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Green tea is good for you, but only if drunk in moderation. While the polyphenols in green tea are credited with preventing heart disease and cancer, it seems they can cause liver and kidney damage if consumed in very large quantities, a review of studies into the toxicity of polyphenols has shown.
"People shouldn't be too alarmed by this, but those taking supplements may experience problems," says lead author Chung Yang of Rutgers, the State University of New Jersey.
He stresses that up to 10 small cups of green tea a day is fine. Problems are likely in people who take supplements, which can contain up to 50 times as much polyphenol as a single cup of tea.
http://www.newscientist.com/article/dn11786-green-tea-can-be-harmful-in-large-quantities.htmlsnip
Although there have been no reports of toxicity in human volunteers enrolled in intervention studies, careful monitoring of liver and kidney function is required until the risk of toxic events associated with tea catechins is established in humans.
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there have been a number of recent case reports of hepatotoxicity related to the consumption of high doses of tea-based dietary supplements (10−29 mg/kg/day po) (18). In nearly all cases (eight out of nine), patients presented with elevated serum alanine aminotransferase (ALT) and bilirubin levels. In two of nine cases, periportal and portal inflammation were observed. All cases resolved following cessation of supplement consumption. A causative role for the green tea preparations is suggested by the fact that reinjury was observed following rechallenge with the same preparations.
http://pubs.acs.org/doi/full/10.1021/tx7000515