Grapefruit Intake and Breast Cancer Risk
August 13, 2007 Grapefruit juice was first found to contain a chemical compound that can inhibit the enzymes the body uses to break down certain medications in 1989. Since then, grapefruit and its juice have been found to interact with a number of different drugs, including estrogen. (You can read more about grapefruit and the medications it is known to interact with here).
Kristine Monroe, PhD, an epidemiologist at the University of Southern California, first learned about the connection between grapefruit and medications when a friend asked her to research a hormone she was taking to treat hot flashes. Intrigued, she began to dig deeper. It didn't take her long to learn that, in 2006, the US Food and Drug Administration (FDA) had mandated that all hormone products for postmenopausal women be labeled to include a warning that grapefruit juice might increase blood levels of estrogen. Well aware of the research showing that estrogen exposure increases breast cancer risk, Monroe decided to explore whether it was possible that eating grapefruit might do the same as well.
To answer this question, Monroe and her colleagues analyzed the food questionnaires collected by the Hawaii-Los Angeles Multiethnic Cohort Study (MEC), a prospective study of diet and cancer involving 212,000 people from five racial-ethnic groups. They identified 46,080 postmenopausal women in the MEC study, 1657 of who had developed breast cancer. Next, they looked at how much grapefruit these women ate. (They could not assess how often the women drank grapefruit juice because it was combined with orange juice in the food questionnaire.)
Monroe and her team found that eating 1/4 of a grapefruit or more per day was associated with a 30 percent increase in breast cancer risk in these postmenopausal women. Although there was a slightly increased risk associated with eating less than that amount, it was not statistically significant. The increased risk from eating 1/4 or more of a grapefruit per day was seen in women who never took hormone therapy to treat menopausal symptoms as well as in women who were taking estrogen alone or combined estrogen and progesterone. Breast cancer risk was also higher in the thin women who ate grapefruit than it was in the overweight women who did.
Monroe acknowledges that while her findings are intriguing and biologically plausible, they must be interpreted with caution. "This is the first report of this association," she underscores, "and it needs to be confirmed in other studies that have better measures of grapefruit—both the fruit itself, and the juice. It also needs to be understood that these findings are only relevant to postmenopausal women because we did not study premenopausal women."
Susan says: This study is intriguing because it is scientifically plausible and well done.
It is interesting to note that the study found that grapefruit intake increased circulating estrogen levels in women not taking any hormones and in women who were on hormone therapy. This appears to indicate that grapefruit juice can affect not only the estrogens women produce naturally but those that they take to treat menopausal symptoms.
It is also interesting that this study found that grapefruit increased breast cancer risk the most in postmenopausal women who had the lowest levels of estrogen to being with, such as women who are thin or not using HRT. This is a finding we've seen in other estrogen studies.
What do this study's findings mean for women who are on hormone therapy to treat menopausal symptoms? Should you not eat grapefruit if you are at high risk for breast cancer? What if you already have had breast cancer? Or are on tamoxifen or an aromatase inhibitor? What about premenopausal women? Should they be worried too?
Given the media attention this study has received, it makes sense that women would be asking questions such as these. But as Dr. Monroe noted, this is the first study done on this topic and, until the findings are replicated, I don't think that postmenopausal women should foreswear grapefruit forever. That said, if you do eat a lot of grapefruit, especially if you are taking hormone therapy to treat menopausal symptoms or are at increased risk for breast cancer, you might want to replace some of that grapefruit with oranges or other citrus fruit.
