There is a growing list of drugs that interact with grapefruit and grapefruit juice, due to its ability to alter liver metabolism. Grapefruit juice slows activity of the liver enzyme that metabolizes Lipitor, for instance, thus increasing the possible side-effects of the drug.
Now there is preliminary evidence that regular consumption of grapefruit in a post-menopausal woman's diet can increase breast cancer incidence, due to grapefruit's inhibition of CYP3A4, an isoenzyme that metabolizes estrogen.
In the Hawaii-Los Angeles Multiethnic Cohort Study, women in the highest intake category—60 grams or more per day, equal to slightly more than one-quarter grapefruit per day or half a grapefruit every other day—had a relative risk of breast cancer 30% higher than women who ate no grapefruit. The trend of increasing risk with increasing consumption was significant after adjusting for weight, exercise, use of postmenopausal hormone therapy, family history of breast cancer, and other factors that could impact risk.
"The results are consistent with a biological effect of grapefruit on estrogen metabolism. At least 2 previous studies have found higher estrogen levels in women consuming grapefruit or grapefruit juice, the authors note, and grapefruit is known to elevate serum concentrations of many drugs, including hormone replacement therapies. The US Food and Drug Administration requires hormone replacement products to carry warning labels stating that grapefruit juice may increase plasma concentrations of estrogen."
Lead study author Kristine R. Monroe, PhD, Research Associate in the Department of Preventive Medicine at the University's Keck School of Medicine stresses, however, that there's not yet enough evidence to recommend women stop eating grapefruit as a means of lowering their breast cancer risk. "I would use caution until further studies are done and a scientific conclusion can be reached."
http://caonline.amcancersoc.org/cgi/content/full/57/6/321
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