Polyphenols and disease risk in epidemiologic studies.

Polyphenols, which are found in high concentrations in fruits and vegetables, have consistently been reported to possess chemo – and cardioprotective properties.  Flaxseed contains a number of different polyphenols. Aside from their antioxidant properties, polyphenols exert a multitude of beneficial physiological effects.  Included is their ability to trap and scavenge free radicals, regulate nitric oxide, decrease leukocyte immobilization, induce apoptosis, inhibit cell proliferation and angiogenesis, and exhibit phytoestrogenic activity.  A large variety of plant polyphenols exist, including cinnamic acids, benzoic acids, flavonoids, stilbenes, couarins, lignans, and lignins.  Epidemiologic data on the effect of long-term intake of polyphenols is limited.  However, data appears to be strongest for the flavonoid subclasses including flavanols, flavones, catechins, and lignans.  The objective of the present study is to provide an overview of epidemiologic studies conducted to date that investigate the health effects (primarily CVD and cancer) of flavonoids (flavonols, flavones, catechins) and lignans. 

Flavanoids and CVD:  12 cohort studies regarding intake of flavonoids and risk of coronary artery disease (CAD) have been published to date.  A protective effect of flavonols, flavones, or catchins with respect to fatal or nonfatal CAD was observed in 7 of the 12 studies.  In these studies, reductions in mortality risk were reported to be as high as 65%.  Five cohort studies on flavanoid intake and risk of stroke have been published.  Of these, 2 found an inverse association between flavanoid intake and risk of stroke. 

Flavanoids and Cancer:  Seven prospective and 4 case-control studies have investigated the association between flavonoid intake and incidence of various cancers.  Of these studies, significant inverse associations were observed only for lung and colorectal cancer. 

Lignans and CVD:  Two publications have investigated enterolactone concentrations in relation to CVD risk.  In both these studies, a significant inverse association between enterolactone concentrations and risk of CVD were reported.  No studies on lignan intake and CVD risk have been published to date.  However, 2 cross-sectional studies observed an association between lignan intake and CVD risk factors including waist-hip ratio and metabolic syndrome score. 

Lignans and Cancer:  Three prospective, nested, case-control studies and 3 case-control studies have investigated the association between plasma or urinary lignan concentrations and risk of cancer.  All but one of these studies (investigating prostate cancer) investigated the incidence of breast cancer risk.  The nested case control studies (n=2) found no association between plasma or urinary enterolactone levels and breast cancer risk, while all 3 case-control studies observed an inverse association between lignan intake and breast cancer risk.  The one study investigating prostate cancer risk found no association between plasma enterolactone concentrations and risk among a large cohort of men residing in Finland, Norway, and Sweden.  Intake of the lignans secoisolariciresinol (SDG) and matairesinol was investigated in relation to risk of several cancers in prospective cohort (n=1) and case-control (n=3) studies.  A significant inverse association was observed for breast cancer in premenopausal, but not postmenopausal women, in a Western New York State study.  A borderline significant association was observed in women participating in the Bay Area Breast Cancer Study and California Teachers Study.  Significant or borderline significant protective associations have also been reported in females for endometrial cancer, ovarian cancer, and thyroid cancer, while no associations have been observed between lignan intake and incidence of prostate or testicular cancer in men.