Which form of iron is best absorbed?

Haem iron, the form of iron found in meat, fish and poultry, is absorbed considerably better than the form found in plant-based foods, dairy and eggs. Your body absorbs 15% to 35% of haem iron, depending on how well stocked your iron stores are. For non-haem iron that figure is only 2% to 20%. In addition, haem iron is barely inhibited by other substances in a meal, whereas that factor plays a significant role for non-haem iron.

What you eat at the same time makes a big difference for non-haem iron. Vitamin C is the most powerful enhancer: it can increase absorption up to fourfold, especially in combination with meat or fish. Tea, coffee and phytates (the substances naturally present in grains and legumes) substantially inhibit absorption. A practical rule of thumb: if you eat little meat, combine your plant-based iron sources with a glass of orange juice or another vitamin C-rich source, and preferably drink tea or coffee between meals rather than with them.

People who have eaten little haem iron for a long time, such as vegans, may partially adapt. A small study (27 participants) found that vegans absorbed non-haem iron better than people on a mixed diet. Lower blood levels of the hormone hepcidin, which normally suppresses iron absorption, likely played a role in this. This is an interesting indication, but the study is too small to draw firm conclusions from.

More haem iron is not automatically better. There are indications that a high intake of haem iron from red meat is associated with a higher risk of certain forms of cancer. This association has not yet been well quantified, but it is a reason not to treat haem iron as a case of 'the more the better'. A recent review does argue for research into haem iron supplements for people with iron deficiency, but two of the four authors of that review are affiliated with companies that sell such products, which should be taken into account when reading those conclusions.