Polyphenols and disease risk in epidemiologic studies

¹ From RIKILT-Institute of Food Safety, Wageningen University and Research Centre, Wageningen, The Netherlands

² Presented at the 1st International Conference on Polyphenols and Health, held in Vichy, France, November 18–21, 2004.

³ Supported by grant QLK1-CT-1999-00505 from the European Community, Framework V Programme (POLYBIND), by the Dutch Ministry of Agriculture, Nature Management, and Fisheries, and by the Netherlands Organisation for Health Research and Development (grant 014-12-014).

⁴ Address correspondence to ICW Arts, RIKILT-Institute of Food Safety, Wageningen University and Research Centre, PO Box 230, 6700 AE, Wageningen, The Netherlands. E-mail: ilja.arts{at}wur.nl.

ABSTRACT

Plant polyphenols, a large group of natural antioxidants, are serious candidates in explanations of the protective effects of vegetables and fruits against cancer and cardiovascular diseases. Epidemiologic studies are useful for evaluation of the human health effects of long-term exposure to physiologic concentrations of polyphenols, but reliable data on polyphenol contents of foods are still scarce. The aim of this review is to summarize available epidemiologic data on the health effects of polyphenols, focusing on the flavonoid subclasses of flavonols, flavones, and catechins and on lignans. Data obtained to date suggest beneficial effects of both flavonoids and lignans on cardiovascular diseases but not on cancer, with the possible exception of lung cancer. There is a need for more research on stroke and lung diseases such as asthma and chronic obstructive pulmonary disease. Most studies to date have included only flavonols and flavones. With data becoming available for other polyphenols, these compounds should be included in future studies. Careful design of prospective studies is important to offset some of the major drawbacks of epidemiologic studies, including residual confounding (by smoking and other dietary factors) and exposure assessment.

Key Words: Review • epidemiology • polyphenols • flavonoids • flavonols • catechins • lignans • antioxidants • phytoestrogens • cancer • cardiovascular diseases • stroke

INTRODUCTION

Epidemiologic studies suggest a protective effect of vegetables and fruits against cancer and cardiovascular diseases (CVDs) (1, 2). Various hypotheses have been suggested to explain these beneficial effects of increased consumption of vegetables and fruits. An attractive hypothesis is that vegetables and fruits contain compounds that have protective effects, independent of those of known nutrients and micronutrients. Plant polyphenols, a large group of natural antioxidants ubiquitous in a diet high in vegetables and fruits, certainly are serious candidates. All plant phenols are derived from the common intermediate phenylalanine, or its close precursor shikimic acid, through the shikimic acid pathway in plants. They can be divided into at least 10 different classes on the basis of their general chemical structures, with the common characteristic being at least one aromatic ring structure with one or more hydroxyl groups. A large variety of plant (poly)phenols exist, including cinnamic acids, benzoic acids, flavonoids including proanthocyanidins, stilbenes, coumarins, lignans, and lignins. Within each family of plant phenols, many compounds may be present. For example, > 6000 different flavonoids occurring in plants have been described.

In addition to their antioxidant properties, polyphenols show several interesting effects in animal models and in vitro systems; they trap and scavenge free radicals, regulate nitric oxide, decrease leukocyte immobilization, induce apoptosis, inhibit cell proliferation and angiogenesis, and exhibit phytoestrogenic activity (3–6). These effects may contribute to their potentially protective role in cancer and CVDs. The question remains of whether these data are relevant for human disease outcomes, where exposure to polyphenols is chronic and at relatively low concentrations, depending on bioavailability and metabolism. An important phenomenon is that, after absorption, polyphenols are subject to phase II metabolism, yielding methoxylated, glucuronidated, and sulfated compounds (7). This may greatly influence their bioactivity, but only a few studies have examined this to date. In addition, bacteria present in the human colon metabolize polyphenols. The major polyphenol metabolites are a variety of phenolic acids such as homovanillic acid (8). As a consequence, body tissues are exposed to high concentrations of these phenolic acids.

Although no information on causality can be obtained, epidemiologic studies are useful for evaluation of the human health effects of long-term exposure to physiologic concentrations of polyphenols. Reliable data on polyphenol contents of foods are needed for studies of the potential role of dietary polyphenols in cancer and CVD prevention. Comprehensive data are available only for the flavonoid subclasses of flavonols, flavones, and catechins, but data on other polyphenols, such as lignans, are forthcoming. In this article, we provide an overview of epidemiologic studies on the health effects of flavonols, flavones, catechins, and lignans conducted to date.

FLAVONOIDS

Studies
Of the 6 major classes of flavonoids, comprehensive data on their contents in foods are available only for the flavonols (quercetin, kaempferol, and myricetin), flavones (apigenin and luteolin), and catechins [(+)-catechin, (+)-gallocatechin, (–)-epicatechin, (–)-epigallocatechin, (–)-epicatechin gallate, and (–)-epigallocatechin gallate]. The flavonoid data used in most epidemiologic studies were based on analyses conducted in the Netherlands (9–12) but were supplemented in some studies with data for additional food items. Recently, data became available for flavanones in Finnish foods (hesperetin, naringenin, and eriodictyol) (13). Except for one study from the United Kingdom (14), all epidemiologic studies of flavonoids are from the Netherlands, Finland, or the United States.

CVDs
To date, 12 cohort studies on flavonoid intake and the risk of coronary artery disease (CAD) and 5 cohort studies on the risk of stroke have been published (Table 1). Seven of these prospective studies found protective effects of flavonols and flavones or of catechins with respect to fatal or nonfatal CAD, and reductions of mortality risk were up to 65%. These studies were as follows: the Zutphen Elderly Study, with a small cohort of 805 men in the Netherlands after 5 and 10 y of follow-up monitoring (21, 24), the Finnish Mobile Clinic Health Examination Survey (significant among men only) (22), the Iowa Women's Health Study, a cohort study of 34 500 women in the United States (20), the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study among 25 000 male smokers (19), the Dutch Zutphen Elderly Study for catechins (18), and the Rotterdam Study in the Netherlands, a cohort study of 4800 men and women (16). In a large cohort study of 35 000 male US health professionals, a suggestion of a reduction in coronary mortality rates with high flavonol intake was found only among men with a previous history of CAD [relative risk (RR): 0.63; 95% CI: 0.33, 1.20] (23). In contrast, a trend for increased CAD mortality rates (P for trend = 0.12) was found in the Caerphilly Study, a cohort study of 1900 Welsh men (14). It was suggested that the English habit of adding milk to tea (the major source of flavonols for this cohort) could inhibit the absorption of flavonols, thus explaining the lack of protection of tea flavonols against CAD. Proteins bind phenols efficiently and therefore might inhibit absorption from the gastrointestinal tract when consumed together with flavonoids. However, it was shown that the absorption of flavonols was not impaired with the addition of milk (27). Residual confounding by lifestyle factors might have affected evaluation of the results of this study among Welsh men.

View this table:
TABLE 1. Prospective studies of flavonoid intake and risk of CVDs¹

 
Two of 5 studies of flavonoid intake and stroke risk found an inverse association, ie, the Zutphen Elderly Study and the Finnish Mobile Clinic Health Examination Survey (Table 1). In the Zutphen Elderly Study, a protective effect was observed for flavonols and flavones (26) but not for catechins (18).

Cancer
Associations between the intake of flavonoids and the incidences of a variety of cancers have been studied in 7 prospective cohort studies (Table 2) and 4 case-control studies. Significant associations were observed only for lung cancer and colorectal cancer. Two Finnish studies with relatively low intakes of flavonols and flavones, ie, the Finnish Mobile Clinic Health Examination Survey (30) and the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study (32), found inverse associations with lung cancer risk (RR: 0.53; 95% CI: 0.29, 0.97; and RR: 0.56; 95% CI: 0.45, 0.69, respectively). In contrast, a borderline positive association was found for colorectal cancer risk in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study (RR: 1.70; 95% CI: 1.00, 2.70; P for trend = 0.10). For catechins, an inverse association was reported for rectal cancer (RR: 0.55; 95% CI: 0.32, 0.95; P for trend = 0.02), but not for colon cancer, among postmenopausal women in the United States. No evidence for an effect of flavonoid intake was found for the incidence of any epithelial cancer or cancer of the stomach, urinary tract, prostate, or breast. None of the case-control studies of prostate (34), lung (35), testicular (36), and ovarian (37) cancer found significant associations.

View this table:
TABLE 2. Prospective studies of flavonoid intake and risk of incident cancer and other chronic diseases¹

 
Other chronic diseases
Because of their antioxidant and antiinflammatory properties, flavonoids may also beneficially influence other chronic diseases involving oxidative stress or inflammation, such as rheumatoid arthritis and chronic obstructive pulmonary disease (COPD). Knekt et al (13) studied the associations between the intake of flavonols, flavones, and flavanones and the incidences of rheumatoid arthritis, type 2 diabetes mellitus, cataracts, and asthma among a cohort of 10 000 male and female participants in the Finnish Mobile Clinic Health Examination Survey (Table 2). A significant inverse association was observed only for asthma (RR: 0.65; 95% CI: 0.47, 0.90; P for trend = 0.04). This finding supported an earlier cross-sectional study, in which intake of flavonoids was beneficially associated with pulmonary function and symptoms of COPD. Pulmonary function (measured as forced expiratory volume in 1 s) was better among subjects in the highest quintile, compared with the lowest quintile, of intake of flavonols, flavones, and catechins (44 mL; 95% CI: 18, 69 mL). Catechin intake alone was most strongly associated with the forced expiratory volume in 1 s (130 mL; 95% CI: 101, 159 mL) and with all 3 symptoms of COPD [cough odds ratio (OR): 0.72; 95% CI: 0.58, 0.90; phlegm OR: 0.60; 95% CI: 0.47, 0.75; breathlessness OR: 0.69; 95% CI: 0.52, 0.90] (38).

LIGNANS

Studies
Plant lignans can be converted by human intestinal bacteria into the so-called enterolignans, ie, enterolactone and enterodiol. Enterolignans are found in biological fluids of humans and animals (6, 39). It was shown, that in addition to the well-known enterolignan precursors secoisolariciresinol and matairesinol, several other plant lignans were converted into enterolactone and enterodiol, although with varying degrees of efficiency (40). The health effects of lignans were evaluated in epidemiologic studies that used both the intake of secoisolariciresinol and matairesinol and plasma or urinary concentrations of enterolactone and enterodiol as exposure estimates. The calculated intake of secoisolariciresinol and matairesinol was based on published food composition tables (41–43), of which that provided by De Kleijn et al (41) is the most comprehensive. Of 8 published studies on enterolignan concentrations and the risk of CVD or cancer, only 2 measured enterodiol in addition to enterolactone. Enterolactone is usually measured with a time-resolved fluoroimmunoassay, which is not available for enterodiol (44).

CVDs
Two publications (45, 46) from one Finnish cohort study in which plasma enterolactone concentrations were studied in relation to CVD risk reported significant inverse associations (Table 3). In the Finnish Kuopio Heart Disease Risk Factor Study, a 65% lower risk of incident CAD was observed (46). With 2 additional years of follow-up monitoring, the risk of CAD death was of the same order of magnitude (RR: 0.44) and similar, although results were only borderline significant for total CVD deaths (RR: 0.55) (45). The association between serum enterolactone concentrations and plasma F₂-isoprostane concentrations (a biomarker of in vivo lipid peroxidation) was studied cross-sectionally in a subset of 100 male participants in the Antioxidant Supplementation in Atherosclerosis Prevention Study (47). With higher enterolactone concentrations, F₂-isoprostane concentrations were significantly lower (P = 0.02).

View this table:
TABLE 3. Prospective studies of serum lignans and risk of CVDs¹

 
No studies on lignan intake and CVD risk have been published to date, but 2 cross-sectional studies related lignan intake to CVD risk factors. Of several risk factors studied among postmenopausal US women, only the waist-hip ratio (difference between extreme quartiles: –0.017; 95% CI: –0.030, –0.002; P for trend = 0.03) and the metabolic syndrome score, a summary score of several risk factors (difference between extreme quartiles: –0.55; 95% CI: –0.82, –0.28; P for trend = 0.0001), were associated with intake of secoisolariciresinol and matairesinol (48). Aortic stiffness, assessed with pulse-wave velocity measurements of the aorta, was borderline significantly inversely associated with lignan intake among postmenopausal Dutch women (49). The regression coefficient for those with a high intake of lignans was –0.41 (95% CI: –0.93, 0.11; P for trend = 0.06), compared with those with a low intake. The protective effect was most pronounced and significant among women with a postmenopausal time of > 20 y.

Cancer
To date, 3 prospective, nested, case-control studies and 3 case-control studies have studied plasma or urinary lignan concentrations and cancer risk (Table 4); all except one investigated breast cancer incidence. The 2 prospective, nested, case-control studies on breast cancer risk, among Dutch postmenopausal women (54) and among female participants in 3 cohorts in northern Sweden (53), found no relationship with plasma or urinary enterolactone concentrations. In contrast, all 3 case-control studies found an inverse association between lignan concentrations and breast cancer risk (52, 55, 56). When a subset of postmenopausal women only was included in the analysis of the Shanghai Breast Cancer Study, the inverse association was no longer significant and the RR increased from 0.40 (95% CI: 0.24, 0.64) to 0.50 (95% CI: 0.23, 1.10) (51). In all of these case-control studies, plasma or urine was collected after diagnosis and sometimes even after initiation of treatment of the disease, which might have influenced lignan concentrations through changes in the diet as a result of disease or through other mechanisms. The only study on prostate cancer risk conducted to date found no association with plasma enterolactone concentrations among a large cohort of male residents of Finland, Norway, and Sweden (50).

View this table:
TABLE 4. Prospective and case-control studies on plasma or urinary lignan concentrations or dietary lignan intake and incident cancer¹

 
Intake of the lignans secoisolariciresinol and matairesinol was studied in relation to the risk of several cancers in 1 prospective cohort and 3 case-control studies, all from the United Stated (Table 4). A significant inverse association was observed for breast cancer among premenopausal women (RR: 0.49) but not postmenopausal women (RR: 0.72) in western New York State (58). In contrast, breast cancer risk was borderline significantly elevated with a high intake of secoisolariciresinol and matairesinol among a large group of women participating in the multiethnic Bay Area Breast Cancer Study (RR: 1.3) (59) and of secoisolariciresinol only in the prospective California Teachers Study (RR: 1.4) (57). In the latter study, the association was substantially attenuated to a RR of 1.2 (95% CI: 0.9, 1.6) after adjustment for wine consumption. This led the authors to conclude that the increased risk with secoisolariciresinol was attributable to confounding by alcohol consumption.

Significant or borderline significant protective associations were also reported for endometrial cancer (60), ovarian cancer (37), and thyroid cancer (61) among female participants. No associations between lignan intake and incident prostate (34) or testicular (36) cancer were found.

DISCUSSION

In the past decade, several well-designed, prospective, cohort studies in which the health effects of flavonoids were studied have been published. The data regarding CVD suggest protective effects of high intakes of flavonols and flavones and possibly of catechins. However, only a few studies are available for catechins and for stroke; given the results obtained to date, these deserve more study. A meta-analysis of tea consumption in relation to CAD and stroke, including all studies published up to October 2000, was conducted by Peters et al (62). Most studies included in the current review were also included in that meta-analysis, because they provided data not only for tea, which is a major flavonoid source, but also for flavonoids. Peters et al (62) found evidence for publication bias, particularly with respect to stroke, and therefore urged caution in interpreting the results for this endpoint. Publication bias might have occurred for flavonoid epidemiologic studies as well. Another striking finding is that studies of CAD or myocardial infarction conducted in continental Europe reported strong inverse associations, whereas studies conducted elsewhere did not. Summarized RRs for drinking 3 cups per day compared with no tea were 1.62 (95% CI: 1.15, 2.30) for 2 studies from the United Kingdom, 0.27 (95% CI: 0.16, 0.44) for 3 studies from continental Europe, and 0.95 (95% CI: 0.84, 1.08) for 8 studies from the United States. Explanations for this phenomenon, which also seems to occur for flavonoids, include differing associations with a healthy lifestyle and publication bias. However, no satisfactory explanation has been provided, and research into these differences seems worthwhile.

Attempts to distinguish the effects of flavonols and flavones from those of catechins were undertaken with data from the Zutphen Elderly Study (18, 29) and demonstrate one of the major limitations of component-based epidemiologic studies. Each food contains a large number of different compounds, some known and quantified, some less well characterized, and some unknown or unmeasurable. Many compounds tend to be present in the same foods or families of foods. The intake of catechins, for example, was positively correlated with the intake of fruits and vegetables and their constituents, eg, vitamin C, vitamin E, carotenoids, folate, and fiber. For the intake of vitamin C, ß-carotene, and fiber, correlations in several European populations on the order of 0.40-0.70 were reported (63). When the correlation is too high, it is impossible to ascertain independent effects of dietary components, because of multicollinearity. This was the case for flavonols and catechins in the Zutphen Elderly Study. Tea supplied 87% of this population's intake of catechins and 61% of the intake of flavonols and flavones (24). To circumvent multicollinearity problems but still distinguish the effects of catechins from those of flavonols, subgroups were defined, ie, tea, catechins from sources other than tea, and flavonols from sources other than tea. Independent effects on CAD mortality rates were borderline significant for tea (P = 0.06) and catechins from other sources than tea (P = 0.11). For correct interpretation of results of dietary component-based epidemiologic studies, adjustment for other dietary factors (both nutrient and nonnutrient) is of major importance.

Data are less convincing for cancer. Of several cancers studied, protective effects have been reported only for lung cancer in relation to flavonol and flavone intake. Together with data from one cohort study and one cross-sectional study suggesting beneficial effects on asthma and lung function, these data suggest a role for flavonoids in lung health that merits additional investigation. For colorectal cancer, data are inconsistent, with 1 positive, 1 inverse, and 4 null associations. Residual confounding by smoking is the most serious drawback of the flavonoid studies published to date. Unhealthy (or healthy) behaviors tend to cluster. Smoking, which is the single most important risk factor for many cancers and an important determinant of CVDs, is associated with higher intakes of energy, alcohol, and fat, lower intakes of fruits and vegetables, lower socioeconomic status, and physical inactivity (64–66). Previous studies showed that consumption of important sources of flavonoids, such as tea in the Netherlands (67) and in Japan (68) and wine in Denmark (69), is associated with healthy dietary patterns. Residual confounding occurs if confounders such as smoking are insufficiently accounted for in statistical analyses. Insufficient control for confounders can occur as a result of misclassification of the confounding factors, and control thus depends on the quality and amount of detail with which the confounders are measured. In particular, if the confounding is strong, as is usually the case for smoking, then misclassification of the confounder can yield spurious associations (70). Studying associations among lifelong nonsmokers is an effective way of ruling out residual confounding by smoking, and this should be done in future studies.

The strong inverse associations found for plasma enterolactone concentrations and the risk of CAD in a prospective study in Finland and supportive data from several cross-sectional studies make this an exciting new area of research that requires more investigation. Lignans are phytoestrogens, and their effect on breast cancer is a more traditional area of research in which several studies have been conducted to date. Inverse associations were reported only for case-control studies, whereas no associations between lignans and breast or prostate cancer were found in 3 prospective studies. Case-control studies may suffer from several drawbacks that make them less suitable for studying the effects of diet on the risk of disease. Recall bias, with misclassification of subjects because case subjects remember their diet differently, compared with control subjects, if this is assessed with questionnaires after diagnosis of the disease, is one hazard. If biomarkers of exposure are used instead of questionnaires, then the possibility exists that the biomarker levels are influenced by the disease state if the samples are collected after onset of the disease. Because significant associations were reported only for the case-control studies, these biases might have influenced the lignan data reported to date. Therefore, more prospective studies should be conducted on the lignan-cancer association before any conclusions are drawn.

The quality of dietary intake assessment and of food composition tables is crucial in the component-based epidemiologic approach. Imprecision of exposure measurement is an important limitation of these studies. If valid biomarkers are available, then these may replace questionnaire-based assessment of exposure; for most compounds, however, no biomarkers that reflect long-term exposure are available. No studies using biomarkers of flavonoid intake have been conducted to date. In contrast, several studies have used plasma or urinary enterolactone concentrations as biomarkers of lignan exposure. A major drawback of most of those studies was that enterodiol, the other enterolignan produced by the colonic microflora, was not measured. It was recently shown that not only secoisolariciresinol and matairesinol but also several other plant lignans, including pinoresinol and lariciresinol, are converted into enterolignans (71). We determined the concentrations of these 2 additional lignans in plant foods and found that their intake is severalfold higher than that of secoisolariciresinol and matairesinol (IEJ Milder, ICW Arts, and PCH Hollman, unpublished results, 2004). These compounds should be included in future evaluations of the health effects of dietary lignans.

Epidemiologic studies can be useful tools to study the health effects of polyphenols. Data obtained to date suggest of a beneficial effect on CVD but not on cancer, with the possible exception of lung cancer. There is a need for more research on stroke and lung diseases such as asthma and COPD. Most studies have included flavonols and flavones only. With data becoming available for other polyphenols, these should be included in future studies. Careful design of prospective studies is important to offset some of the major drawbacks of epidemiologic studies, including residual confounding (by smoking and other dietary factors) and exposure assessment.

REFERENCES

1. World Cancer Research Fund. Vegetables and fruits. In: Food, nutrition and the prevention of cancer: a global perspective. Washington, DC: World Cancer Research Fund/American Institute for Cancer Research, 1997:436–46.
2. Law MR, Morris JK. By how much does fruit and vegetable consumption reduce the risk of ischaemic heart disease? Eur J Clin Nutr 1998;52:549–56.
3. Higdon JV, Frei B. Tea catechins and polyphenols: health effects, metabolism, and antioxidant functions. Crit Rev Food Sci Nutr 2003;43:89–143.
4. Yang CS, Landau JM, Huang MT, Newmark HL. Inhibition of carcinogenesis by dietary polyphenolic compounds. Annu Rev Nutr 2001;21:381–406.
5. Nijveldt RJ, van Nood E, van Hoorn DEC, Boelens PG, van Norren K, van Leeuwen PAM. Flavonoids: a review of probable mechanisms of action and potential applications. Am J Clin Nutr 2001;74:418–25.
6. Adlercreutz H, Mazur W. Phyto-oestrogens and Western diseases. Ann Med 1997;29:95–120.
7. Hollman PCH, Arts ICW. Flavonols, flavones and flavanols: nature, occurrence and dietary burden. J Sci Food Agric 2000;80:1081–93.
8. Olthof MR, Hollman PCH, Buijsman MNCP, van Amelsvoort JMM, Katan MB. Chlorogenic acid, quercetin-3-rutinoside and black tea phenols are extensively metabolized in humans. J Nutr 2003;133:1806–14.
9. Hertog MGL, Hollman PCH, Katan MB. Content of potentially anticarcinogenic flavonoids of 28 vegetables and 9 fruits commonly consumed in the Netherlands. J Agric Food Chem 1992;40:2379–83.
10. Hertog MGL, Hollman PCH, Van de Putte B. Content of potentially anticarcinogenic flavonoids of tea infusions, wines, and fruit juices. J Agric Food Chem 1993;41:1242–6.
11. Arts ICW, van de Putte B, Hollman PCH. Catechin contents of foods commonly consumed in the Netherlands. 1. Fruits, vegetables, staple foods, and processed foods. J Agric Food Chem 2000;48:1746–51.
12. Arts ICW, van de Putte B, Hollman PCH. Catechin contents of foods commonly consumed in the Netherlands. 2. Tea, wine, fruit juices, and chocolate milk. J Agric Food Chem 2000;48:1752–7.
13. Knekt P, Kumpulainen J, Jarvinen R, et al. Flavonoid intake and risk of chronic diseases. Am J Clin Nutr 2002;76:560–8.
14. Hertog MGL, Sweetnam PM, Fehily AM, Elwood PC, Kromhout D. Antioxidant flavonols and ischemic heart disease in a Welsh population of men: the Caerphilly Study. Am J Clin Nutr 1997;65:1489–94.
15. Sesso HD, Gaziano JM, Liu S, Buring JE. Flavonoid intake and the risk of cardiovascular disease in women. Am J Clin Nutr 2003;77:1400–8.
16. Geleijnse JM, Launer LJ, Van der Kuip DA, Hofman A, Witteman JC. Inverse association of tea and flavonoid intakes with incident myocardial infarction: the Rotterdam Study. Am J Clin Nutr 2002;75:880–6.
17. Arts ICW, Jacobs DR Jr, Harnack LJ, Gross M, Folsom AR. Dietary catechins in relation to coronary heart disease death among postmenopausal women. Epidemiology 2001;12:668–75.
18. Arts ICW, Hollman PCH, Feskens EJM, Bueno de Mesquita HB, Kromhout D. Catechin intake might explain the inverse relation between tea consumption and ischemic heart disease: the Zutphen Elderly Study. Am J Clin Nutr 2001;74:227–32.
19. Hirvonen T, Pietinen P, Virtanen M, et al. Intake of flavonols and flavones and risk of coronary heart disease in male smokers. Epidemiology 2001;12:62–7.
20. Yochum L, Kushi LH, Meyer K, Folsom AR. Dietary flavonoid intake and risk of cardiovascular disease in postmenopausal women. Am J Epidemiol 1999;149:943–9.
21. Hertog MGL, Feskens EJ, Kromhout D. Antioxidant flavonols and coronary heart disease risk. Lancet 1997;349:699 (letter).
22. Knekt P, Järvinen R, Reunanen A, Maatela J. Flavonoid intake and coronary mortality in Finland: a cohort study. Br Med J 1996;312:478–81.
23. Rimm EB, Katan MB, Ascherio A, Stampfer MJ, Willett WC. Relation between intake of flavonoids and risk for coronary heart disease in male health professionals. Ann Intern Med 1996;125:384–9.
24. Hertog MGL, Feskens EJ, Hollman PCH, Katan MB, Kromhout D. Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study. Lancet 1993;342:1007–11.
25. Hirvonen T, Virtamo J, Korhonen P, Albanes D, Pietinen P. Intake of flavonoids, carotenoids, vitamins C and E, and risk of stroke in male smokers. Stroke 2000;31:2301–6.
26. Keli SO, Hertog MGL, Feskens EJ, Kromhout D. Dietary flavonoids, antioxidant vitamins, and incidence of stroke: the Zutphen study. Arch Intern Med 1996;156:637–42.
27. Hollman PCH, van het Hof KH, Tijburg LBM, Katan MB. Addition of milk does not affect the absorption of flavonols from tea in man. Free Radic Res 2001;34:297–300.
28. Arts ICW, Jacobs DR Jr, Gross M, Harnack LJ, Folsom AR. Dietary catechins and cancer incidence among postmenopausal women: the Iowa Women's Health Study (United States). Cancer Causes Control 2002;13:373–82.
29. Arts ICW, Hollman PCH, Bueno de Mesquita HB, Feskens EJM, Kromhout D. Dietary catechins and epithelial cancer incidence: the Zutphen Elderly Study. Int J Cancer 2001;92:298–302.
30. Knekt P, Järvinen R, Seppänen R, et al. Dietary flavonoids and the risk of lung cancer and other malignant neoplasms. Am J Epidemiol 1997;146:223–30.
31. Hertog MGL, Feskens EJM, Hollman PCH, Katan MB, Kromhout D. Dietary flavonoids and cancer risk in the Zutphen Elderly Study. Nutr Cancer 1994;22:175–84.
32. Hirvonen T, Virtamo J, Korhonen P, Albanes D, Pietinen P. Flavonol and flavone intake and the risk of cancer in male smokers (Finland). Cancer Causes Control 2001;12:789–96.
33. Goldbohm RA, Hertog MGL, Brants HAM, van Poppel G, van den Brandt PA. Intake of flavonoids and cancer risk: a prospective cohort study. In: Armado R, Andersson H, Bardócz S, Serra F, eds. Polyphenols in food. Luxembourg: Office for Official Publications of the European Communities, 1998:159–66.
34. Strom SS, Yamamura Y, Duphorne CM, et al. Phytoestrogen intake and prostate cancer: a case-control study using a new database. Nutr Cancer 1999;33:20–5.
35. Le Marchand L, Murphy SP, Hankin JH, Wilkens LR, Kolonel LN. Intake of flavonoids and lung cancer. J Natl Cancer Inst 2000;92:154–60.
36. Walcott FL, Hauptmann M, Duphorne CM, Pillow PC, Strom SS, Sigurdson AJ. A case-control study of dietary phytoestrogens and testicular cancer risk. Nutr Cancer 2002;44:44–51.
37. McCann SE, Freudenheim JL, Marshall JR, Graham S. Risk of human ovarian cancer is related to dietary intake of selected nutrients, phytochemicals and food groups. J Nutr 2003;133:1937–42.
38. Tabak C, Arts ICW, Smit HA, Heederik D, Kromhout D. Chronic obstructive pulmonary disease and intake of catechins, flavonols, and flavones: the MORGEN Study. Am J Respir Crit Care Med 2001;164:61–4.
39. Nesbitt PD, Lam Y, Thompson LU. Human metabolism of mammalian lignan precursors in raw and processed flaxseed. Am J Clin Nutr 1999;69:549–55.
40. Heinonen S, Nurmi T, Liukkonen K, et al. In vitro metabolism of plant lignans: new precursors of mammalian lignans enterolactone and enterodiol. J Agric Food Chem 2001;49:3178–86.
41. De Kleijn MJJ, Van der Schouw YT, Wilson PWF, et al. Intake of dietary phytoestrogens is low in postmenopausal women in the United States: the Framingham Study. J Nutr 2001;131:1826–32.
42. Horn-Ross PL, Barnes S, Lee M, et al. Assessing phytoestrogen exposure in epidemiologic studies: development of a database (United States). Cancer Causes Control 2000;11:289–98.
43. Pillow PC, Duphorne CM, Chang S, et al. Development of a database for assessing dietary phytoestrogen intake. Nutr Cancer 1999;33:3–19.
44. Stumpf K, Uehara M, Nurmi T, Adlercreutz H. Changes in the time-resolved fluoroimmunoassay of plasma enterolactone. Anal Biochem 2000;284:153–7.
45. Vanharanta M, Voutilainen S, Rissanen TH, Adlercreutz H, Salonen JT. Risk of cardiovascular disease-related and all-cause death according to serum concentrations of enterolactone: Kuopio Ischaemic Heart Disease Risk Factor Study. Arch Intern Med 2003;163:1099–104.
46. Vanharanta M, Voutilainen S, Lakka TA, van der Lee M, Adlercreutz H, Salonen JT. Risk of acute coronary events according to serum concentrations of enterolactone: a prospective population-based case-control study. Lancet 1999;354:2112–5.
47. Vanharanta M, Voutilainen S, Nurmi T, et al. Association between low serum enterolactone and increased plasma F2-isoprostanes, a measure of lipid peroxidation. Atherosclerosis 2002;160:465–9.
48. de Kleijn MJJ, van der Schouw YT, Wilson PWF, Grobbee DE, Jacques PF. Dietary intake of phytoestrogens is associated with a favorable metabolic cardiovascular risk profile in postmenopausal US women: the Framingham Study. J Nutr 2002;132:276–82.
49. van der Schouw YT, Pijpe A, Lebrun CEI, et al. Higher usual dietary intake of phytoestrogens is associated with lower aortic stiffness in postmenopausal women. Arterioscler Thromb Vasc Biol 2002;22:1316–22.
50. Stattin P, Adlercreutz H, Tenkanen L, et al. Circulating enterolactone and prostate cancer risk: a Nordic nested case-control study. Int J Cancer 2002;99:124–9.
51. Dai Q, Franke AA, Yu H, et al. Urinary phytoestrogen excretion and breast cancer risk: evaluating potential effect modifiers endogenous estrogens and anthropometrics. Cancer Epidemiol Biomarkers Prev 2003;12:497–502.
52. Dai Q, Franke AA, Jin F, et al. Urinary excretion of phytoestrogens and risk of breast cancer among Chinese women in Shanghai. Cancer Epidemiol Biomarkers Prev 2002;11:815–21.
53. Hulten K, Winkvist A, Lenner P, Johansson R, Adlercreutz H, Hallmans G. An incident case-referent study on plasma enterolactone and breast cancer risk. Eur J Nutr 2002;41:168–76.
54. den Tonkelaar I, Keinan-Boker L, Van't Veer P, et al. Urinary phytoestrogens and postmenopausal breast cancer risk. Cancer Epidemiol Biomarkers Prev 2001;10:223–8.
55. Pietinen P, Stumpf K, Mannisto S, Kataja V, Uusitupa M, Adlercreutz H. Serum enterolactone and risk of breast cancer: a case-control study in eastern Finland. Cancer Epidemiol Biomarkers Prev 2001;10:339–44.
56. Ingram D, Sanders K, Kolybaba M, Lopez D. Case-control study of phyto-oestrogens and breast cancer. Lancet 1997;350:990–4.
57. Horn-Ross PL, Hoggatt KJ, West DW, et al. Recent diet and breast cancer risk: the California Teachers Study (USA). Cancer Causes Control 2002;13:407–15.
58. McCann SE, Moysich KB, Freudenheim JL, Ambrosone CB, Shields PG. The risk of breast cancer associated with dietary lignans differs by CYP17 genotype in women. J Nutr 2002;132:3036–41.
59. Horn-Ross PL, John EM, Lee M, et al. Phytoestrogen consumption and breast cancer risk in a multiethnic population: the Bay Area Breast Cancer Study. Am J Epidemiol 2001;154:434–41.
60. Horn-Ross PL, John EM, Canchola AJ, Stewart SL, Lee MM. Phytoestrogen intake and endometrial cancer risk. J Natl Cancer Inst 2003;95:1158–64.
61. Horn-Ross PL, Hoggatt KJ, Lee MM. Phytoestrogens and thyroid cancer risk: the San Francisco Bay Area thyroid cancer study. Cancer Epidemiol Biomarkers Prev 2002;11:43–9.
62. Peters U, Poole C, Arab L. Does tea affect cardiovascular disease? A meta-analysis. Am J Epidemiol 2001;154:495–503.
63. Riboli E, Slimani N, Kaaks R. Identifiability of food components for cancer chemoprevention. IARC Sci Publ 1996;(139):23–31.
64. Burke V, Milligan RA, Beilin LJ, et al. Clustering of health-related behaviors among 18-year-old Australians. Prev Med 1997;26:724–33.
65. McPhillips JB, Eaton CB, Gans KM, et al. Dietary differences in smokers and non-smokers from two southern New England communities. J Am Diet Assoc 1994;94:287–92.
66. Veenstra J, Schenkel JA, Erp-Baart AM, et al. Alcohol consumption in relation to food intake and smoking habits in the Dutch National Food Consumption Survey. Eur J Clin Nutr 1993;47:482–9.
67. Hulshof KFAM, Wedel M, Löwik MRH, et al. Clustering of dietary variables and other lifestyle factors (Dutch Nutritional Surveillance System). J Epidemiol Community Health 1992;46:417–24.
68. Tsubono Y, Takahashi T, Iwase Y, Iitoi Y, Akabane M, Tsugane S. Dietary differences with green tea intake among middle-aged Japanese men and women. Prev Med 1997;26:704–10.
69. Tjonneland A, Gronbaek M, Stripp C, Overvad K. Wine intake and diet in a random sample of 48 763 Danish men and women. Am J Clin Nutr 1999;69:49–54.
70. Marshall JR, Hastrup JL. Mismeasurement and the resonance of strong confounders: uncorrelated errors. Am J Epidemiol 1996;143:1069–78.
71. Milder IEJ, Arts ICW, Venema DP, Lasaroms JJP, Wähälä K, Hollman PCH. Optimization of a liquid chromatography-tandem mass spectrometry method for quantification of the plant lignans secoisolariciresinol, matairesinol, lariciresinol, and pinoresinol in foods. J Agric Food Chem 2004;52:4643–51.