Few epidemiologic studies have investigated the potential relation between flavonoids and breast cancer risk. We have applied recently published data on the composition of foods and beverages in terms of six principal classes of flavonoids (i.e., flavanones, flavan-3-ols, flavonols, flavones, anthocyanidines, and isoflavones) on dietary information collected in a large-case control study of breast cancer conducted in Italy between 1991 and 1994. The study included 2,569 women with incident, histologically confirmed breast cancer, and 2,588 hospital controls. Odds ratios (OR) and 95% confidence intervals were estimated by multiple logistic regression models. After allowance for major confounding factors and energy intake, a reduced risk of breast cancer was found for increasing intake of flavones (OR, 0.81, for the highest versus the lowest quintile; P-trend, 0.02), and flavonols (OR, 0.80; P-trend, 0.06). No significant association was found for other flavonoids, including flavanones (OR, 0.95), flavan-3-ols (OR, 0.86), anthocyanidins (OR, 1.09), as well as for isoflavones (OR, 1.05). The findings of this large study of an inverse association between flavones and breast cancer risk confirm the results of a Greek study.
Flavonoids are polyphenols present in vegetables, fruit, and beverages of plant origin which have antioxidant, antimutagenic, and antiproliferative properties (1-5). They may thus have a potential protective role in various chronic diseases, including common cancers (6-9), and explain, at least in part, the well-established associations between high consumption of vegetables and fruit and reduced risk of several neoplasms (10, 11).
With reference to breast cancer, particular interest has been given to the investigation of the relation with isoflavones, contained mainly in soy products, given their antiestrogenic effects, and their consequent potential role in breast cancer prevention (12-16). A few epidemiologic studies on isoflavone intake—mainly based on urinary excretion measurements in Asian populations with high soy consumption—have suggested an inverse association with breast cancer risk (16-23). Other studies conducted in non-Asian populations with low and limited range in isoflavone intake, however, did not confirm these results (24-28).
As reliable data on the flavonoid content of foods has become available only recently, the epidemiologic evidence on the association between other flavonoids and breast cancer risk is scanty. In a cohort study from Finland (29), including 87 breast cancer cases, a nonsignificant inverse association was reported for total flavonoids (relative risk, 0.72, for the highest quintile of intake). In a subsequent follow-up of the same cohort (30), including 125 cases, a reduced breast cancer risk was found for high intake however, of quercetin (relative risk, 0.62), but not for other flavonoids. In a Greek case-control study (27) based on 820 women with breast cancer and 1,548 controls, a significant inverse association was found for the intake of flavones [odds ratio (OR), 0.87], but not for other classes of flavonoids.
We have thus applied recently published data on the composition of foods and beverages in terms of the six principal classes of flavonoids (i.e., flavanones, flavan-3-ols, flavonols, flavones, anthocyanidines, and isoflavones; refs. 31, 32) on dietary information collected in the context of a large multicentric case-control study of breast cancer from Italy.
Materials and Methods
Data were derived from a case-control study of breast cancer conducted between 1991 and 1994 in six Italian areas: the greater Milan area, the province of Pordenone, the urban area of Genoa, the province of Forlì, the province of Latina, and the urban area of Naples (33). Briefly, cases were 2,569 women with incident, histologically confirmed breast cancer (median age 55, range 23-74 years), admitted to major teaching and general hospitals of the study areas and controls were 2,588 women (median age 56, range 20-74 years) with no history of cancer, admitted to the same hospitals for acute, non-neoplastic, nongynecological conditions, unrelated to hormonal or digestive tract diseases or to long-term modifications of diet. Among controls, 22% had traumas, 33% other orthopedic diseases, 15% acute surgical condition, 18% eye diseases, and 12% other miscellaneous diseases. Less than 4% of cases and controls approached for interview refused to participate.
Cases and controls were interviewed in the hospital by centrally trained interviewers, using a standard structured questionnaire. This included information on sociodemographic factors, anthropometric variables, tobacco smoking, alcohol drinking, and other life-style habits, physical activity, obstetric, gynecologic and a selected medical history, and history of breast cancer in first-degree relatives. A validated (34) and reproducible (35) food frequency questionnaire was used to assess the patients' usual diet in the previous 2 years, including 78 foods or food groups, plus questions aimed at assessing fat intake and general dietary habits. Subjects were asked to indicate their average weekly consumption of single food items or food groups. Intakes lower than once a week, but at least once per month were coded as 0.5 per week.
Energy was computed using an Italian food composition database, appropriately checked and supplemented with other published data (36). Food and beverage content in terms of six classes of flavonoids (i.e., flavanones, flavan-3-ols, flavonols, flavones, anthocyanidines, and isoflavones) was obtained from the U.S. Department of Agriculture (31, 32), further integrated with other sources (37-39). Major flavonoids included in these classes were hesperitin and narigerin for flavanones, epicatechin and catechin for flavan-3-ols, quercetin, myricetin and kaempferol for flavonols, apigenin and luteolin for flavones, cyanidin and malvidin for anthocyanidines, and genistein and daizeina for isoflavones. In our population, flavanones come mainly from oranges and other citrus fruits, flavan-3-ols from red wine, grapes, and other fruits, flavonols from various common vegetables and fruits, flavones from aromatic herbs, anthocyanidines from onion and garlic, and isoflavones from pulses.
ORs and 95% confidence intervals (CI) were estimated by multiple unconditional logistic regression models (40), including terms for age (quinquennia), study center, years of education (<7, 7-11, ≥12 years), parity (0, 1, 2, 3, ≥4 births), alcohol consumption (tertiles), and nonalcohol energy intake (quintiles). Flavonoids were included in the models as quintiles based on the distribution of controls. Models including the residuals of the regression of flavonoids on energy (41) yielded similar results; thus, only the former estimates were presented. Tests for trend for quintiles of flavonoids were based on the likelihood ratio test between the models with and without a linear term for flavonoid quintile. Flavonoids were also included into the models as continuous variables, with a measurement unit equal to the difference between the upper cutpoint of the fourth quintile and that of the first.
Table 1 gives the median daily intake of six major classes of flavonoids among controls, and the ORs of breast cancer according to quintile of intake. The median daily intake was 33.7 mg for flavanones, 36.4 mg for flavan-3-ols, 15.6 mg for flavonols, 0.5 mg for flavones, 10.4 mg for anthocyanidins, and 21.7 μg for isoflavones. After allowance for major confounding factors and energy intake, a reduced risk of breast cancer was found for increasing intake of flavones (OR, 0.81, for the highest versus the first quintile; P-trend, 0.02), and of flavonols (OR, 0.80; P-trend, 0.06). No meaningful association emerged for other flavonoids, including flavanones (OR, 0.95), flavan-3-ols (OR, 0.86), anthocyanidins (OR, 1.09), as well as for isoflavones (OR, 1.05). The continuous ORs for an increment equal to the difference between the upper cutpoint of the fourth quintile and that of the first were 0.95 for flavanones, 0.93 for flavan-3-ols, 0.94 for flavonols, 0.90 flavones, 1.06 for anthocyanidins, and 0.97 for isoflavones. The estimates were not substantially changed after mutual adjustment of the various classes of flavonoids.
In Table 2, the relation between the six classes of flavonoids and breast cancer risk were analyzed in strata of selected covariates. The estimates were consistent across strata of menopausal status, body mass index, and parity.
The results of the present study indicate that flavones are inversely related to breast cancer risk. An inverse—although not significant—association was also found for flavonols, whereas no evidence that other flavonoids, including isoflavones, had a major role on breast cancer risk was found. These findings are in agreement with those of another case-control study from Greece (27), which found a protective effect of flavones on breast cancer risk.
The Italian population of our study is characterized by a high and varied consumption of vegetables (42), including herbs and aromatic plants rich in flavones (2). Thus, our results indicate that these substances could be, at least in part, responsible for the potential beneficial properties of vegetables in breast cancer risk (43, 44). Some recent studies have challenged the inverse association of vegetable intake with breast cancer risk (45). Observational studies, however, are susceptible to the negative confounding likely to be generated by the fact that health-conscious women, who tend to consume more vegetables and fruits, also tend to undergo mammographic examinations more frequently and hence are more easily diagnosed with subclinical breast cancer.
With respect to isoflavones, our data did not confirm the results from other studies, mainly from Asian populations (17, 18, 20, 21). The absence of any meaningful association with isoflavone intake in our study may be due to the extremely limited intake of soya or soya products—and consequently of isoflavonoids—in the Italian population. It is also possible that an association between isoflavones and breast risk may not be captured through a dietary intake study, but only through measurement of urinary excretion (46). Thus, the association between isoflavones and breast cancer risk remains unclear, with several studies reporting no relation (15, 16).
The usual strengths and weaknesses of hospital-based case-control studies should be considered (40). Dietary recall can be influenced by recent diagnosis of cancer. However, the information collected refers to the habitual diet in the 2 years before the diagnosis or hospital admission. Furthermore, potential recall bias in the intake of flavonoids should be limited, given the limited appreciation by the lay population in Italy of a link between vegetable and fruit intake and breast cancer risk. The dietary habits of hospital controls may differ from those of the general population, but we took great care to include as controls only patients admitted to hospital for acute conditions not related to major changes in diet and other life-style factors. Moreover, the same interview setting and catchment areas for cases and controls, and the almost complete participation rate are reassuring. The well recognized risk factors for breast cancer (including age at menarche, age at menopause, parity, family history, alcohol drinking, body mass index, etc.) were consistently observed in the present data (47, 48). Among the strengths of this study are the uniquely large dataset, the high intake of fruit and vegetables in the population, the satisfactory reproducibility and validity of the food frequency questionnaire (34, 35), and the ability to control for total energy intake and major potential confounding factors. Among the limitations of the study are the questions concerning the adaptability of U.S. flavonoid food composition tables to the Italian diet, and the fact that the questionnaire was not specifically designed to investigate flavonoids.
In conclusion, we found evidence of an inverse association of flavones and—to a lesser extent—flavonols with breast cancer risk, which may, at least in part, explain the inverse association of vegetable consumption with breast cancer risk in this population (33).
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Note: This work was conducted with the contribution of the Italian Association for Cancer Research and the Italian League Against Cancer.