Common iron compounds, often used in dietary supplements and as a food additively, can increase the risk of colon cancer, a study has found.
The researchers from Chalmers University of Technology in Sweden studied ferric citrate and ferric EDTA, which have both previously been shown to worsen tumour formation in mice with colon cancer.
The science behind this has been little understood until now, and possible effects on human cells were not previously investigated.
Researchers, including those from the UK Medical Research Council and Cambridge University in the UK, looked at the effect of normal supplemental doses of these compounds on two types of cultured human colon cancer cells.
As a comparison, they also measured the effects of ferrous sulphate, another very commonly available iron compound.
While ferrous sulphate had no effect, both ferric citrate and ferric EDTA caused an increase in cellular levels of amphiregulin, a biomarker for cancer. This was the case even at low doses.
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"We can conclude that ferric citrate and ferric EDTA might be carcinogenic, as they both increase the formation of amphiregulin, a known cancer marker most often associated with long-term cancer with poor prognosis," said Nathalie Scheers, assistant professor at Chalmers University of Technology.
Today there are many different types of iron supplements on the market. These can be based on at least 20 different iron compounds, and sold under a wide range of brands.
Ferric sulphate is one of the most common, but ferric citrate, which is said to be gentler for the stomach, is also widely available in stores and online. It is also more easily absorbed by the body through foods such as granary bread, beans and nuts.
However, for consumers looking to make an informed choice, it can often be difficult to know what exactly they are buying.
"Many stores and suppliers don't actually state what kind of iron compound is present - even in pharmacies. Usually it just says 'iron' or 'iron mineral', which is problematic for consumers," said Scheers.
Iron is also added to some foods, to combat iron deficiency. Ferric EDTA is approved as a fortifying agent in both the US and the EU. It is also used in countries such as China, Pakistan, Brazil, Mexico and The Philippines, where it is added to flour and powdered drinks.
Additionally, it is present in certain medicines for children with low iron levels in countries such as the UK and France.
"We must bear in mind that the study was done on human cancer cells cultured in the laboratory, since it would be unethical to do it in humans. But, the possible mechanisms and effects observed still call for caution. They must be further investigated," said Scheers.
Research in the field has so far been limited, even concerning the more common ferrous sulphate.
"We need to consider that different forms can have different biological effects," said Scheers.
Most of the iron that the body needs is obtained through food such as meat, fish, vegetables, fruits and whole grains. However, sometimes this is not enough.
Pregnant women may need additional iron, as well as people who have lost blood or have low haemoglobin levels for other reasons. In patients with kidney disease, high doses of iron may be needed to bind phosphates into the bloodstream.