The relationship between grapefruit and breast cancer risk is highly speculative and has to do with how estrogen is metabolized in the body. Elevated levels of estrogen are associated with an increased risk of breast cancer.[1] Normally, the cytochrome P450 enzyme CYP3A4 metabolizes estrogen into a form that is more easily removed from the body. However, grapefruit inhibits this enzyme, raising the question of whether grapefruit might lead to elevated estrogen in the body and potentially an increased risk of breast cancer.
Although biologically plausible, research has not provided a clear answer. In one observational study, higher grapefruit consumption was associated with an increased risk of breast cancer. However, in two other studies, no association was found.[2][3] Furthermore, a study examining the effects of grapefruit on endogenous estrogen levels in postmenopausal women reported unclear and contradictory effects. Whole grapefruit did not affect levels of estrone (E1) or estradiol (E2), while fresh grapefruit juice, bottled juice, and soda all lowered E2 — the opposite of what was hypothesized.[4]
Grapefruit has been reported to increase the plasma concentration of estrogen taken orally.[5] Because grapefruit inhibits CYP3A4 primarily in the intestines rather than the liver,[5] the interaction between grapefruit and estrogen might only be relevant when estrogen is being taken orally, rather than being produced naturally in the body.