Green Tea Consumption and Liver Disease

Xi Jin; Ruo-heng Zheng; You-ming Li

Disclosures

Liver International. 2008;28(7):990-996. 

In This Article

Discussion

The present systematic review demonstrates that increased green tea consumption is associated with a reduced risk of liver disease. Of 472 studies we selected for examination, 10 studies were finally included. The low inclusion rate may be because of the relatively wider range of MESH words, the strict selection criteria and current situation wherein most studies investigating therapeutic tea effect are on animal levels. The initial search strategy involved data from both the Western and the Chinese literature. However, we found only two articles based on a Japanese population[25] and a US population,[26] both of which showed beneficial effects of tea consumption against chronic liver disease. It is not surprising to see such phenomenon for tea originated and is much more widely consumed in China than in any other country and, hence, raises more research enthusiasm. Moreover, two special articles from the Chinese literature[27,28] using the attenuation rate of fatty liver disease as outcome measurements, both showed a protective effect of green tea against fatty liver disease (P<0.05).

A protective effect of tea consumption against liver disease, especially liver cancer, is biologically plausible. Tea and its main component tea polyphenol have been shown to be safe and effective in inhibiting carcinogenesis both in in vitro bioassays and in in vivo animal models.[29,30] This effect may be achieved through their ability of capture and detoxification of reactive oxygen species from in vivo biochemical reactions such as carcinogen metabolism and inflammation.[31] It has been shown that green tea polyphenol can prevent oxygen free radical-induced hepatocyte lethality, prevent lipopolysaccharide-induced liver injury through inhibition of inducible nitric oxide synthase and tumour necrosis factor-α expression, inhibit carcinogen or toxin-induced liver oxidative DNA damage and so on.[32,33,34] It is not surprising that green tea has a beneficial effect on fatty liver disease as hyperlipidaemia is considered to be one contributor and tea polyphenol could stimulate lipid metabolism in the liver and decrease blood lipid concentration.[35,36] The lipid-lowering effect might be achieved through inhibiting the intestinal absorption of lipids.[4] Besides, several studies have supported the protective effect of tea extracts or tea polyphenol against liver fibrosis and cirrhosis in rats,[37,38] which might be exerted through cleaning up lipid oxidation and reducing the level of endotoxin.[39]

Before concluding, several limitations of the systematic review and this study itself should be considered. Firstly, the possibility of bias and cofounders cannot be excluded. Most of the studies controlled for confounders such as sex, age, smoking and alcohol intake but little information on the control of previous liver disease was reported, which might have partially influenced the results. Secondly, the dosage and duration of green tea consumption varied in different studies and such non-uniform green tea consumption data might affect the results. Thirdly, various tea dosages, types of liver disease and study designs caused significant heterogeneity and hence precluded further application of meta-analysis, which would have provided information on the overall effect of green tea against liver disease. Fourthly, as eight of the 10 selected studies were conducted in China, the results might not be well applicable to other populations. Besides, because they were not published on SCI journals, their credibility, reliability and availability to the Western world should be carefully considered and further high-quality double-blinded RCTs in this area are urgently needed. Fifthly, as tea has subtypes and all studies presented here used Chinese green tea, the effect of black tea and oolong tea is still not clear and needs to be explored in the future. Finally, although methodological quality has been assessed, the methods from most Chinese articles were still not described sufficiently. For instance, the method of randomization has not been clearly described in most of the randomized trials.

In summary, the findings from this systematic review indicate that green tea consumption may reduce the risk of liver disease. However, as the studies used here differed in the design, outcome, tea dosage and other aspects, more rigorous studies such as double-blinded RCTs in different countries are needed. In addition, lack of an objective measurement is a common flaw of questionnaire-based studies where tea intake is provided according to subjects' memories. Therefore, finding objective markers for tea consumption is a priority and we would like to see such research work is now under way where several urinary biomarkers have been established.[40,41]


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