Dietary flavonols and flavonol-rich foods intake and the risk of breast cancer.

Laboratory and animal studies suggest that dietary flavonols may reduce breast cancer risk but there are limited epidemiological studies. We computed flavonol intakes from dietary data collected by validated food frequency questionnaires in 1991 and 1995 from 90,630 women in the Nurses Health Study II. Using multivariate relative risks (RR) and 95% confidence intervals (95% CI), we evaluated the association of flavonol intake with breast cancer risk in women who were premenopausal and aged between 26 and 46 years at baseline in 1991. During 8 years of follow-up, we documented 710 cases of invasive breast cancer. The multivariate RR (95% CI), comparing highest to lowest quintiles of cumulative average intake, was 1.05 (0.83, 1.34; p-value for test of trend=0.96) for the sum of flavonols and there were no associations seen between individual flavonols such as kaempferol, quercetin and myricetin and breast cancer risk. The multivariate RR (95% CI), comparing highest to lowest quintiles of cumulative average intake, was 0.94 (0.72, 1.22; p-value for test of trend=0.54) for sum of flavonol-rich foods. Among the major food sources of flavonols, we found a significant inverse association with intake of beans or lentils but not with tea, onions, apples, string beans, broccoli, green pepper and blueberries. The multivariate RR (95% CI), comparing the highest category (2 or more times a week) of cumulative average beans or lentils intake with the lowest category (less than once a month), was 0.76 (0.57, 1.00; p-value for test of trend=0.03). While we found no overall association between intake of flavonols and risk of breast cancer, there was an inverse association with intake of beans or lentils that merits further evaluation.