A nutritional supplement used for weight loss and body building is partially converted into a carcinogenic form when it enters cells, say Australian researchers.
Chromium supplements also are used by people with metabolic disorders such as insulin resistance and type 2 diabetes.
The UNSW and University of Sydney findings, published in the chemistry journal Angewandte Chemie, raise concerns over the risks of taking chromium pills long term or in high doses.
Chromium is a trace mineral sold as nutritional supplements, with some containing up to 500 micrograms per tablet.
The US National Academy of Sciences has estimated up to 200 micrograms of chromium is a safe and adequate daily dietary intake for adults.
Australia's current National Health and Medical Research Council Nutrient Reference Values, which are under review, recommend 25-35 micrograms of chromium daily as an adequate adult intake.
UNSW's Dr Lindsay Wu said the researchers treated animal fat cells with chromium (III) in the laboratory.
A map was created of every chemical element contained within the cell using a synchrotron's intense X-ray beam.
This allowed them to not only see the chromium spots throughout the cell but also to determine whether they were the carcinogenic form, Dr Wu said.
"We were able to show that oxidation of chromium inside the cell does occur, as it loses electrons and transforms into a carcinogenic form," he said.
"This is the first time oxidation was observed in a biological sample with the same results expected in human cells."
They say more research is needed to ascertain whether the supplements significantly alter cancer risk.
