Dietary flavonoid intake and breast cancer risk among women on Long Island.

Flavonoids are a group of polyphenolic compounds found in high concentrations in fruits and vegetables, as well as other foods of plant origin.  Experimental studies have shown that flavonoids inhibit a number of mechanisms responsible for the development of certain cancers, particularly cancer of the breast.  Until recently, epidemiologic research regarding flavoinoid intake and breast cancer development has been limited due to difficulties in estimating flavonoid intake.  Previous studies based in Italy and Greece investigating the association between flavonoid intake and breast cancer incidence have used flavanoid databases from the US Department of Agriculture (USDA) to assess intake.  Based on this information, a reduced risk of breast cancer was observed for intake of 2 flavonoid classes – flavones and flavonols.  The objective of the present study was to investigate the association between flavonoid intake and breast cancer risk in women participating in the Long Island Breast Cancer Study Project (LIBCSP). 

The LIBCSP was a case-control study conducted in Nassau and Suffolk counties on Long Island, New York.  1434 cases and 1440 controls participated in the present study.  Cases were English-speaking women with either in-situ or invasive breast cancer that was diagnosed between August 1, 1996 and July 31, 1997.  Controls were frequency matched to cases in 5-year age groups.  Women were administered a standardized questionnaire and requested to report on a variety of known and suspected breast cancer risk factors.  Both cases and controls provided a diet history for the previous 12 months, including frequency and portion size.  The content of total flavonoids and seven classes of flavonoids (flavonols, flavones, flavan-3-ols, flavanones, anthocyanidins, isoflavones, and lignans) were estimated via a database created for use in the LIBCSP.  This database included both the USDA Database for the Flavonoid Content of Selected Foods and the USDA – Iowa State University Database on the Isoflavone Content of Selected Foods. 

An inverse association between flavonoid intake and breast cancer risk was observed.  This decrease in risk was most pronounced among postmenopausal women for intake of flavonols (odds ratio (OR) = 0.54, 95% confidence interval (CI): 0.40, 0.73; flavones (OR=0.61, 95% CI: 0.45, 0.83), flavan-3-ols (OR=0.74, 95% CI: 0.55, 0.99); and lignans (OR=0.69, 95% CI: 0.51, 0.94). No association between intake of flavanones, anthocyanidins, and isoflavones and breast cancer risk was observed in postmenopausal women.  In premenopausal women, no association between risk of breast cancer and intake of any class of flavonoids or lignans was observed. 

This case-control study provides evidence that an increased intake of flavonoids and lignans is associated with a reduced risk of breast cancer in US postmenopausal women.  These results are similar, but more modest, to previous findings observed in Mediterranean women.