Temperance in Green Tea
Throughout China and Japan, green tea is considered a staple beverage.
Many epidemiologic studies have linked frequent tea intake with a lower
incidence of cancer, cardiovascular disease, and neurodegenerative
disorders. Consumer interest in the tea’s health benefits has led to
the inclusion of green tea extracts in multivitamins and other dietary
supplements. But too much of a good thing could prove harmful,
according to a review in the April 2007 issue of Chemical Research in
Toxicology that analyzed the toxic potential of green tea polyphenols.
Did you know? Field
workers pluck only two leaves and a bud (shown
above) during the thrice-yearly tea harvest. To make green tea, leaves
of the Camellia sinensis plant are specially processed to prevent
oxidation. Black, oolong, and white teas are made from the same plant,
just using different processes.
Currently there are no published epidemiologic studies on the toxicity
of green tea supplements. But laboratory research with both rodents and
dogs has shown that high doses of the most heavily studied green tea
polyphenol, (-)-epigallocatechin-3-gallate (EGCG), cause liver, kidney,
and gastrointestinal toxicities.
Case reports on the toxic effects of green tea extracts in humans are
also beginning to emerge. “To date, there have been nine anecdotal case
reports of liver toxicity in humans associated with consumption of high
doses [700–2,000 mg/day] of green tea from dietary supplements,” says
lead author Joshua Lambert, an assistant research professor in the
Department of Chemical Biology at Rutgers University. “In some cases,
the subject stopped taking the supplement and the symptoms resolved,
and then the subject started taking the supplement again and liver
toxicity returned.” Such observations, albeit anecdotal, suggest that
green tea supplements are not without risk.
Cell culture studies have shown that EGCG can cause oxidative stress,
although these data now need to be confirmed in animal models. The
Rutgers team speculates that some susceptible individuals may carry a
particular polymorphism of the gene that codes
catecholamine-o-methyltransferase, an enzyme critical to the protection
of cells against EGCG-mediated oxidative stress and hepatotoxicity.
About a quarter of the population have a polymorphism that is
associated with low activity of this enzyme. “This is just a hypothesis
that we are testing,” says coauthor Chung S. Yang, a chemical biology
professor at Rutgers.
Toxic effects tend to arise when people take green tea supplements,
which can contain more than 50 times as much polyphenol as a single cup
of tea. “People who take less than 500 mg [of green tea concentrate or
preparation] per day and spread the dose out over the course of the day
are unlikely to have toxic side effects,” says Yang.
Yang adds that some Japanese publications report beneficial effects for
the consumption of 10 cups of green tea a day with no apparent harmful
effects. At most, people may experience stomach irritation after
drinking strongly brewed green tea on an empty stomach. Commercial
preparations such as the bottled green teas found in the United States
and green tea–flavored gum, bread, candy, ice cream, and desserts found
in Asia have very low levels of polyphenols.
At the present time there is no established upper tolerable limit for
green tea consumption. The Rutgers review points to the need for
epidemiologic studies to test the potential concerns of taking green
tea supplements at 500-mg doses or higher. Yang and Lambert hypothesize
that people with oxidative stress–related liver diseases such as
hepatitis or cirrhosis may be at greater risk of toxic side effects
from ingesting high doses of green tea polyphenols. “When a person’s
liver is already under stress, toxic effects tend to become amplified,”
Yang says. Conversely, he notes there are data showing that low or
moderate amounts of green tea have a protective effect against both
toxicity and carcinogenesis in target organs—once again supporting the
adage “everything in moderation, nothing to excess.”