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Registered User
Join Date: Jul 2005
Location: Spring, TX
Posts: 47
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Green tea and cancer
Hundreds of studies have been done in recent years on the effect green tea has on both the prevention and treatment of cancer, as well as cardiovascular disease. Epidemiological studies tend to yield different results, with some finding no effect [27-28] and others finding that green tea drinkers have less incidence of certain types of cancer [29-30]. However, when a less specific variable such as total cancer deaths or lifespan is measured in a larger population it can be shown that green tea has a definite effect.
One of the more comprehensive studies was conducted on a Japanese population over 13 years and the results were published this year in Ageing Research Reviews. It measured 90 lifestyle factors in a population of 8552 individuals over 40 years of age and determined the correlation of various factors with death from any cause. Consumption of over ten cups a day of green tea, when compared with less than three cups, was correlated with an increase in lifespan of 4.3 and 3.8 years in men and women respectively. Greater associations were seen in smokers (implying green tea had a protective effect) and those who died before age 80. Green tea was correlated with a 7.5 year longer lifespan in male smokers. Other large scale studies by this research group found that drinking green tea delayed cancer onset by 4.1 and 7.6 years in males and females respectively, cancer death by 3.9 and 5.9 years, and cardiovascular death by 1.9 and 1.4 years. Not surprisingly, these studies also found that smoking was correlated with earlier deaths from cancer and cardiovascular disease. The conclusion of this group was that green tea increases lifespan and decreases the risk of pre-mature death, particularly that caused by cancer [31].
Many animal and in vitro studies have also been conducted on the effect green tea has on specific types of cancer. In mice, both oral and topical administration of green tea significantly decreases the risk of UV-induced skin cancer [32]. However, it is possible that this is due to decreased tissue fat [33], and caffeine is a necessary constituent in this case [17, 33], so more research is needed in this area. Green tea selectively destroys breast cancer cells [34, 35] and epidemiologic data implies that it aids in the prevention of early stage breast cancer [30]. A case control study also found that green tea decreases the risk of developing ovarian cancer [36], while other in vitro data has found that green tea inhibits the proliferation of cervical cancer [37], prostate cancer [38], leukemia [39], head and neck carcinoma cells [35], and pancreatic carcinoma cells [40]. In the case of lung cancer, green tea has a definite and significant effect in smokers and nonsmokers alike. In vitro data [41] and epidemiologic data in both smokers [42] and nonsmokers [29] support this. Finally, green tea may have the ability to prevent cancer in the gastrointestinal tract. An epidemiologic study in China found that green tea drinkers had less incidence of gastrointestinal cancer [43], although a different study in Japan found no statistically significant difference [28]. In vitro and in rats, polyphenols also protect against colon cancer [44, 45].
As if this wasn't enough, green tea has also been tested against many carcinogens and almost always proves successful. Two toxins that humans are very commonly exposed to, tobacco and alcohol, are probably of most importance. An epidemiological study in China compared the incidence of various types of cancer among cigarette smokers and alcohol drinkers. In alcohol drinkers, those that regularly consumed green tea had 81%, 78%, and 39% decreased risk of gastric, liver and esophageal cancer, while smokers that drank green tea had 16%, 43%, and 31% decreased risk of these cancers [42]. In both mice and rats, green tea significantly protects against tumorgenesis induced by nitrosamines, which are found in tobacco as well as many food products [46, 47]. It also protects against the toxic effects of the environmental pollutant pentachlorphenol (PCP) and the chemotherapeutic drug cyclophosphamide (CP) in mice [48, 49] and the cancer induced by the carcinogens DMBA and DMH in other rodents [45, 50].
As with the thermogenic effect, there are multiple mechanisms for the proctive effect green tea has against cancer, and many of them are tissue specific. One of the more important ones is the protection against oxidative stress, which may play a primary role in certain types of cancer [48, 49-52]. The antioxidant properties of green tea and their importance will be discussed in more detail later on. Of more importance is that when it comes to cancer cells, green tea is actually a pro-oxidant which results in the death of these cells [53]. This condition-dependent antioxidant/pro-oxidant duality is a common feature of many antioxidants.
EGCG plays a fundamental role in cancer prevention as it inhibits many proteins and the activity of many protein kinases involved in tumor cell proliferation and survival. These include (but are by no means limited to) the large multi-catalytic protease and metaloproteionases involved in tumor survival and metastasis and the epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGF), platelet-derived growth factor receptor, mitogen-activated protein kinase, and IkB kinase [52]. The inhibition of EGFR and VEGF are of importance because they are known to play roles in angiogenesis [35, 28], which contributes to tumor formation in many cases [54]. The inhibition of metalloproteinases MMP-2 and MMP-9 are also known to play important roles in the anti-carcinogenic action of green tea [38, 41].
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