The mineral selenium may help protect some women from developing breast cancer, research has suggested.
The element, which can be found in brazil nuts, liver and kidneys, may help the body defend itself.
Scientists from the University of Illinois believe they may have worked out how selenium interacts with a natural body chemical to offer protection.
This is not the first health claim to be made about selenium.
Studies have suggested that it can reduce the likelihood of other types of cancer, and some have linked it to a lowered chance of heart disease.
The latest study looked at tissue samples from more than 500 women who did not have breast cancer, and compared their genetic makeup with those in 79 breast cancer tissue samples.
The scientists were looking for genes which are responsible for the production of an enzyme which the team believe has cancer-fighting properties.
They found that different versions of these genes were more common in the breast cancer tissue samples.
Genetic fingerprint
Their conclusion was that certain women - with a certain genetic makeup - might benefit from extra selenium in their diet to make sure their "cancer-fighting" enzyme worked properly.
However, the researchers stopped short of recommending selenium supplementation, saying that their research was at far too early a stage.
Professor Alan Diamond said: "For over 20 years, animal studies have shown that tiny amounts of selenium in the diet can suppress cancer in several types of organs.
"The animal data is very strong, but human data is just emerging.
"We believe there are certain proteins in mammalian cells that contain selenium that can mediate the protective effects, but proving that is difficult."
Cautionary note
However, experts at CancerBacup said that it was important not to read too much into the study.
"Scientists don't yet know whether these genes actually have a significant impact on cancer risk, says Charlotte Augst, its Genetic Information Project Manager.
"Secondly, there is not yet a firm evidence base showing that selenium actually reduces people's risk of developing cancer.
"Both these areas of uncertainty require more research. There's obviously no harm in women eating brazil nuts, but people should consult their GP's before taking selenium supplements as selenium itself has toxic effects if too much of it is taken."
The study was published in the journal Cancer Research.
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