To our knowledge, this is the first prospective study examining the association between urinary levels of polyphenol flavanols and flavonols and risk of breast cancer. Strikingly, we found urinary excretion of tea polyphenols increased with increasing tea leaves consumed among controls, but not breast cancer cases. We found levels of polyphenol EC and M4 to be significantly higher among controls than their matched cases. However, no statistically significant linear dose-response associations were found between urinary excretion of flavanols and flavonols and risk of breast cancer in categorized analyses. In the restricted cubic spline function model with nonlinear terms, we found an overall non-linear dose-response relationship between level of EC and risk of breast cancer, although it was not apparent in low urinary excretion range.
We found in the current study that the dose-response associations between tea drinking amount and urinary tea polyphenols were observed only among healthy controls, which is consistent with a previous study [43
]. We found the dose-response relationship with EC was not as strong as that for EGC in controls. Unlike EGC, EC could also be derived from certain other sources of fruits and vegetables, such as apples. On the other hand, no clear dose-response relationships were seen among breast cancer cases. It is possible that breast cancer patients have a different hormone metabolism pattern from healthy subjects. For example, previous studies found that tea polyphenols and estrogens were metabolized by the same enzymes such as catechol-o-methyltransferase (COMT) [44
] whereas polymorphisms in the COMT
gene may be associated with breast cancer risk [46
] . Therefore, the differing associations between tea drinking amount and urinary tea polyphenols by case-control status may be because polyphenols are metabolized differently in cases and controls. This finding, if confirmed, has very important implications as it can potentially be utilized to develop tests in indentify high-risk women.
Three previous case-control studies evaluated the associations between dietary intakes of flavonols and flavanols and risk of breast cancer and found inconsistent results [50
]. Very recently, a cohort study found dietary intakes of flavonols and flavones not related to the breast cancer risk [53
]. Epidemiologic studies have also generated discordant findings for the association between tea drinking (abundant in flavonols, particularly flavanols) and breast cancer. In a recent meta-analysis of epidemiologic studies, black tea consumption was inversely associated with breast cancer risk in eight case-control studies, whereas in five cohort studies a moderately increased risk was observed [54
]. In accordance with an earlier meta-analysis, a non-significant inverse association was observed between green tea and breast cancer risk in three cohort studies, and only in one case-control study the inverse association reached statistical significance[54
]. We found in a population-based case-control study conducted among Chinese women in Shanghai that regular green tea drinking was related to a 12% reduction in risk of breast cancer [56
], in consistent with another case-control study conducted among Chinese women [57
]. In the current prospective study, we did not find an overall association for green tea drinking [58
]. In addition, previous studies also generated inconclusive results on the associations between dietary intake of allium vegetables (rich in flavonols) and risk of breast cancer [32
Possible explanations for the inconsistencies across previous studies include 1) food preparation processes which may vary by population were not collected in these previous studies; and 2) substantial inter-person variation in absorption of polyphenols have not been considered [26
]. In contrast to the observed significant difference in mean level of EC between cases and controls, we did not find a significant linear dose-response relationship in categorical analyses (by tertile). This may be due to the fact that in the restricted cubic spline function model with nonlinear terms, we also found the dose-response association was non-linear. Furthermore, in the spline curve, the urinary level of EC for most of cases and controls was in the low range, in which we did not find an apparent dose-response association. However, when urinary level of EC further increased, the risk of breast cancer more appreciably reduced. The stronger inverse association observed for EC than EGC and other tea polyphenols is biologically plausible because previous in vitro
and in vivo
studies found EC was more potent than EGC in inhibiting breast cancer cell lines and breast cancer progression [64
]. We, however, cannot exclude the possibility that this finding is solely due to chance or other underlying confounding factors.
The present study has several notable strengths. Levels of bioavailable polyphenols were measured together with its major metabolites using a sensitive method (LC/PDA/ESI-MS). The parent study, the population-based Shanghai Women's Health Study, had high rates for baseline participation and follow-up, which minimized selection bias. However, one spot urine was used to measure urinary levels of polyphenols, which may not reflect well the long-term exposure levels. Intra-person variation should lead to non-differential misclassification, which usually biases the result to the null. To the extent that intra-person variation levels of these polyphenols exist in our data, the true associations of EC and other tea polyphenol levels with breast cancer risk could be stronger than those we observed.
We found the relationship between tea consumption and urinary excretion levels of tea polyphenol differed in healthy subjects from breast cancer cases. If confirmed, this finding may be used to develop strategies to identify high-risk women. In addition, our finding suggests a possible link between urine EC level and a reduced risk of breast cancer. Future studies are necessary to confirm this finding, particularly longitudinal studies with multiple collections of biological samples at different time window.