Full mechanism, in text

Mechanism · why heme iron absorbs well

1 · Two doors, two fates

Food iron comes in two forms, and they use two different doors.The heme iron in lamb, beef and liver sits inside a ring-shaped cage, and the gut wall has a dedicated channel that takes in the whole molecule; it is absorbed several times more efficiently than plant iron, and the rest of the meal barely affects it.

The non-heme iron in plants first has to be reduced before it can get in, and tannins in tea and coffee and phytate in grains hold it back — vitamin C gives it a clear boost.

Across a whole day's diet the gap is smaller than a single meal suggests: Hurrell & Egli 2010 estimate that, in people with no iron stores, a mixed diet delivers about 14-18% iron absorption and a vegetarian diet about 5-12%. The big swings that inhibitors and enhancers produce in single-meal isotope studies shrink a lot in multi-meal studies with a varied diet, and a person's own iron status matters more than what the diet is made of.

So for efficiency of getting iron in, a small portion of red meat matches a big plate of spinach. But that is about efficiency, not about more being better — why becomes clear once you see what happens after the door.

2 · Past the door, one room

Once the ring cage is taken whole into the intestinal-wall cell, an enzyme inside opens the ring, and only then is the iron released.The iron that comes out joins the same pool as plant iron.

This step is easy to skip past, but everything below depends on it: the two irons walk through two different doors, and once inside they are in the same room. From here on the body no longer cares whether this iron atom arrived from lamb or from spinach — only how much iron is in the room.

3 · To the blood, a single exit

For iron to be handed from the intestinal-wall cell into the blood, it has to pass a dedicated exit protein on the other side of the cell.There is no second route. Every iron atom in the room, whichever door it came through, queues for this same exit.

Hold that picture: two entrances, one room, one exit. What happens next is that something goes and moves that exit — and that turns out to be the body's entire method of regulating iron.

4 · The gate is at the entrance

When the liver hormone hepcidin rises, it takes the exit to the blood off the membrane of the intestinal-wall cell.With the exit gone, iron is left inside the intestinal-wall cell; and intestinal lining cells turn over fast, so within days the cell and its iron drop off together in the stool.

Two things make it rise: the body has enough iron, or the body is inflamed. Which makes all three of these make sense:

Topping up iron is not more-is-better — there is no routine channel for putting surplus iron out, apart from bleeding, menstruation and cell shedding. And it has been measured: an oral dose of iron itself pushes hepcidin up, so the next dose is absorbed less.Inflammation brings anemia, and iron often does not fix it — iron is held at the entrance and cannot come in, and iron already in the blood is hidden into stores. This is the body competing with bacteria for iron. What needs treating is the inflammation.Inflammation also makes the lab report lie — is an acute-phase reactant, lifted by roughly a third when inflamed, which covers up iron deficiency that should have been found.
Back to the table: for someone who needs iron, lamb's efficiency is a real advantage. For someone already iron-replete, eating more red meat to top up a little more has no point — the absorption end closes itself down.

Full mechanism, in text

1 · Two doors, two fates

2 · Past the door, one room

3 · To the blood, a single exit

4 · The gate is at the entrance

Lamb — the one-way iron gate

Full mechanism, in text

1 · Two doors, two fates

2 · Past the door, one room

3 · To the blood, a single exit

4 · The gate is at the entrance

Lamb — the one-way iron gate