Historical or
traditional use (may or
may not be supported by scientific
studies)
According to Chinese legend, tea was
discovered accidentally by an emperor 4,000
years ago. Since then, Traditional Chinese Medicine has
recommended green tea for headaches, body
aches and
pains, digestion, depression, immune enhancement,
detoxification, as an energizer, and to prolong
life.
Active
constituents
Green
tea contains volatile oils, vitamins, minerals,
and caffeine, but the primary
constituents of interest are the polyphenols,
particularly the catechin called
epigallocatechin gallate (EGCG). The polyphenols
are believed to be responsible for most of green
tea’s roles in promoting good
health.
Green
tea has been shown to mildly lower total
cholesterol levels and improve the
cholesterol profile (decreasing LDL “bad”
cholesterol and increasing HDL “good”
cholesterol) in most, but not all, studies.
Green tea may also promote cardiovascular
health by making platelets in the blood less
sticky.
Green
tea has also been shown to protect against
damage to LDL (“bad”) cholesterol caused by
oxygen.7 Consumption of green tea
increases antioxidant activity in the blood.
Oxidative damage to LDL can promote atherosclerosis. While
population studies have suggested that
consumption of green tea is associated with
protection against atherosclerosis, the evidence
is still preliminary.
Several animal and test tube studies have
demonstrated an anticancer effect of polyphenols
from green tea. In one of these studies, a
polyphenol called catechin from green tea
effectively inhibited metastasis (uncontrolled
spread) of melanoma (skin cancer) cells. The
polyphenols in green tea have also been
associated with reduced risk of several types of
cancer in humans.However,
some human studies have found no association
between green tea consumption and decreased
cancer risk.
In a
double-blind trial, people with leukoplakia (a
pre-cancerous oral condition) took 3 grams
orally per day of a mixture of whole green tea,
green tea polyphenols, and green tea pigments
orally, and also painted a mixture of the tea on
their lesions three times daily for six months.
As compared to the placebo group, those in the
green tea group had significant decreases in the
pre-cancerous condition.
Compounds in green tea, as well as
black tea, may reduce the risk of dental caries.
Human volunteers rinsing with an alcohol extract
of oolong tea leaves before bed each night for
four days had significantly less plaque
formation, but similar amounts of plaque-causing
bacteria, compared to those with no
treatment.
Green
tea polyphenols have been shown to stimulate the
production of several immune system
cells, and have topical antibacterial
properties—even against the bacteria that cause
dental plaque.
One
study found that intake of 10 cups or more of
green tea per day improved blood test results,
indicating protection against liver damage.
Further studies are needed to determine if
taking green tea helps those with liver
diseases.
Tea
flavonoids given by capsule reduced
fecal odor and favorably altered the gut
bacteria in elderly Japanese with feeding tubes
living in nursing homes. The study was repeated
in bedridden elderly not on feeding tubes, and
green tea was again shown to improve their gut
bacteria. These studies raise the
possibility of using green tea in other settings
where gut bacteria are disturbed, such as after
taking antibiotics. Further studies are
needed to clarify the role of green tea in this
respect, however.
High-tannin tea has been shown to reduce
the need for blood removal from people with iron
overload, or hemochromatosis, in an open study.
The tea had to be taken with meals and without
lemon or milk to be effective. Tea is believed
to help in hemochromatosis by preventing iron
absorption.
In a
double-blind trial, men with precancerous
changes in the prostate received a green tea
extract providing 600 mg of catechins per day or
a placebo for one year. After one year, prostate
cancer had developed in 3.3% of the men
receiving the green tea extract and in 30% of
those given the placebo, a statistically
significant difference. These results suggest
that drinking green tea or taking green tea
catechins may help prevent prostate cancer in
men at high risk of developing the
disease.
There are four case reports in
which certain types of leukemia or lymphoma (low
grade B-cell malignancies) improved after the
patients began taking green tea
extracts.
Are there any side
effects or interactions?
Green
tea is generally free of side effects. The most
common adverse effects reported from consuming
large amounts (several cups per day) of green
tea are insomnia, anxiety, and other symptoms caused
by the caffeine content in the
herb.
An
extract of green tea taken by healthy women with
a meal inhibited the absorption of non-heme iron
(e.g., the form of iron in plant foods) by 26%.
Frequent use of green tea could, in theory,
promote the development of iron deficiency in
susceptible individuals.
There
are several case reports of people developing
liver damage while consuming weight-loss
products that contained concentrated extracts of
green tea. A cause–effect relationship was not
proven, and most of the products contained other
ingredients in addition to green tea extract.
Nevertheless, researchers have cautioned against
the use of large amounts, or concentrated
extracts, of green
tea.