Eating grapefruit might actually increase breast cancer risk, but the evidence here is preliminary and needs more study before we can draw any definitive conclusions. What we do know comes from a study published in the July 31, 2007 British Journal of Cancer. More than 46,000 women from five ethnic groups in Los Angeles and Hawaii participated in the study. All were past menopause, and none had ever had breast cancer. They all filled out questionnaires about their eating habits, including the amount of grapefruit or pomelo they consumed (pomelo is an Asian fruit that is bigger than grapefruit with coarser texture; the fruit we know as grapefruit is actually a pomelo/orange cross).
Researchers at the University of Southern California followed the women from 1993 to 2002. When they analyzed their data, they found that those who ate the most grapefruit (about one quarter per day or half a grapefruit every other day) had a 30 percent higher risk of breast cancer than women who ate no grapefruit. This risk did not change even after researchers controlled for such factors as weight, family history of breast cancer, and use of hormone replacement therapy after menopause.
The investigators noted that estrogen (the hormone that promotes most breast cancers) interacts with grapefruit and that at least two earlier studies found higher levels of estrogen in women who eat a lot of grapefruit or drink a lot of grapefruit juice. This is such a well known effect that the FDA requires hormone replacement products to carry a label warning that grapefruit juice may increase their potency. We also know that grapefruit increases the potency of some other prescription drugs.
It's much too soon to conclude that grapefruit and grapefruit juice increase the risk of breast cancer. We need to know how long grapefruit's effects on estrogen last in the body, whether grapefruit itself or grapefruit juice is responsible for the change and what effects grapefruit and grapefruit juice have in premenopausal women. In the meantime, if you're worried, you might want to substitute other citrus fruits for grapefruit.
Andrew Weil, M.D.