Consuming grapefruit appears to increase risk of breast cancer in post-menopausal women, according to a study in British Journal of Cancer (July, 2007) (Prospective study of grapefruit intake and risk of breast cancer in postmenopausal women: the Multiethnic Cohort Study)

One quarter grapefruit per day compared to none increased risk by about one third. The significance of this slightly increased risk is underscored by 2 factors — women are taught lifelong to associate grapefruit intake with health and with weight-control (the “Grapefruit diet”).

The factor involved in the increased risk is a chemical in grapefruit (CYP3A4) that blocks the body’s breakdown of estrogen.

Since vitamin C and other antioxidants found in grapefruit are readily available from other fruits, with this new information women can choose to switch to oranges, tangerines, lemon juice, and other fruits and avoid grapefruit.

Grapefruit (including grapefruit juice) contains a natural chemical that blocks Cytochrome P450 3A4 (CYP3A4), an enzyme which is involved in the metabolism of estrogens. “There is evidence that grapefruit, an inhibitor of CYP3A4, increases plasma estrogen concentrations,” the researchers write. “Since it is well established that estrogen is associated with breast cancer risk, it is plausible that regular intake of grapefruit would increase a woman’s risk of breast cancer.”

The researchers investigated the association of grapefruit intake with breast cancer risk in the Hawaii-Los Angeles Multiethnic Cohort Study, which follows over 50 000 postmenopausal women from five racial ethnic groups.

“A total of 1657 incident breast cancer cases were available for analysis. Grapefruit intake was significantly associated with an increased risk of breast cancer (relative risk=1.30, 95% confidence interval 1.06 — 1.58) for subjects in the highest category of intake, that is, one-quarter grapefruit or more per day, compared to non-consumers.”

“An increased risk of similar magnitude was seen in users of estrogen therapy, users of estrogen plus progestin therapy, and among never users of hormone therapy. Grapefruit intake may increase the risk of breast cancer among postmenopausal women.”