A nutritional supplement popular with dieters and bodybuilders is partially converted into a carcinogenic form when it enters cells, new Australian research has found.
Chromium supplements, which are available over the counter, are also taken by some people with diabetes.
The findings raise concerns about the risks of taking chromium pills long term or in high doses, adding to a growing body of evidence indicating they are unsafe.
The research, led by Lindsay Wu from the University of NSW's School of Medical Sciences and Peter Lay from the University of Sydney's School of Chemistry, was supported by the Australian Research Council, with the findings recently published in the chemistry journal Angewandte Chemie.
"All the evidence is there's very little benefit, particularly at low doses, of taking chromium supplements, and at higher doses they're potentially hazardous," Professor Lay said.
"We can't say for certain whether it increases the risk of cancer ... [but] the fact that we can generate the carcinogenic form in living cells is quite a concern."
Professor Lay said the latency period for chromium-induced cancer was often more than 20 years.
"Chromium supplements have only been widely taken for the last couple of decades," he said. "If there are problems, they'd only be starting to emerge."
Chromium is a trace mineral found in two main forms: trivalent chromium (III) and hexavalent chromium (VI).
Trivalent chromium (III) picolinate and a range of other chromium (III) forms are sold as nutritional supplements.
Hexavalent chromium (VI) is classified as carcinogenic to humans by the International Agency for Research on Cancer. It was the contaminant in the drinking water of the Californian town of Hinkley, linked to a spate of illnesses and made notorious by the film Erin Brockovich.
Despite debate over its effectiveness, people take chromium to lose weight and treat metabolic disorders, such as insulin resistance and type 2 diabetes. Some contain up to 500 micrograms per tablet, when 25-35 micrograms is the daily intake recommended for adults.
"People tend to think that, if a little bit's good, a lot's better, but we know that's not the case," Professor Lay said. "The other concern is people tend to think that anything you can buy over the counter is safe."
The research team treated animal fat cells in the laboratory with a chromium (III) supplement, then mapped the chemical elements within them under an intense X-ray beam at a synchrotron facility.
"We were able to show that oxidation of chromium inside the cell does occur, as it loses electrons and transforms into a carcinogenic form," Dr Wu said. "It's the first time anyone's seen it in a cell."
The same results are expected in human cells. However, they are very unlikely to apply to trace amounts of chromium (III) found in food.
The chief executive of Diabetes Australia, Professor Greg Johnson, said doctors might recommend chromium supplements for patients with an identified chromium deficiency but generally they were "not recommended anywhere in the world for routine management of diabetes because there's insufficient evidence of benefit".
He advised anyone taking chromium supplements to "make sure your doctor or diabetes educator or health professional knows about it at your next visit".