ReviewAll teas are not created equal: The Chinese green tea and cardiovascular health
Introduction
Tea is one of the most widely consumed beverages in the world, next only to water [1], [2] and well ahead of coffee, beer, wine and carbonated soft drinks [3]. It can be categorized into three types, depending on the level of fermentation, i.e., green (unfermented), oolong (partially fermented) and black (fermented) tea. Although this process is often assumed, incorrectly, to be fermentation which usually implies additives, the more correct term should be oxidation which means exposure to air while drying [4]. In general, green tea has been found to be superior to black tea in terms of antioxidant activity owing to the higher content of (−)-epigallocatechin gallate [5], [6], [7]. The processes used in the manufacture of black tea are known to decrease levels of the monometric catechins to a much greater extent than the less severe conditions applied to other teas [5], [6]. The production and consumption of the partially fermented oolong tea are confined to China [1].
The cardioprotective effect of flavonoids from green tea can be attributed to not only antioxidant [5], antithrombogenic [8], [9] and anti-inflammatory [10] properties but also improvement of coronary flow velocity reserve [1], [11]. In this article, I will discuss the effects of green tea on atherosclerosis, coronary heart disease, hypertension, diabetes, metabolic syndrome and obesity, and, finally, its comparison with black tea.
Section snippets
Antioxidant effect
The protective effect of tea (Camellia sinensis) against atherosclerosis has been attributed to the antioxidant activity of the flavonoids [3], [5], [12]. Over 4000 different flavonoids have been described, and they are categorized into flavonols, flavones, catechins, flavanones, anthocyanidins and isoflavonoids [13]. Green tea has been shown to retard atherosclerosis in both experimental animals [14], [15], [16], [17], [18], [19] and humans [20], [21]. However, although the results in animals
Hypertension
In ancient Chinese medicine, tea has long been believed to possess hypotensive effects [50]. However, conflicting results have been shown among human trials as well as animal studies on the relation between tea consumption and blood pressure. The controversy, which is partly due to the fact that tea contains caffeine that can transiently increase blood pressure, has largely been resolved by the recent studies of Hodgson and Puddey [51]. They found that in population studies people generally
Diabetes
Atherosclerosis accounts for some 80 % of all diabetic mortality; about three-quarters of the cardiovascular deaths from diabetes result from coronary artery disease [58]. Plants containing flavonoids have been used to treat diabetes in Indian medicine [59]. The green tea flavonoid has been shown to have insulin-like activities [59] as well as insulin-enhancing activity [60]. However, epigallocatechin gallate, which is the principal catechin in green tea, differs from insulin in that it affects
Metabolic syndrome
The metabolic syndrome, formerly called metabolic ‘syndrome X’ [67], has reached an epidemic proportion [68] since its original description in 1988 [69]. A cluster of abnormalities defines the metabolic syndrome, including insulin resistance, hypertension, obesity, hypertriglyceridemia, low HDL cholesterol and inflammation [70].
Green tea has been shown recently to be a promising tool against the metabolic syndrome in China [66]. According to Campbell [66], Chinese researchers have experimented
Obesity
Obesity has increased at an alarming rate in recent years and is now a worldwide health problem [71], including China [71], [72]. It has been known for some time that tea helps to control obesity, and this is common knowledge in China. A Chinese classical pharmaceutical book called the Bencao Shiyi (The Conpendium of Materia Medica) states: “Drinking tea for a long time will make one live long to stay in good shape without becoming too fat and too heavy” [73].
The mechanisms of action of tea in
Green tea vs. black tea
While there are hundred varieties of teas, most can be categorized into 3 types, depending on the level of fermentation or oxidation, i.e., green (unfermented), oolong (partially fermented) and black (fermented) tea. Green teas are not fermented during processing and thus retain the original color of the tea leaves; the most famous green tea is the Lung Chen Tea (Dragon Well Tea), grown in the hillside of Hangzhou, Zhejiang Province, China. Black teas are made from fermented leaves which
Conclusion
According to Chinese legends, tea was discovered by the Emperor Shen Nong in about 2700 BC, when a gust of wind blew tea leaves into a kettle of boiling water [92]. Emperor Shen Nong reputedly said in 2737 BC that tea can provide “vigor of body, contentment of mind, and determination of purpose” [106]. Since then green tea has been a staple product and the main beverage in China [12], [92]. It is the Chinese national drink served at all meals of the day to everyone from the president to the
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