|
Green
Tea Information:
Archeological evidence suggests
that people consumed tea leaves steeped in boiling water as
many as 500,000 years ago. Botanical evidence indicates that
India and China were among the first countries to cultivate
tea. Today, hundreds of millions of people drink tea around
the world, and studies are now suggesting that green tea
(Camellia sinesis) in particular has many health benefits.
There are four main varieties of tea – white, green, black,
and oolong. The difference between the teas is in their
processing. White tea is picked before the leaves of the plant
fully open. Green tea is made from unfermented leaves and
reportedly contains the highest concentration of powerful
antioxidants called polyphenols. Antioxidants are substances
that scavenge free radicals — damaging compounds in the body
that alter cells, tamper with DNA (genetic material), and even
cause cell death. Free radicals occur naturally in the body,
but environmental toxins (including ultraviolet rays from the
sun, radiation, cigarette smoke, and air pollution) also give
rise to these damaging particles. Many scientists believe that
free radicals contribute to the aging process as well as the
development of a number of health problems, including cancer
and heart disease. Antioxidants such as polyphenols in green
tea can neutralize free radicals and may reduce or even help
prevent some of the damage they cause.
Green tea has been consumed throughout the ages in India,
China, Japan, and Thailand. In traditional Chinese and Indian
medicine, practitioners used green tea as a stimulant,
diuretic (to promote the excretion of urine), astringent (to
control bleeding and help heal wounds), and to improve heart
health. Other traditional uses of green tea include treating
flatulence (gas), regulating body temperature and blood sugar,
promoting digestion, and improving mental processes.
Green tea has been extensively studied in people, animals, and
laboratory experiments. Results from these studies suggest
that green tea may be useful for the following health
conditions:
Atherosclerosis
Population-based clinical studies indicate that the
antioxidant properties of green tea may help prevent
atherosclerosis, particularly coronary artery disease.
(Population-based studies means studies that follow large
groups of people over time or studies that are comparing
groups of people living in different cultures or with
different dietary habits.) In May 2006, however, the U.S. Food
and Drug Administration (FDA) rejected a petition from
teamakers to allow tea labels to claim that green tea reduces
the risk of heart disease. The FDA concluded that there is no
credible evidence to support qualified health claims for green
tea or green tea extract reducing the risk of heart disease.
High cholesterol
Research shows that green tea lowers total cholesterol and
raises HDL (”good”) cholesterol in both animals and people.
One population-based clinical study found that men who drink
green tea are more likely to have lower total cholesterol than
those who do not drink green tea. Results from one animal
study suggest that polyphenols in green tea may block the
intestinal absorption of cholesterol and promote its excretion
from the body. In another small study of male smokers,
researchers found that green tea significantly reduced blood
levels of harmful LDL cholesterol.
Cancer
Several population-based clinical studies have shown that
green tea helps protect against cancer. For example, cancer
rates tend to be low in countries such as Japan where people
regularly consume green tea. However, it is not possible to
determine from these population-based studies whether green
tea actually prevents cancer in people. Emerging clinical
studies suggest that the polyphenols in green tea may play an
important role in the prevention of cancer. Researchers also
believe that polyphenols help kill cancerous cells and stop
their progression.
Bladder cancer.
Only a few clinical studies have examined the relationship
between bladder cancer and green tea consumption. In one study
that compared people with and without bladder cancer,
researchers found that women who drank black tea and powdered
green tea were less likely to develop bladder cancer. A
follow-up clinical study by the same group of researchers
revealed that bladder cancer patients (particularly men) who
drank green tea had a substantially better 5-year survival
rate than those who did not.
Breast cancer.
Clinical studies in animals and test tubes suggest that
polyphenols in green tea inhibit the growth of breast cancer
cells. In one study of 472 women with various stages of breast
cancer, researchers found that women who consumed the most
green tea experienced the least spread of cancer (particularly
premenopausal women in the early stages of breast cancer).
They also found that women with early stages of the disease
who drank at least 5 cups of tea every day before being
diagnosed with cancer were less likely to suffer recurrences
of the disease after completion of treatment. However, women
with late stages of breast cancer experienced little or no
improvement from drinking green tea. In terms of breast cancer
prevention, the studies are inconclusive. In one very large
clinical study from Japan, researchers found that drinking
green tea was not associated with a reduced risk of breast
cancer.
Ovarian cancer.
In a clinical study conducted on ovarian cancer patients in
China, researchers found that women who drank at least one cup
of green tea per day survived longer with the disease than
those who didn’t drink green tea. In fact, those who drank the
most tea, lived the longest.
Colorectal cancer.
Clinical studies on the effects of green tea on colon or
rectal cancer have produced conflicting results. Some clinical
studies show decreased risk in those who drink the tea, while
others show increased risk. Further research is needed before
researchers can recommend green tea for the prevention of
colorectal cancer.
Esophageal cancer.
Studies in laboratory animals have found that green tea
polyphenols inhibit the growth of esophageal cancer cells.
However, clinical studies in people have produced conflicting
findings. For example, one large-scale population-based
clinical study found that green tea offered significant
protection against the development of esophageal cancer
(particularly among women). Another population-based clinical
study revealed just the opposite — green tea consumption was
associated with an increased risk of esophageal cancer. In
fact, the stronger and hotter the tea, the greater the risk.
Given these conflicting results, further research is needed
before scientists can recommend green tea for the prevention
of esophageal cancer.
Lung cancer.
While green tea polyphenols have been shown to inhibit the
growth of human lung cancer cells in test tubes, few clinicial
studies have investigated the link between green tea
consumption and lung cancer in people and even these studies
have been conflicting. One population-based clinical study
found that Okinawan tea (similar to green tea but partially
fermented) was associated with decreased lung cancer risk,
particularly among women. A second clinical study revealed
that green tea and black tea significantly increased the risk
of lung cancer. As with colon and esophageal cancers, further
clinical studies are needed before researchers can draw any
conclusions about green tea and lung cancer.
Pancreatic cancer.
In one large-scale clinical study researchers compared green
tea drinkers with non-drinkers and found that those who drank
the most tea were significantly less likely to develop
pancreatic cancer. This was particularly true for women —
those who drank the most green tea were half as likely to
develop pancreatic cancer as those who drank less tea. Men who
drank the most tea were 37% less likely to develop pancreatic
cancer. However, it is not clear from this population-based
study whether green tea is solely responsible for reducing
pancreatic cancer risk. Further studies in animals and people
are needed before researchers can recommend green tea for the
prevention of pancreatic cancer.
Prostate cancer.
Laboratory studies have found that green tea extracts prevent
the growth of prostate cancer cells in test tubes. In a large
clinical study conducted in Southeast China researchers found
that the risk of prostate cancer declined with increasing
frequency, duration and quantity of green tea consumption.
However, both green and black tea extracts also stimulated
genes that cause cells to be less sensitive to chemotherapy
drugs. Given this potential interaction, people should not
drink black and green tea (as well as extracts of these teas)
while receiving chemotherapy.
Skin cancer.
The main polyphenol in green tea is epigallocatechin gallate (EGCG).
Scientific studies suggest that EGCG and green tea polyphenols
have anti-inflammatory and anti-cancer properties that may
help prevent the onset and growth of skin tumors.
Stomach cancer.
Laboratory studies have found that green tea polyphenols
inhibit the growth of stomach cancer cells in test tubes, but
clinical studies in people have been less conclusive. In two
studies that compared green tea drinkers with non-drinkers,
researchers found that people who drank tea were about half as
likely to develop stomach cancer and gastritis (inflammation
of the stomach) as those who did not drink green tea. However,
a clinicial study including more than 26,000 men and women in
Japan found no association between green tea consumption and
stomach cancer risk. Some clinicial studies even suggest that
green tea may increase the risk of stomach cancer.
Further clinicial studies are underway to determine whether
green tea helps reduce the risk of stomach cancer. Although
green tea is considered safe for people at risk for stomach
cancer, it is too soon to tell whether green tea reduces the
likelihood of developing this disease.
Inflammatory Bowel Disease (IBD)
Green tea may help reduce inflammation associated with Crohn’s
disease and ulcerative colitis, the two types of IBD. If green
tea proves to be helpful for preventing colon cancer, this
would be an added benefit for those with IBD because they are
at risk for colon cancer.
Diabetes
Green tea has been used traditionally to control blood sugar
in the body. Animal studies suggest that green tea may help
prevent the development of type 1 diabetes and slow the
progression once it has developed. People with type 1 diabetes
produce little or no insulin, a hormone that converts glucose
(sugar), starches, and other foods into energy needed for
daily life. Green tea may help regulate glucose in the body.
A few small clinical studies have found that daily
supplementation of the diet with green tea-extract powder
lowered the hemoglobin A1c level in individuals with
borderline diabetes.
Liver disease
Population-based clinical studies have shown that men who
drink more than 10 cups of green tea per day are less likely
to develop disorders of the liver. Green tea also seems to
protect the liver from the damaging effects of toxic
substances such as alcohol. Animal studies have shown that
green tea helps protect against the development of liver
tumors in mice.
Results from several animal and human studies suggest that one
of the polyphenols present in green tea, known as catechin,
may help treat viral hepatitis (inflammation of the liver from
a virus). In these studies, catechin was isolated from green
tea and used in very high concentrations. It is not clear
whether green tea (which contains a lower concentration of
catechins) confers these same benefits to people with
hepatitis.
Weight loss
Clinical studies suggest that green tea extract may boost
metabolism and help burn fat. One study confirmed that the
combination of green tea and caffeine improved weight loss and
maintenance in overweight and moderately obese individuals.
Some researchers speculate that substances in green tea known
as polyphenols, specifically the catechins, are responsible
for the herb’s fat-burning effect.
Other uses
Drinking green tea has been found effective in a small
clinical study for dental caries, or tooth decay. More studies
need to be performed. Green tea may also be useful in
inflammatory diseases, such as arthritis. Research indicates
that green tea may benefit arthritis by reducing inflammation
and slowing cartilage breakdown. Chemicals found in green tea
may also be effective in treating genital warts and preventing
symptoms of colds and influenza.
Plant Description:
Green, black, and oolong tea are all derived from the leaves
of the Camellia sinensis plant. Originally cultivated in East
Asia, this plant grows as large as a shrub or tree. Today,
Camellia sinensis grows throughout Asia and parts of the
Middle East and Africa.
People in Asian countries more commonly consume green and
oolong tea while black tea is most popular in the United
States. Green tea is prepared from unfermented leaves, the
leaves of oolong tea are partially fermented, and black tea is
fully fermented. The more the leaves are fermented, the lower
the polyphenol content (See: “What’s It Made Of?”) and the
higher the caffeine content. Green tea has the highest
polyphenol content while black tea has roughly two to three
times the caffeine content of green tea.
What’s It Made Of?:
The healthful properties of green tea are largely attributed
to polyphenols, chemicals with potent antioxidant properties.
In fact, the antioxidant effects of polyphenols appear to be
greater than vitamin C. The polyphenols in green tea also give
it a somewhat bitter flavor.
Polyphenols contained in teas are classified as catechins.
Green tea contains six primary catechin compounds: catechin,
gallaogatechin, epicatechin, epigallocatechin, epicatechin
gallate, and apigallocatechin gallate (also known as EGCG).
EGCG is the most studied polyphenol component in green tea and
the most active.
Green tea also contains alkaloids including caffeine,
theobromine, and theophylline. These alkaloids provide green
tea’s stimulant effects. L-theanine, an amino acid compound
found in green tea, has been studied for its calming effects
on the nervous system.
Available Forms:
Most green tea dietary supplements are sold as dried leaf tea
in capsule form. Standardized extracts of green tea are
preferred. There are also liquid extracts made from the leaves
and leaf buds. The average cup of green tea contains between
50 – 150 mg polyphenols (antioxidants). Decaffeinated green
tea products contain concentrated polyphenols. Caffeine-free
supplements are available.
How to Take It:
Pediatric
There are no known scientific reports on the pediatric use of
green tea, so it is not recommended for children.
Adult
Depending on the brand, two to three cups of green tea per day
(for a total of 240 – 320 mg polyphenols) or 100 – 750 mg per
day of standardized green tea extract is recommended.
Caffeine-free products are available and recommended.
Precautions:
The use of herbs is a time-honored approach to strengthening
the body and treating disease. However, herbs contain active
substances that can trigger side effects and interact with
other herbs, supplements, or medications. For these reasons,
people should take herbs with care, under the supervision of a
practitioner knowledgeable in the field of botanical medicine.
People with heart problems, kidney disorders, stomach ulcers,
and psychological disorders (particularly anxiety) should not
take green tea. Pregnant and breast-feeding women should also
avoid green tea.
People who drink excessive amounts of caffeine (including
caffeine from green tea) for prolonged periods of time may
experience irritability, insomnia, heart palpitation, and
dizziness. Caffeine overdose can cause nausea, vomiting,
diarrhea, headaches, and loss of appetite. If you are drinking
a lot of tea and start to vomit or have abdominal spasms, you
may have caffeine poisoning. Lower your caffeine intake and
see your health care provider if your symptoms are severe.
Possible Interactions:
If you are being treated with any of the following
medications, you should not drink green tea or take green tea
extract without first talking to your health care provider:
Adenosine — Green tea may inhibit the actions of adenosine, a
medication given in the hospital for an irregular (and usually
unstable) heart rhythm.
Antibiotics, Beta-lactam — Green tea may increase the
effectiveness of beta-lactam antibiotics by reducing bacterial
resistance to treatment.
Benzodiazepines — Caffeine (including caffeine from green tea)
has been shown to reduce the sedative effects of
benzodiazepines (medications commonly used to treat anxiety,
such as diazepam and lorazepam).
Beta-blockers, Propranolol, and Metoprolol — Caffeine
(including caffeine from green tea) may increase blood
pressure in people taking propranolol and metoprolol
(medications used to treat high blood pressure and heart
disease).
Blood Thinning Medications (Including Aspirin) — People who
take warfarin, a blood thinning medication, should not drink
green tea. Since green tea contains vitamin K, it can make
warfarin ineffective. Meanwhile, you should not mix green tea
and aspirin because they both prevent platelets from clotting.
Using the two together may increase your risk of bleeding.
Chemotherapy — The combination of green tea and chemotherapy
medications, specifically doxorubicin and tamoxifen, increased
the effectiveness of these medications in laboratory tests.
However, these results have not yet been demonstrated in
studies on people. On the other hand, there have been reports
of both green and black tea extracts stimulating a gene in
prostate cancer cells that may cause them to be less sensitive
to chemotherapy drugs. Given this potential interaction,
people should not drink black and green tea (as well as
extracts of these teas) while receiving chemotherapy for
prostate cancer in particular.
Clozapine — The anti-psychotic effects of the medication
clozapine may be reduced if taken fewer than 40 minutes after
drinking green tea.
Ephedrine — When taken together with ephedrine, green tea may
cause agitation, tremors, insomnia, and weight loss.
Lithium — Green tea has been shown to reduce blood levels of
lithium (a medication used to treat manic/depression).
Monoamine Oxidase Inhibitors (MAOIs) — Green tea may cause a
severe increase in blood pressure (called a “hypertensive
crisis”) when taken together with MAOIs, which are used to
treat depression. Examples of MAOIs include phenelzine and
tranylcypromine.
Oral Contraceptives — Oral contraceptives can prolong the
amount of time caffeine stays in the body and may increase its
stimulating effects.
Phenylpropanolamine — A combination of caffeine (including
caffeine from green tea) and phenylpropanolamine (an
ingredient used in many over-the-counter and prescription
cough and cold medications and weight loss products) can cause
mania and a severe increase in blood pressure. The FDA issued
a public health advisory in November 2000 to warn people of
the risk of bleeding in the brain from use of this medication
and has strongly urged all manufacturers of this drug to
remove it from the market.
|
