This may come as a surprise to lovers of grapefruit.
An epidemiological study published in the July 2007 British Journal of Cancer suggets there could be a correlation between grapefruit consumption and breast cancer in menopausal and post-menopausal women.
The biological plausible mechanism of action for the observed effect is that grapefruit is a well known cytochrome CYP34A inhibitor (membrane-bound hemoprotein enzyme). As a result, regular consumption of grapefruit can increase circulating concentrations of estrogens, which are otherwise metabolized by these enzymes.
Higher systemic levels of estrogens are associated with a increased risk of breast cancer in older women.
The study conclusion suggests the information in this study might be sufficient for menopausal and post-menopausal women to limit their grapefruit consumption.
FYI: The Texas red grapefruit was designated the offiicial state fruit in 1993. Given the large number of people that could be affected by the study above; they may want to rethink this decision.