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Tea and Cardiovascular Health
Although the field of research that encompasses tea in relation to its impact on cardiovascular health is relatively young, it is a rapidly growing area of investigation. To date, most of the literature supports an inverse relationship in which increased consumption of tea is associated with a reduced risk of cardiovascular disease (myocardial infarction – MI, stroke and coronary heart disease – CHD). The mechanisms by which tea may offer these benefits are only beginning to be understood, but numerous hypotheses have been put forth. One of the major mechanisms by which tea consumption is thought to confer health benefits is via antioxidant protection, as the flavonoids (catechins as well as derived tannins) present in tea have been shown both in vitro as well as in vivo to have powerful antioxidant properties. For example, consumption of a single dose of green or black tea in humans has been shown to improve plasma antioxidant capacity, and repeated consumption has been shown to decrease markers of oxidative stress including oxidative damage to DNA. Most notably, the catechin components of green tea, especially epigallocatechin gallate (EGCG), as well as components present black tea such a theaflavins have demonstrated significant antioxidant capacity in numerous model systems. However, in addition to having the ability to directly quench free radicals, flavonoids have been shown to chelate metals, inhibit smooth muscle cell proliferation, enhance vasodilation and perfusion, enhance endothelial function, inhibit pro-oxidant enzymes, enhance phase II detoxification and enhance gap junction communication between cells. An interesting example of tea's ability to enhance blood vessel (endothelial cell) function was provided by a study published last year in the journal Clinical Science. In this study subjects consumed either 5 cups of black tea/day or a placebo (hot water) for 4 weeks. Endothelial function was assessed by measuring the ability of the brachial artery to dilate after being constricted. Changes in the ability of the artery to dilate were assessed before and after tea consumption. An improvement in the ability of the artery to dilate after being constricted signifies an improvement in blood vessel function. Subjects that ingested the black tea experienced a significant and consistent increase in endothelial function compared to those consuming hot water.
A recent review of the observational data on tea consumption and cardiovascular disease was published in the American Journal of Epidemiology. This report reviewed 17 observational studies (10 cohort and 7 case-control) on tea consumption and cardiovascular disease and reported an overall decrease in the risk for cardiovascular disease with increasing consumption of tea. This analysis found that for each 3 cups of tea consumed per day, there was an 11% reduction in the risk for MI and a 26 - 66% reduction in the risk for stroke. When the country in which the study was conducted was accounted for, there was a trend for those studies published in Europe to report a greater inverse effect of tea on cardiovascular disease than studies published elsewhere. One particularly impressive prospective study published last year in the American Journal of Clinical Nutrition investigated the association of tea and flavonoid intake with the incidence of MI in almost 5,000 men and women in the Netherlands. This study, known as the Rotterdam Study, reported that during a 5.6 year follow up, those that consumed > 375 mL/day of tea had a 43% reduction in the risk of MI and a 70% reduction in the risk for a fatal MI. These reductions in risk remained significant after adjustment for numerous variables including age, sex, body mass index (BMI), smoking status, education level, alcohol, coffee and fat consumption. When the data were divided by sex, a stronger inverse relationship was observed in women compared with men. This finding is intriguing as tea contains natural phytoestrogens, and these compounds may have a more pronounced affect in women compared with men. In addition to tea consumption, the total daily intake of flavonoids in the Rotterdam Study was also inversely correlated with the risk of MI. In support of these findings is another study that was published in January of this year examining tea consumption in men and women in the Middle East. Over 3,000 men and women aged 30 - 70 were followed to investigate the link between tea drinking and cardiovascular disease. A dose response relationship was observed between the consumption of tea and a reduction in risk of coronary heart disease. The data revealed that those subjects who consumed > 480 mL tea/day had a significantly lower prevalence of coronary heart disease compared with non-tea drinkers. After adjustment for other variables associated with heart disease, those that consumed > 6 cups/day were shown to have almost a 50% reduction in the risk of coronary heart disease. These data are compelling and support a real potential for a protective effect of tea in cardiovascular disease. It is important to note that in the Middle Eastern study as well as in the Rotterdam study the type of tea consumed was black, as that is the form most commonly consumed in those regions. However, similar data also exist linking the consumption of green tea to a reduction in the risk of coronary artery disease (CAD) and MI. Data published last year in The American Journal of Cardiology followed a group of subjects in Japan over a two-year period who had received coronary angiography for suspected heart disease. The participants in the study were divided into 3 groups based on green tea consumption (<1 cup/d, 1 - 3 cups/d, >3 cups/d). While tea consumption did not significantly affect the prevalence of CAD, it was significantly and inversely correlated to MI. Those that consumed > 1 cup/d of green tea had a 42% reduction in the risk for MI compared with non-tea drinkers after adjustment for traditional risk factors as well as for the intake of fruits and coffee.
Interpretation of data with respect to tea and cardiovascular disease can often be challenging and there are a number of factors that must be kept in mind. First, it should be noted that these studies are observational or case-control in design and it is impossible to control for all potential confounding variables. A very limited number of studies have reported that tea consumption was associated with an increased risk for heart disease or stroke. For example, the Caerphilly Study, conducted in South Wales reported an increase in the risk for ischemic heart disease with increasing consumption of tea, with those consuming > 1.2 L/day having the greatest increase in risk. While this does not fit with our biological understanding of how tea may affect health, an understanding of the significant cultural differences in the manner in which tea is prepared and consumed as well as the socioeconomic differences among countries with respect to tea consumption may help to explain these findings. In many countries tea consumption increases together with increasing affluence. However in South Wales, where the Caerphilly Study was conducted, t |