Flavonoids and the Risk of Renal Cell Carcinoma

Renal cell carcinoma (RCC) has been consistently linked to cigarette smoking, overweight, and hypertension (1, 2). Dietary aspects of RCC risk remain largely undefined, although a diet rich in vegetables and fruit has been inversely related to the risk of RCC in several studies (2-4). It is, however, unclear which component of vegetables and fruit may account for this apparent inverse relation.

Flavonoids are a large group of polyphenolic compounds present in foods and beverages of plant origin, which have antioxidant, anti-inflammatory, and, possibly, antimutagenic and antiproliferative activities (5, 6). In most, although not all, studies, intake of various flavonoids has been inversely related to the risk of cancers of the upper aerodigestive tract (7-9), stomach (10, 11), colorectum (12, 13), lung (14-17), breast (18, 19), and prostate (20), but only scanty information is available on their possible role on kidney cancer (17).

The Alpha-Tocopherol Beta-Carotene Cancer Prevention study in male smokers from Finland (7) reported a nonsignificant reduction of RCC for increasing intake of flavones and flavonols. Similarly, another Finnish cohort (15) reported a nonsignificant risk reduction of urinary organ cancers for specific flavonoids of the flavanone and flavonol classes, and the Iowa Women's Health Study (13) showed an inconsistent risk relation between kidney and renal pelvis cancer and the flavan-3-ol catechin in postmenopausal women.

Because estimates of food and beverage content of six major classes of flavonoids have recently become available (21, 22), we had the opportunity to investigate the relation between major classes of flavonoids and RCC using data from a large case-control study conducted in Italy.

We conducted a case-control study of RCC between 1992 and 2004 in four areas of Italy, including the greater Milan area, the provinces of Pordenone and Gorizia in northern Italy, the province of Latina in central Italy, and the urban area of Naples in southern Italy (3). Cases were 767 patients (494 men and 273 women; median age, 62 years; range, 24-79 years) with incident, histologically confirmed RCC (International Classification of Diseases version 9, 189.0) admitted to major teaching and general hospitals in the study areas. Cancers of the renal pelvis and ureter (International Classification of Diseases version 9, 189.1-189.2) were not considered. Controls were 1,534 subjects (988 men and 546 women; median age, 62 years; range, 22-79 years) admitted to the same hospitals as the RCC cases for a wide spectrum of acute, nonneoplastic conditions, unrelated to known risk factors for RCC and not associated with long-term dietary modifications. Controls were matched with cases by study center, sex, and 5-year age groups, with a case to control ratio of 1:2. Twenty-six percent of controls were admitted for traumas, 32% for other orthopedic disorders, 14% for surgical conditions, and 27% for other miscellaneous illnesses. Less than 5% of both cases and controls contacted refused to participate.

Trained interviewers interviewed cases and controls during their hospital stay using a structured questionnaire. This included information on sociodemographic characteristics, lifestyle habits, such as tobacco smoking and alcohol drinking, anthropometric measures, personal medical history, and family history of cancer.

The subjects' usual diet during the 2 years before cancer diagnosis or hospital admission (for controls) was assessed using an interviewer-administered food frequency questionnaire, which included 78 foods and beverages as well as a range of the most common Italian recipes. Subjects were asked to indicate their average weekly frequency of consumption for each dietary item; intakes lower than once a week, but at least once a month, were coded as 0.5 per week. The food frequency questionnaire was satisfactorily reproducible (23) and valid (24). To estimate total energy intake, an Italian food composition database was used, complemented with other sources when needed (25). Food and beverage content of six classes of flavonoids (i.e., isoflavones, anthocyanidins, flavan-3-ols, flavanones, flavones, and flavonols) was estimated based on information obtained from the U.S. Department of Agriculture databases (21, 22), supplemented by data from other sources (26-28). No evaluation was made for the validity and reproducibility of the intake of flavonoids. Major flavonoids included in these classes were genistein and daidzein for isoflavones; cyanidin and malvidin for anthocyanidins; epicatechin and catechin for flavan-3-ols; hesperitin and narigerin for flavanones; apigenin and luteolin for flavones; and quercetin, myricetin, and kaempferol for flavonols. In this population, isoflavones were derived mainly from vegetable or bean soups and pulses; anthocyanidins from wine and red fruits; flavan-3-ols from tea, wine, and fruits; flavanones from citrus fruit; flavones from green leafy vegetables; and flavonols from various fruit, wine, and common vegetables. Total flavonoids were calculated by summing up the six classes of flavonoids. Quintiles based on the control distribution were computed both directly on the flavonoids and on the residuals of the regression of flavonoids on total energy (29). Because both analyses yielded similar results, only the former is presented.

Odds ratios (OR) and the corresponding 95% confidence intervals (95% CI) were calculated using conditional multiple logistic regression models (30) conditioned on study center, sex, and 5-year age groups. All the regression models included terms for years of education, tobacco smoking, alcohol drinking, body mass index (BMI), occupational physical activity, and family history of kidney cancer in first-degree relatives, in addition to period of interview and total energy intake. In additional models, further terms for fruit and vegetable consumption were added.

Table 1 shows the distribution of 767 cases of RCC and 1,534 controls according to sex, age, and other selected covariates. By design, cases and controls had the same sex and age distribution. Cases were more educated than controls, had a higher BMI, were more frequently heavy smokers, and reported more often first-degree relatives with kidney cancer. Cases and controls were comparable in terms of alcohol consumption and total energy intake.

Table 2 shows the distribution of RCC cases and controls according to intake quintile of the six classes of flavonoids and the mean daily intake of the six classes of flavonoids and total flavonoids among controls. The mean daily intake was 25.5 μg for isoflavones, 20.1 mg for anthocyanidins, 59.5 mg for flavan-3-ols, 34.7 mg for flavanones, 0.5 mg for flavones, 22.4 mg for flavonols, and 137.2 mg for total flavonoids.

The ORs of RCC according to quintile of intake of flavonoids are given in Table 3. All the ORs for the highest versus the lowest quintile of intake were below unity. In particular, the ORs were 0.80 (95% CI, 0.58-1.11) for total flavonoids, 0.76 (95% CI, 0.56-1.03) for isoflavones, 0.94 (95% CI, 0.60-1.47) for anthocyanidins, 0.77 (95% CI, 0.56-1.06) for flavan-3-ols, 0.90 (95% CI, 0.67-1.21) for flavanones, 0.68 (95% CI, 0.50-0.93) for flavones, and 0.69 (95% CI, 0.50-0.95) for flavonols. After the addition of vegetable and fruit consumption to the models, the ORs for the highest quintile of intake become 0.76 (95% CI, 0.55-1.05) for flavones and 0.76 (95% CI, 0.52-1.11) for flavonols (data not shown).

In Table 4, the relation between flavonoids and RCC risk was analyzed in strata of selected covariates. Most ORs were below unity among strata of sex, age, and BMI. For most classes of flavonoids (except anthocyanidins and flavanones), risk reductions were stronger in women than in men. Moreover, for all classes of flavonoids, the ORs were consistently lower among smokers than among nonsmokers, although the heterogeneity tests were not significant.

In the present study, intake of various classes of flavonoids, particularly flavones and flavonols, was inversely related to the risk of RCC. Allowance for fruit and vegetable consumption (the main sources of flavonoids) only partly accounted for the associations observed.

The class of flavonoids most strongly related to RCC in this data set was flavones. Flavones, along with flavonols, have been slightly inversely associated with the risk of RCC in a Finnish study (7). They have also been inversely associated with breast cancer risk (19), and incombination with flavonols with lung cancer risk (7, 31). Flavones are derived mainly from (green) vegetables, which have been inversely related to the risk of RCC (2-4), as well as breast cancer (32-34), at least in Mediterranean populations. Levels and variability of vegetable consumption are greater in Mediterranean than in other populations (35), and greens, which are rich in flavones, are much more common in the Mediterranean countries (36).

Flavonols, which derived from various fruit and vegetables, have been inversely related to the risk of cancer of the stomach in a case-control study from Spain (10), of the colorectum in an Italian case-control study (12), of the lung in two Finnish cohort investigations (7, 15), and of the prostate in one of them (15).

The association of RCC and isoflavones was of borderline significance, but the very low intake of isoflavones in this Italian population makes this finding difficult to interpret. In our study, isoflavones derived mainly from beans, soy, and soy products, the consumption of which is limited in Italy.

The inverse associations between flavonoids and RCC were consistent across strata of age and BMI. However, for most flavonoids considered, the associations were stronger in women than in men and in smokers than in never smokers. There is no clear biological explanation for these apparent heterogeneities. In particular, the stronger risk reduction in smokers, if not due to chance alone, may be related to the antioxidant, anti-inflammatory action of flavonoids, which may have a greater effect in smokers, due to their higher baseline risk.

Among the strengths of the study are its large size, the high consumption and diversity of fruits and vegetables in this Mediterranean population (35), the use of a comprehensive food frequency questionnaire, with a satisfactory reproducibility (23) and validity (24), and proper allowance for total energy intake as well as for other major identified confounding factors.

Among the limitations of the study are inherent problems related to the food composition data, which, for flavonoids, may not satisfactorily reflect the variability in content attributable to factors such as crop conditions, sunlight, and temperature. Further, the suitability of U.S. flavonoid food composition tables to the Italian diet is unknown, and the dietary questionnaire was not specifically designed to investigate flavonoid intake. Although total flavonoid intake may have been underestimated in our study, these limitations should not have affected the comparisons between cases and controls. Dietary recall of cases can be influenced by recent diagnosis of cancer. However, the information collected refers to the habitual diet in the 2 years before diagnosis or hospital admission. Furthermore, care was taken to include as controls only patients admitted to hospital for acute conditions not related to major changes in diet and other lifestyle factors. Recall bias in the intake of flavonoids should also be limited, as knowledge of a link between increased vegetable and fruit consumption and lower RCC risk in Italy is not widely known. Moreover, the same interview setting and catchment areas for cases and controls and the almost complete participation are reassuring against any relevant information or selection bias and support therefore the validity of the present findings.

In conclusion, flavonoids, and particularly flavones and flavonols, may, at least in part, explain the inverse association between vegetables and kidney cancer risk in this population.

We thank I. Garimoldi for editorial assistance.