For calcium, reasoning from where it blocks, vitamin C should not help much; that step is inferred from the mechanism and has not been tested directly. So the most reliable move is to put the calcium pill or big glass of milk at a different time from this iron-rich meal. Iron in meat sits inside a heme ring and takes another road, so the clump cannot form on it; calcium is the exception and may lower heme iron a little too.