Tea shows an inverse association with cardiovascular disease risk. The reason for this has been extensively researched and likely mechanisms centre around the high antioxidant content in tea. The cardioprotective effects of green tea may stem from its high concentrations of the catechin (flavan-3-ols) group of flavonoids. Catechins are thought to provide most of the health benefits associated with tea and have shown weight loss effects. However, while black tea contains much lower concentrations of catechins it does contain high concentrations of tannins, chemicals formed from catechins during the fermentation black tea. Tannins in black tea may provide antioxidant effects to humans and thereby provide health benefits. One way that both green tea and black tea may protect from cardiovascular disease is through protection of the endothelial lining of arteries. Free radicals can decrease the activity of nitric oxide synthase, and this leads to dysfunctional response of the arteries to blood flow, resulting in high blood pressure.
Endothelial dysfunction may be associated with fluctuations in blood pressure, with spikes in pressure readings resulting from changing levels of oxidative stress and its subsequent effects on nitric oxide synthase synthesis. Black tea is known to reduce total blood pressure and the variation in blood pressure that may characterise endothelial dysfunction1 For example, subjects were asked to consume 2 cups of black tea per day or a flavonoid-free caffeinated beverage for 6 months while ambulatory blood pressure was measured at baseline and at days 1, 3 and 6 months. Tea consumption resulted in significantly lower diastolic and systolic blood pressure readings compared to the control. In addition, the subjects consuming black tea had significant reductions in the night time variation in blood pressure that was estimated to be roughly 10 %. Interestingly the effects were immediate from day 1 and sustained over the entire 6 month period. However, daytime blood pressure was not affected by the consumption of tea.
As with lipoprotein chemistry, the relationship between blood pressure and cardiovascular disease has become more complex as more is understood about the nuances of the parameters. Blood pressure variability is now known to be one such nuance that had been overlooked previously regarding its effect on cardiovascular disease. Black tea is known to reduce blood pressure, and this study now shows that the variability on nighttime blood pressure is also reduced with black tea consumption. While the tannins in tea could be responsible for some or all of this effect, other constituents of black tea may also play a significant role. Black tea is a rich source of L-theanine, an amino acid that is known to induce relaxation through increased alpha wave activity. The fact that the effects of black tea consumption were apparent from day 1 are consistent with the known effects of L-theanine. Of course it is possible that both L-theanine and tannins act synergistically to improve blood pressure.
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