Polyphenols, the most abundant dietary
antioxidants, also possess many other anticarcinogenic activities. Urinary metabolites of
polyphenols could
complement dietary assessment of the bioavailability of these nutrients. We conducted a study of 353 incident
breast cancer cases and 701 individually matched controls nested within the Shanghai Women's Health Study cohort of women aged 40-70 years at baseline. Liquid chromatography photo-diode array electrospray mass spectrometry was used to measure
tea polyphenols (
epicatechin,
epigallocatechin, and their metabolites) and
flavonols (e.g.,
quercetin and
kaempferol). Multivariate conditional logistic regression analyses were used to assess associations between
breast cancer risk and urinary excretion rates of
polyphenols. Urinary excretion of
tea polyphenols increased with increasing
tea leaves consumed among controls, but not among
breast cancer cases. Compared with cases, controls had higher levels of urinary total
polyphenols and
tea polyphenols, particularly
epicatechin. In contrast, we did not find any dose-response relationship between urinary
polyphenols and
breast cancer risk. Urinary excretion of
epicatechin was inversely associated with
breast cancer risk [odds ratio (OR) and 95% confidence interval (CI) of 0.59 (0.39-0.88) for the intermediate tertile]. In spline regression, we found an overall dose-response relationship between
epicatechin level and risk of
breast cancer, although it was not apparent in low and middle urinary excretion range. In conclusion, high
epicatechin may be related to a reduced risk of
breast cancer. Further studies are warranted to confirm our findings.