General Description
Tea comes from a rounded, evergreen shrub that grows between 4 to
6 feet tall. Tea plants thrive in tropical and semi-tropical climates,
such as those of India, China, and Argentina. Small, white flowers
are present in autumn, but are often hidden by foliage. Tea leaves
are elliptical, serrated, and about 4 inches long. Both green and
black tea come from the same plant. After picking, leaves that are
allowed to oxidize produce black tea. Many of the polyphenols present
in green tea are absent in black tea because of this oxidative process.
In order to produce green tea, the leaves are steamed. This keeps
the polyphenols from turning into compounds with much less activity.
Health applications
History and traditional use
According to Chinese legend, nearly 5,000 years ago in 2737 B.C.,
emperor Shen Nung discovered tea accidentally, when dried leaves
from a nearby bush fell into water he was boiling. The emperor drank
some of the tea-infused water and enjoyed it. Green tea has a long
history in Asia. As a traditional medicine, green tea has been used
for depression, body aches, headaches, immune enhancement, and as
an energizer. In the United States green tea is increasingly popular,
in part because of its potential as a health-enhancing drink.
Chemical constituents
The main constituents of green tea are polyphenols, carotenoids,
polysaccharides, chlorophyll, vitamins E and C, manganese, zinc,
and potassium. The polyphenols are generally believed to be the
most beneficial compounds in green tea, thanks primarily to their
antioxidant activity. The most active polyphenols in green tea
are catechins, in particular Epigallocatechin Gallate (EGCG).1
Green tea supplements are often standardized to contain
specific catechin concentrations.
Antibacterial
Research indicates that green tea extracts may have antibacterial
properties.2-6 For example, studies have shown
that green tea extracts inhibit the growth of the bacteria that
cause gingivitis, dental plaque,7 and various
diarrheal diseases.s,3,5 It is believed that
the polyphenols found in the extracts are largely responsible
for these effects.2
Cancer prevention
Many herbal enthusiasts believe that green tea consumption aids
in the prevention of cancer. Several investigations have been
done on the anticarcinogenic properties of green tea extracts,
however, studies thus far have been inconclusive. Some compounds
found in green tea extracts, particularly EGCG 1,8,9,
show promise in preventing various cancers in animal models, however,
additional human-based studies are needed before any significant
conclusion can be made.10,11,12
Dosage and Toxicity
Over-consumption of green tea can cause mild insomnia and irritability,
due to its natural caffeine content. According to the US Food
and Drug Administration, an 8 ounce serving of green tea has about
15 milligrams of caffeine; in contrast, an 8 ounce serving of
coffee has between 80-135 milligrams of caffeine. Tea infusions
are regularly recommended for those attempting to wean themselves
from coffee because tea has a moderate amount of caffeine. People
seeking the antioxidant benefits of green tea without the caffeine
often take supplemental extracts, which are usually standardized
to provide high polyphenol concentrations. Many herbalists recommend
consuming the polyphenol equivalent of six cups of green tea (480-640
mg catechins) per day, and there are no known side effects at
this dosage. |
