Could eating grapefruit daily raise a woman's risk of breast cancer? Possibly, researchers from the University of Southern California report in the British Journal of Cancer.


But they stress that the finding is preliminary and must be investigated further before any recommendations can be made about how much grapefruit women should eat.


"It's an interesting hypothesis, there's biological plausibility, and we want other researchers to look at it," says lead study author Kristine R. Monroe, PhD, a research associate in the Department of Preventive Medicine at the university's Keck School of Medicine.


Monroe's findings come from data on 46,080 women taking part in a large lifestyle study of people from 5 ethnic/racial groups (African American, Japanese American, Hispanic/Latina, Native Hawaiian, and Caucasian) in Los Angeles and Hawaii. The women were all past menopause and none had ever had breast cancer. They filled out questionnaires about their eating habits that included information on how much grapefruit or pomelo they ate. They were followed from the study's beginning in 1993 to 2002.


The women who ate the most -- about one-quarter grapefruit per day or half a grapefruit every other day -- had a 30% higher risk of breast cancer than women who ate no grapefruit. The risk remained high even after the researchers controlled for other factors that could influence breast cancer, like weight, whether the women used hormone therapy after menopause, and family history of the disease.


Researchers say grapefruit's unique effects on metabolism may be behind the increased risk seen in this study. The fruit is known to interact with many drugs, causing more of the medication to circulate in the blood. In effect, many drugs become more potent if taken with grapefruit, and many carry warning labels about this potential effect.


Estrogen -- the hormone that fuels most breast cancers -- is one of the substances that interacts with grapefruit. At least two previous studies have found higher estrogen levels in women consuming grapefruit or grapefruit juice, and the US Food and Drug Administration requires hormone replacement products to carry warning labels stating that grapefruit juice may increase the concentration of estrogen in the body.


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