tea and cancer prevention
programmed death of cancer cells
modulation of cancer chemotherapy
tea and health diseases
tea and stroke
tea and inflammation and arthritis
Tea is an inexpensive, and next to water, the most
widely-consumed beverage worldwide. Modern science, especially of the last
fifteen years, is beginning to confirm that tea has health-promoting potential,
a concept advocated by Asian traditional for centuries. In fact, the medicinal
value of tea has been recognized for a long time in as much as tea was used as
an elixir by ancient Chinese. When tea was introduced to Europe, people of
Holland considered it a divine herb, a panacea.
Tea is derived from the plant Camellia sinensis, an evergreen
shrub of the theaceae family. The tea plant Camelia sinensis was originated in
Southeast Asia and is presently cultivated in over 30 countries around the
world. Of the total tea produced and consumed in the world, 78% is black tea, 2%
is oolong tea. Black tea is mainly consumed in western countries and some Asian
countries including India. Green tea is consumed mainly in Chine, Japan, India,
and a few countries in North Africa and Middle East. Oolong tea production and
consumption is confined to southeastern China and Taiwan. Both green and black
teas contain main polyphenolic antiooxidants. Polyphenols, in chemical term, are
members of the flavonoid family. They are catechins made of several ring-like
structures.
Each of these structures has chemicals attached to it called
phenol groups, hence the name polyphenols (poly means "many").
Antioxidants are the chemicals endowed with the ability to counteract the
adverse effects of oxidants by scavenging them or by interfering with their
production. The oxidant species are constantly produced within the living system
and are responsible for many diseases. By trapping these oxidant radicals, the
antioxidants reduce their effective bio-availability. This represents a highly
efficient built-in preventive machinery within a living cell. The water-extract
of the dry tea leaves, as is consumed by humans, contains many compounds
including a mixture of polyphenols. Green tea contains flavanols, flavandiols,
flavonoids, and phenolic acids, which are antioxidant in nature. These compounds
account for up to 30% of the dry weight of green tea leaves. Most of the
polyphenols present in green tea are flavanols, commonly known as catechins.
The major catechins present in green tea are:
- epicatechin (EC)
- epicatechin-3-gallate (ECG)
- epigallocatechin (EGC), and
- epigallocatechin-3-gallate (EGCG)
The major polyphenolic antioxidants present in black tea are i) theaflavins
such as theaflavin-3-gallate, theaflavin-3'-gallae, theaflavin-3,3'-digallate,
ii) thearubigens and their derivatives such as theafulvins and theacitrins, and
iii) tannins such as pentagalloylglucose and camelliatanin A. In black tea, in
addition to its own specific poly phenolic antioxidants, some green tea-specific
polyphenolic antioxidnts are also present. Their quantity, however, depends on
the fermentation process employed during the manufacture.
Extensive laboratory research and he epidemiological findings
have revealed that the polyphenolic antioxidant of tea are far superior than the
antioxidants in any other edible produce known to mankind. For example, it has
been shown that the trolox equivalent antioxidant capacity of green tea (1000
ppm) and black tea (1000 ppm) is between 3.5-3.8 and of vitamin C and vitamin E
is 1.0. Because most beneficial or harmful physiological responses are regulated
by the balance between prooxidants and antioxidants, it is no wonder that tea
which is rich in antioxidants possesses numerous health-promoting effects.
Indeed, the experimental findings indicate that it can protect against the risk
of many diseases. The focus of this presentation is consumption of tea in the
prevention of cancer, heart disease, stroke and arthritis.
Tea and Cancer
Prevention
Abundant experimental and epidemiological data accumulated
mainly in the last decade, from several laboratories worldwide, provided
convincing evidence that polyphenolic antioxidants present in green as well as
in black tea are capable of affording protection against cancer risk in a
variety of animal tumor bioassay systems. Initial studies in this area of
research reported from our laboratory showed that the consumption of tea and its
polyphenolic fraction afforded protection against chemical carcinogen - or
ultraviolet radiation - induced skin cancer in mouse model. This work laid the
foundation for many subsequent studies from several laboratories around the
world.
Collectively, these studies show that tea
consumption affords protection against chemical carcinogen-induced lung,
forestomach, esophagus, duodenum, pancreas, liver, breast and colon
carcinogensis in mice, rats and hamsters. These laboratory findings suggests
that tea consumption by humans may result in prevention of some cancer types.
Recent studies from our laboratory have shown that
green as well as black tea may also prevent prostate cancer. These laboratory
findings may explain geographical difference in prostate cancer incidence. It is
important to note that the Japanese and Chinese populations which traditionally
consume a diet having low fat and several cups of tea, have one of the lowest
rates of prostate cancer in the World. Furthermore, the incidence of prostate
cancer in India, a population that consumes tea regularly is lower than the
Western countries.
Our recent study conducted in mice has demonstrated
that intraperitoneal injections of EGCG, known to present in green as well as
black ea, results in a significant reduction in grown of human prostate cancer
cells in nude mice. In this study, prostate tumors were developed by implanting
human prostate cancer cells via injections on the backs of athymic nude mice. It
was found that daily injections of 1 mg of EGCG resulted in a significant
reduction in prostate tumor growth as well as in the levels of prostate-specific
antigen, which is regarded as a gold standard for prostate cancer diagnosis and
to assess prostate cancer body burden in humans.
Tea Antioxidants and a Programmed Death of Cancer Cells
A study from a laboratory has shown that tea antioxidant EGCG
results in an induction of a programmed cell death technically known as
'apoptosis' of many human cancer cells including human prostate cancer cells.
Interestingly,
under similar conditions, EGCG did not kill the normal cells. This
observation was subsequently varified by many laboratories worldwide. In
subsequent studies, black tea antioxidants were also found to cause apoptotic
death of cancer cells. These studis suggest that tea polyphenols may have
advantage in killing cancer cells without harming normal sells.
A recent study has shown that green tea can also modulate
cancer-chemotherapeutic drugs in such a way that their efficacy is increased. In
this study, the oral administration of green tea was found to enhance the
tumor-inhibitory effects of doxorubicin, on Ehrlich ascites carcinomas implanted
in mice.
Green tea treatment resulted in an increased
availability of doxorubicin in tumors, but not in normal tissue. In another
study, theanine, an amino acid present both in black and green tea, was found to
enhance the inhibition of liver metastasis induced by doxorubicin.
Furthermore, in vitro experiments indicated that theanine increased the
intracellular concentration of doxorubicin in cancer cells. If verified in the
human population, these observations may have relevance to cancer chemotherapy.
Tea consumption may lead to the prevention of heart
disease. Tea has been shown to strengthen blood vessels and to decrease the
cholesterol level in the bloodstream. Epidemiological studies has shown the
preventive effect of green tea consumption against atherosclerosis and coronary
heart disease. In some studies, tea consumption has been shown to afford
protection against high cholesterol levels and high blood pressure. More
research in this area is required. Studies have also shown the preventive effect
of green tea against atherosclerosis in experimental animals. In a Japanese
study, the atherogenic index that is a measure for the risk of atherosclerosis
was found to be significantly lower in rats fed with a high-cholesterol diet
supplemented with black tea extract than in the ones not given the extract. In
this study, black tea was also found to inhibit the proliferation of smooth
muscle cells involved in the development and progression of atherosclerosis.
This study suggested that black tea possesses an anti-atherosclerotic action.
In hamster model, both black as well as green tea
has been shown to decrease the risk for heart disease. In this study, the
hamsters were fed a normal or high cholesterol diet and green or black tea. Tea
consumption was found to result in a significant improvement in the plasma lipid
profile and a decrease in the oxidation of lipids. This study suggests that both
black and green teas improve the risk factors for heart disease.
Some studies have shown that tea consumption ma
reduce the risk of stroke. In a study, the effect of EGCG on the life span and
hypertensive lesions in the stroke-prone spontaneously hypertensive rat (SHRSP)
was compared with that of persimmon tannin. In this study, it was found that a
long-term administration of either 0.5% EGCG or 0.5% persimmon tannin to SHRSP
inhibited the incidence of stroke and prolonged the life span. These results
suggested that green tea may prevent incidence of stroke. Similarly, in another
study among 5910 non-drinking and non-smoking women, it was found that the
incidence of stroke and cerebral hemorrhage during a 4-year follow-up of the
study population was twice or more higher in those who took less tea 9less than
5 cups a day) than in those who took more tea (greater than or equal to 5 cups
daily). These studies show a possible connection between of tea-drinking and the
prevention of stroke.
I an epidemiological study, where a cohort of 552
men aged 50 to 69 years was examined in 1970 and followed up for 15 years, it
was found that the habitual intake of black tea may protect humans against stroke.
In several studies from my laboratory and
elsewhere, the polyphenolic fraction prepared from both green and black teas
have been shown to prevent against inflammation caused by chemicals and
ultraviolet radiation. In a recent landmark study, we showed that mice drinking
green tea developed lower incidence and milder form of collagen-induced
arthritis, which is similar to that of rheumatoid arthritis in humans.
Importantly, the biological markers of arthritis progression were negatively
regulated in the animals consuming green tea. This study concluded that the
antioxidants present in green tea might retard the onset and incidence of
inflammatory diseases including arthritis. Data from a study recently completed
in my laboratory examining simultaneously the effects of black and green tea
consumption on arthritis incidence and disease severity in mice will be
presented for the first time in this meeting.
Source: Abstract of talk at
International Millennium Tea Convention
New Delhi, India
by Hasan Kukhtar, PhD
Professor and research Director
Department of Dermatology
Case Western Reserve University
Cleveland, OH-44106, USA
