Heme and non-heme iron: why the form matters more than the number

Almost every interaction in the Atlas that involves iron comes with a qualifier: non-heme iron. That word is doing a lot of work, and it explains why two foods listing the same milligrams of iron can deliver very different amounts to the body.

Two forms, two routes

Heme iron comes bound inside a ring-shaped molecule — the same structure that carries oxygen in blood and muscle. It is found in meat, poultry and seafood. Because it arrives packaged, it is absorbed relatively efficiently and is largely shielded from whatever else is on the plate.

Non-heme iron is everything else: the iron in lentils, beans, leafy greens, nuts, seeds, eggs and fortified flour. It arrives as a bare mineral, and bare minerals are chemically sociable. They bind to whatever they encounter — phytate, polyphenols, calcium — and they can drop out of solution when acidity changes.

That difference in packaging is the reason the Atlas has so many entries about non-heme iron and so few about heme iron. There is simply more to interact with.

What this means for a food label

A label reports how much iron is present, not how much will be absorbed. Estimates based on intake data and isotope studies put overall iron bioavailability at roughly 14–18 per cent for mixed diets and 5–12 per cent for vegetarian diets in people with no iron stores — a difference substantial enough that dietary reference values account for it. US dietary reference intakes set the requirement for people following vegetarian diets at 1.8 times the standard figure, reflecting both the absence of heme iron and the typical presence of phytate and polyphenols in those meals.

This is not an argument for any particular way of eating. It is an argument for reading an iron figure as an upper bound rather than a delivery.

The body adjusts

One more thing separates iron from most nutrients: there is no physiological mechanism for excreting it. Iron balance is regulated entirely at the point of absorption.

The practical consequence is that absorption is not a fixed percentage. Someone with depleted stores absorbs a larger fraction of the iron available; someone replete absorbs less. This regulation is a large part of why interactions that look dramatic in a single test meal often fail to show up as changes in iron status months later — the body has been quietly compensating the whole time.

It is also why blanket statements about iron are difficult. The same meal does different things in different people.

Subjects covered in this chapter

• All interactions involving Bioavailability
• All interactions involving iron