Haem iron vs non-haem iron – how do they differ?

In 1840, F.L. Hunefeld described the crystalline structures of haemoglobin, which later became a key milestone in the history of research into blood proteins and haem-bound iron. In the diet, iron occurs primarily in two forms: haem and non-haem, which differ in chemical structure, dietary sources and bioavailability. This distinction is of practical importance, as the total iron content of a product does not always accurately reflect the amount that can be utilised by the body.

1. Differences between forms of iron
2. Iron in the body
3. Iron in the diet

Differences between forms of iron

Haem iron is a form bound to haem, a structure with a characteristic ring-like shape in which an iron atom is located at the centre. In food, it is found in animal-derived products, as haem is a component of proteins present in animal tissues, primarily haemoglobin and myoglobin. For this reason, its sources include meat, offal, poultry and fish, although the proportion of haem iron may vary depending on the type of product and the part of the carcass.

Non-haem iron, as the name suggests, is not bound to haem and is found mainly in its free form in plant-based products, such as pulses, whole grains, groats, seeds, nuts, leafy vegetables and parsley. However, this form is also found in certain quantities in animal-derived products, so the distinction between ‘animal haem’ and ‘plant non-haem’ is somewhat of a simplification.

The properties of both forms differ primarily in terms of bioavailability and susceptibility to the influence of other dietary components, as haem iron is generally more easily absorbed and less dependent on the composition of the meal, whereas the utilisation of non-haem iron depends to a greater extent on conditions in the gastrointestinal tract and the presence of substances that promote or inhibit its absorption.

"Substances that significantly reduce iron absorption include phytates, which are found in bran, seeds and pulses; absorption is also hindered by certain polyphenols, such as those found in coffee or tea, as well as fibre and large amounts of calcium." Łukasz Domeracki – Dietitian

Iron in the body

Iron status should not be assessed solely on the basis of the reported dietary intake, as there may be a significant discrepancy between the intake, absorption, transport, storage and utilisation of this mineral. In basic laboratory assessment, the complete blood count is important, particularly haemoglobin concentration (a protein containing iron), red blood cell count, haematocrit (the proportion of red blood cells in whole blood) and red blood cell indices such as mean corpuscular volume (mean corpuscular volume; MCV) and mean corpuscular haemoglobin (MCH).

However, these indices mainly reflect the consequences of more advanced disorders, which is why ferritin is often also taken into account when assessing iron stores. Ferritin is a protein that reflects iron stores. However, interpreting its blood levels requires caution, as it is also an acute-phase protein and may rise during inflammation. For this reason, when results are inconclusive, it can be helpful to compare ferritin with C-reactive protein (CRP), as well as with iron transport parameters such as serum iron concentration, transferrin, total iron-binding capacity ( TIBC), and transferrin saturation.

Examples of products with a significant iron content per 100 g of product

Iron in the diet

According to European nutritional standards, the reference intake for iron for adults is 14 mg per day (Nutrient Reference Value, NRV), and this value is used in the labelling of food and food supplements within the European Union. However, the European Food Safety Authority (EFSA) points out that iron requirements vary depending on gender, age and physiological status. In women of childbearing age, requirements are usually higher than in men and postmenopausal women, mainly due to losses associated with menstruation. During pregnancy, the requirement for iron increases further.

In dietary supplements, iron is usually present in the form of salts or organic complexes, including iron fumarate, sulphate, gluconate, pyrophosphate or bisglycinate. Typical daily doses declared by manufacturers are around 7–30 mg of iron, which corresponds to approximately 50–214% of the European Nutrient Reference Value (NRV).

Sources:

• Monsen E. R. (1988). Iron nutrition and absorption: dietary factors which impact iron bioavailability. Journal of the American Dietetic Association, 88(7), 786–790.
• Hallberg, L., & Hulthén, L. (2000). Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron. The American journal of clinical nutrition, 71(5), 1147–1160. https://doi.org/10.1093/ajcn/71.5.1147
• Cappellini, M. D., Musallam, K. M., & Taher, A. T. (2020). Iron deficiency anaemia revisited. Journal of internal medicine, 287(2), 153–170. https://doi.org/10.1111/joim.13004