New European research suggests that two common compounds found in iron supplements could increase the growth of a known biomarker for cancer.

Carried out by Chalmers University of Technology, Sweden in collaboration with researchers from the UK Medical Research Council and the University of Cambridge, the study looked at the effects of ferric citrate and ferric EDTA, often used in dietary supplements, on human cancer cells in the laboratory.

The team used normal supplemental doses of the compounds and compared the effects of both to ferrous sulphate, another commonly available iron compound.

They found that although ferrous sulphate had no effect on the cells, both ferric citrate and ferric EDTA caused an increase of amphiregulin, a biomarker for cancer, even at low doses.

"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," says Nathalie Scheers, lead author on the study.

Both ferric citrate and ferric EDTA are used in products available around the world, including the USA and the EU.

However, with so many different types of iron supplements available, which can include at least 20 different iron compounds, it can be hard for consumers to know what they are buying, and which to avoid.

"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," explained Scheers.

As to what action consumers should now take, Scheers said, "First, 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. At the moment, people should still follow recommended medical advice. As a researcher, I cannot recommend anything -- that advice needs to come from the authorities. But speaking personally, if I needed an iron supplement, I would try to avoid ferric citrate."

Despite many studies on the subject, it is still unclear whether taking vitamin supplements has any benefits for health.

Some research has suggested that they can have a positive effect on health, for example linking vitamin D supplements with a lower risk of asthma, while others have shown that some supplements could at best be ineffective, and at worst dangerous to health.

A 2017 US study found that a link between taking high-dose long term supplementation of vitamins B6 and B12 and an increased risk of lung cancer in men, while a 10-year 2015 study found that taking above the recommended doses of dietary supplements, such as the common multivitamin, could raise the risk of developing cancer and heart disease by more than 20 percent.

The findings of the new study come just days after a report by Kaiser Health News suggested that many Americans are taking supplements, despite little evidence that they work. The report cited a 2013 Gallup poll which found that more than half of adult Americans take supplements, including 68 percent of those age 65 and older, and the results of a 2017 study, which found that 29 percent of older adults take four or more supplements.