A new study by U.S. researchers shows that eating small amounts of grapefruit every day could raise the risk of developing breast cancer by almost a third.

The study looked at 50,000 post-menopausal women and found that eating just a quarter of a grapefruit daily raised the risk by up to 30 percent, according to a report from BBC News.

The fruit is thought to boost levels of oestrogen - the hormone associated with a higher risk of the disease, the British Journal of Cancer reported.

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For the study, women filled out questionnaires asking how often they ate grapefruit and how big their portions were.

The findings showed women who ate one quarter of a grapefruit or more every day had a higher risk of developing breast cancer and higher levels of oestrogen in their bodies than those who did not eat the fruit at all.

"Therefore, if grapefruit intake affects oestrogen metabolism leading to higher circulating levels, then it is biologically plausible that regular intake of grapefruit would increase a woman's risk of breast cancer," the researchers wrote in the study.

But the University of Southern California and University of Hawaii researchers, as well as other experts, said more research is needed.