Orized into six major subclasses based on their range and structural complexity: flavonols, flavones, flavan-3-ols, flavanones, anthocyanins and isoflavones (Figure 1) [14?6]. As reviewed before by Hooper L et al (Table 1), in western diet, major flavonoids included in these subclasses are quercetin, myricetin and kaempferol for flavonols, hesperitin and narigerin for flavanones, epicatechin and catechin for flavan-3-ols, apigenin and luteolin for flavones, cyanidin, delphinidin and malvidin forFlavonoids and Breast Cancer Riskthe risk of breast cancer by performing a meta-analysis of epidemiologic studies. Given that many reviews have been conducted to assess the association between the dietary isoflavone intake and the breast cancer risk, we excluded this issue in the present study.Methods Search StrategyWe conducted a systematic search of literature published before July 
1st 2012 using the Cochrane Library, MEDLINE, and EMBASE Databases and the following search terms: “flavonoid”, “flavonols”, “flavones”, “flavanones”, “flavan-3-ols”, “flavanols”, “anthocyanidins”, “phytoestrogens”, “polyphenolic compounds” and “breast cancer”. We also performed a manual search using reference lists of original articles and relevant reviews. Only fulllength original journal articles were considered and no attempt was made to include abstracts or unpublished studies.Study SelectionFigure 1. Structures of six flavonoid subclasses. doi:10.1371/journal.pone.0054318.ganthocyanidines, and genistein and daizeina for isoflavones [16?18]. Flavonols mainly exist in onions, broccoli, tea, and various GSK -3203591 site common fruits, flavones in aromatic herbs, celery, and chamomile tea, flavan-3-ols in cocoa, red wine, grapes, apples, green tea, and other fruits, flavanones in oranges and other citrus fruits, anthocyanidines in colored berries, black currants, and isoflavones in soy food [19?1]. In decades, studies have suggested the AKT inhibitor 2 site chemopreventive effects of flavonoids on carcinogenesis, the anticancer activity of dietary flavonoids has become an important and interesting topic. Yet in fact, a number of epidemiologic studies assessing the association between dietary flavonoid intake and the risk of breast cancer have yielded inconsistent results and have identified controversial evidence [22?4]. One possible explanation is that different flavonoid subclasses in diet may have different properties and effects in vivo. So it is important to elucidate the different role of each 15755315 flavonoid subclass in the chemopreventive effect of these dietary compounds. However no systematic analysis has been performed to assess the association between dietary intake of flavonoid subclasses and the risk of breast cancer. Thus, we aimed to examine the association of each flavonoid subclass intake withStudies were eligible for our analysis if: (1) the study design is cohort or case-control study; (2) data related to dietary consumption or exposure assessment (blood/urinary levels) of total flavonoids or one of flavonoid subclasses (isoflavones excluded) were available; (3) the association of flavonoids or one 
of flavonoid subclasses with breast cancer risk was specifically evaluated; (4) relatively complete assessment of total flavonoids or flavonoid subclass intake was performed; (5) relative risk (RR), hazard ratio (HR), or odds ratio (OR), and corresponding 95 confidence intervals (95 CI) were available. Because isoflavones have been studied extensively, including meta-analy.Orized into six major subclasses based on their range and structural complexity: flavonols, flavones, flavan-3-ols, flavanones, anthocyanins and isoflavones (Figure 1) [14?6]. As reviewed before by Hooper L et al (Table 1), in western diet, major flavonoids included in these subclasses are quercetin, myricetin and kaempferol for flavonols, hesperitin and narigerin for flavanones, epicatechin and catechin for flavan-3-ols, apigenin and luteolin for flavones, cyanidin, delphinidin and malvidin forFlavonoids and Breast Cancer Riskthe risk of breast cancer by performing a meta-analysis of epidemiologic studies. Given that many reviews have been conducted to assess the association between the dietary isoflavone intake and the breast cancer risk, we excluded this issue in the present study.Methods Search StrategyWe conducted a systematic search of literature published before July 1st 2012 using the Cochrane Library, MEDLINE, and EMBASE Databases and the following search terms: “flavonoid”, “flavonols”, “flavones”, “flavanones”, “flavan-3-ols”, “flavanols”, “anthocyanidins”, “phytoestrogens”, “polyphenolic compounds” and “breast cancer”. We also performed a manual search using reference lists of original articles and relevant reviews. Only fulllength original journal articles were considered and no attempt was made to include abstracts or unpublished studies.Study SelectionFigure 1. Structures of six flavonoid subclasses. doi:10.1371/journal.pone.0054318.ganthocyanidines, and genistein and daizeina for isoflavones [16?18]. Flavonols mainly exist in onions, broccoli, tea, and various common fruits, flavones in aromatic herbs, celery, and chamomile tea, flavan-3-ols in cocoa, red wine, grapes, apples, green tea, and other fruits, flavanones in oranges and other citrus fruits, anthocyanidines in colored berries, black currants, and isoflavones in soy food [19?1]. In decades, studies have suggested the chemopreventive effects of flavonoids on carcinogenesis, the anticancer activity of dietary flavonoids has become an important and interesting topic. Yet in fact, a number of epidemiologic studies assessing the association between dietary flavonoid intake and the risk of breast cancer have yielded inconsistent results and have identified controversial evidence [22?4]. One possible explanation is that different flavonoid subclasses in diet may have different properties and effects in vivo. So it is important to elucidate the different role of each 15755315 flavonoid subclass in the chemopreventive effect of these dietary compounds. However no systematic analysis has been performed to assess the association between dietary intake of flavonoid subclasses and the risk of breast cancer. Thus, we aimed to examine the association of each flavonoid subclass intake withStudies were eligible for our analysis if: (1) the study design is cohort or case-control study; (2) data related to dietary consumption or exposure assessment (blood/urinary levels) of total flavonoids or one of flavonoid subclasses (isoflavones excluded) were available; (3) the association of flavonoids or one of flavonoid subclasses with breast cancer risk was specifically evaluated; (4) relatively complete assessment of total flavonoids or flavonoid subclass intake was performed; (5) relative risk (RR), hazard ratio (HR), or odds ratio (OR), and corresponding 95 confidence intervals (95 CI) were available. Because isoflavones have been studied extensively, including meta-analy.