Overview:
Tea
has been cultivated for centuries, beginning in India and China. Today,
tea is the most widely consumed beverage in the world, second only to
water. Hundreds of millions of people drink tea, and studies suggest
that green tea (Camellia sinesis) in particular has many health benefits.
There
are three main varieties of tea -- green, black, and oolong. The
difference is in how the teas are processed. Green tea is made from
unfermented leaves and reportedly contains the highest concentration of
powerful antioxidants called polyphenols. Antioxidants are substances
that fight free radicals -- damaging compounds in the body that change
cells, damage DNA, and even cause cell death. 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.
In traditional
Chinese and Indian medicine, practitioners used green tea as a
stimulant, a diuretic (to help rid the body of excess fluid), an
astringent (to control bleeding and help heal wounds), and to improve
heart health. Other traditional uses of green tea include treating 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 help
treat the following health conditions:
Atherosclerosis
Clinical
studies that look at populations of people indicate that the
antioxidant properties of green tea may help prevent atherosclerosis,
particularly coronary artery disease. Population-based studies are
studies that follow large groups of people over time or studies that
compare groups of people living in different cultures or with different
diets.
Researchers aren't
sure why green tea reduces the risk of heart disease by lowering
cholesterol and triglyceride levels. Studies show that black tea has
similar effects. In fact, researchers estimate that the rate of heart
attack decreases by 11% with consumption of 3 cups of tea per day.
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 that claim.
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
cholesterol from being absorbed in the intestine and also help the body
get rid of cholesterol. 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 both green and black
teas may help 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 know for sure from these
population-based studies whether green tea actually prevents cancer in
people.
Early clinical
studies suggest that the polyphenols in tea, especially green tea, may
play an important role in the prevention of cancer. Researchers also
believe that polyphenols help kill cancerous cells and stop them from
growing.
Bladder cancer.
Only a few clinical studies have examined the relationship between
bladder cancer and drinking tea. 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
people with bladder cancer -- particularly men -- who drank green tea
had a 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 drank the most green tea had the least spread of cancer. It
was especially true in 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 have the cancer come back after they finished
treatment. However, women with late stages of breast cancer had little
or no improvement from drinking green tea.
There
is no clear evidence one way or the other about green tea and breast
cancer prevention. In one very large study, researchers found that
drinking tea, green or any other type, was not associated with a reduced
risk of breast cancer. However, when the researchers broke down the
sample by age, they found that women under the age of 50 who consumed 3
or more cups of tea per day were 37% less likely to develop breast
cancer compared to women who didn't drink tea.
Ovarian cancer.
In a clinical study done with ovarian cancer patients in China,
researchers found that women who drank at least one cup of green tea per
day lived longer with the disease than those who didn' t drink green
tea. In fact, those who drank the most tea, lived the longest. But other
studies found no beneficial effects.
Colorectal cancer.
Clinical studies on the effects of green tea on colon or rectal cancer
have showed conflicting results. Some studies show decreased risk in
those who drink the tea, while others show increased risk. In one study,
women who drank 5 or more cups of green tea per day had a lower risk of
colorectal cancer compared to non-tea-drinkers. There was no protective
effect for men, however. Other studies show that drinking tea regularly
may reduce the risk of colorectal cancer in women. More 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, studies in
people have produced conflicting findings. For example, one large-scale
population-based clinical study found that green tea offered protection
against the development of esophageal cancer, particularly among women.
Another population-based clinical study found 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, more 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 clinical studies have looked
at the link between drinking green tea and lung cancer in people. And
even these studies have been conflicting. One population-based study
found that Okinawan tea -- similar to green tea but partially fermented
-- was associated with lower lung cancer risk, particularly among women.
But a second clinical study found that green tea and black tea
increased the risk of lung cancer. More 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 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 lowering pancreatic cancer risk. More 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
in Southeast China researchers found that the risk of prostate cancer
went down 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.
People who are undergoing chemotherapy should ask their doctors before
drinking green or black tea, or taking tea supplements.
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 anticancer properties that may help prevent the
development 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 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 stomach inflammation
as those who did not drink green tea. However, a clinical study with
more than 26,000 men and women in Japan found no association between
green tea and stomach cancer risk. Some studies even suggest that green
tea may increase the risk of stomach cancer.
More clinical studies are underway to see whether green tea helps reduce the risk of stomach cancer.
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 help
prevent colon cancer, it would also help those with IBD because they are
at higher risk for colon cancer.
Diabetes
Green
tea has been used traditionally to control blood sugar levels. Animal
studies suggest that green tea may help prevent the development of type 1
diabetes and slow the progression once it has developed. In people with
type 1 diabetes, their bodies make little or no insulin, which helps
convert glucose or sugar into energy. Green tea may help regulate
glucose in the body.
A few
small clinical studies have found that taking a green tea extract daily
lowered the hemoglobin A1c level in people 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 liver problems. 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 liver tumors in mice.
Results
from several animal and human studies suggest that one of the
polyphenols in green tea, known as catechin, may help treat viral
hepatitis, an inflammation of the liver. In these studies, catechin was
used by itself in very high amounts. It is not clear whether green tea,
which has a lower concentration of catechins, would have the same
benefits.
10 cups of green
tea a day could cause problems because of the high level of caffeine
consumed. Ask your doctor about the best way to include green tea in
your treatment.
Weight loss
Clinical
studies suggest that green tea extract may boost metabolism and help
burn fat. One study found that the combination of green tea and caffeine
improved weight loss and maintenance in people who were overweight and
moderately obese. Some researchers think that substances in green tea
known as catechins are responsible for the herb's fat-burning effect.
Other uses
One
small study found that drinking green tea helped prevent dental
cavities. More studies need to be done. Green tea may also be useful in
inflammatory diseases, such as arthritis. Research suggests that green
tea may help arthritis by reducing inflammation and slowing the
breakdown of cartilage. Chemicals in green tea may also be help treat
genital warts and prevent symptoms of colds and flu. Studies also show
that drinking green tea is associated with reduced risk of dying from
any cause.
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 2 - 3 times the caffeine
content of green tea.
What's It Made Of?:
Researchers
think the health-giving properties of green tea are mostly due to
polyphenols, chemicals with potent antioxidant properties. In fact, the
antioxidant effects of polyphenols seem 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. They 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. Look for standardized extracts of green tea. There are also liquid
extracts made from the leaves and leaf buds. The average cup of green
tea contains 50 - 150 mg polyphenols (antioxidants). Decaffeinated green
tea products contain concentrated polyphenols. Caffeine-free
supplements are available.
How to Take It:
Pediatric
Green tea has not been studied in children, so it is not recommended for pediatric use.
Adult
Depending
on the brand, 2 - 3 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 or high blood pressure, kidney problems, liver
problems, stomach ulcers, and psychological disorders, particularly
anxiety, should not take green tea. Pregnant and breastfeeding women
should also avoid green tea.
People
with anemia, diabetes, glaucoma, or osteoporosis should ask their
health care provider before drinking green tea or taking an extract.
People
who drink large amounts of caffeine, including caffeine from green tea,
for long periods of time may experience irritability, insomnia, heart
palpitations, 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. If your symptoms are severe, lower your caffeine
intake and see your health care provider.
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.
Beta-lactam -- Green tea may increase the effectiveness of beta-lactam antibiotics by making bacteria less resistant to treatment.
Benzodiazepines
-- Caffeine, including caffeine from green tea, may reduce the sedative
effects of these medications commonly used to treat anxiety, such as
diazepam (Valium) and lorazepam (Ativan).
Beta-blockers, Propranolol, and Metoprolol
-- Caffeine, including caffeine from green tea, may increase blood
pressure in people taking propranolol (Inderal) and metoprolol
(Lopressor, Toprol XL). These medications are used to treat high blood
pressure and heart disease.
Blood Thinning Medications
-- People who take warfarin (Coudamin) should not drink green tea.
Since green tea contains vitamin K, it can make this medication
ineffective. You should not mix green tea and aspirin because they both
prevent blood 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, the same results have
not been found in studies on people. On the other hand, there have been
reports of both green and black tea extracts affecting a gene in
prostate cancer cells that may make them less sensitive to chemotherapy
drugs. For that reason, people should talk to their doctors before
drinking black and green tea or taking tea extracts while undergoing
chemotherapy.
Clozapine (Clozaril) -- The effects of the clozapine may be reduced if taken within 40 minutes after drinking green tea.
Ephedrine -- When taken 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 bipolar disorder. That can make lithium less
effective.
Monoamine Oxidase Inhibitors (MAOIs)
-- Green tea may cause a severe increase in blood pressure, called a
"hypertensive crisis," when taken together with these drugs used to
treat depression. Examples of MAOIs include:
- Isocarboxazid (Marplan)
- Moclobemide (Manerix)
- Phenelzine (Nardil)
- Tranylcypromine (Parnate)
Birth control pills -- Oral contraceptives can prolong the amount of time caffeine stays in the body, which may increase its stimulating effects.
henylpropanolamine
-- A combination of caffeine, including caffeine from green tea, and
phenylpropanolamine, used in many over-the-counter and prescription
cough and cold medications and weight loss products, may 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 urged all manufacturers of this
drug to remove it from the market. Most drugs that contained
phenylpropanolamine have been reformulated without it.
Quinolone antibiotics -- Green tea may makes these medications more effective and also increase the risk of side effects. These medications include:
- Ciprofloxacin (Cipro)
- Enoxacin (Penetrex)
- Grepafloxacin (Raxar)
- Norfloxacin (Chibroxin, Noroxin)
- Sparfloxacin (Zagam)
- Trovafloxacin (Trovan)
Other medications -- Green tea, especially caffeinated green tea, may interact with a number for medications, including:
- Acetaminophen (Tylenol)
- Carbamazepine (Tegretol)
- Dipyridamole (Persatine)
- Estrogen
- Fluvoxamine (Luvox)
- Methotrexate
- Mexiletine (Mexitil)
- Phenobarbital
- Theophylline
- Verapamil (Bosoptin, Calan, Covera- HS, Verelan, Verelan PM)
To be safe, check with your health care provider before drinking or taking green tea if you also take other medications.
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