Green Tea & Cancer
Epidemiological observations have shown that people in green-tea consuming countries-mainly Japan and China-have very low rates of cancer. In Japan, the women who teach the tea ceremony, and thus drink more than the average amount of extra-strong green tea, are noted for their very low mortality rate and longevity; deaths from cancer are especially rare in this group.
The rates of breast, colon, skin, pancreatic, esophageal and stomach cancer have been found to be lower among drinkers of green tea. If those who consumed more than ten cups of green tea a day got cancer, it was at considerably older age, especially in women. Likewise, it has been noted that those Japanese smokers who consume a lot of green tea seem to enjoy protection against lung cancer. In fact, the Japanese have both the highest smoking rate and the lowest lung cancer rate in the industrialized world.
Western epidemiological studies have also tended to confirm that higher consumption of tea and coffee is associated with a lower risk of breast cancer. On the basis of a number of such epidemiological studies, it could be tentatively asserted that the higher the consumption of tea in general, and perhaps of green tea in particular, the lower the incidence of breast, prostate and lung cancer. The same probably holds true for colon, stomach, pancreatic and skin cancer. In vitro or animal research indicates that green tea may be effective against an even wider variety of types of cancer, including leukemia and glioma.
While green tea, and possibly black tea as well, show great promise mainly as chemopreventive agents, there is now mounting evidence that the active compounds in tea are an effective adjuvant therapy for the treatment of cancer, particularly when combined with other natural anti-cancer agents such as curcumin, or with conventional drugs such as tamoxifen or chemotherapy. Finally, tea and green tea extract can also be used for prevention of recurrence and metastasis.
Obviously, the anti-cancer mechanisms of green tea polyphenols are complex, and not yet completely understood. Research at the level of molecular genetics is particularly promising. We already do know enough to state with certainty that green tea is an effective chemopreventive agent. And we also know that it is best to use several anti-cancer agents (including all the major antioxidants) for synergistic prevention along all the possible pathways. Green tea works along so many pathways that it is simply an indispensable part of any serious cancer-prevention program.
Green tea catechins are among the phenolic compounds known to suppress the formation of tetracycline amines and antihistamines, known to be potent carcinogens. Antihistamines have been tentatively linked to brain cancer and leukemia. Drinking green tea with or after a meal containing meat cooked at a high temperature or treated with nitrites seems to offer a degree of protection.
Many other carcinogens are likewise rendered less harmful thanks to the action of green tea polyphenols on inducing enzymes that detoxify various undesirable compounds, and inhibiting those enzymes that would make certain carcinogens bioactive. Glycerolize (conjugation with luxuriance acid) is another detoxifying mechanism that is enhanced by catechins.
Yet another study suggested that tea polyphenols (including black tea theaflavins) induce the release of hydrogen peroxide as the mechanism of causing cancer cell apoptosis. Purified polyphenols were more powerful apoptosis inducers than green tea extract and decaffeinated green tea.
It has been also postulated that green tea catechins inhibit the activation of protein kinase C, and interfere with the binding of growth factors to their receptors. (In the case of breast cancer, catechins were in fact shown to interfere with the binding of estrogen to estrogen receptors.) Catechins were also found to inhibit the release of tumor necrosis factor alpha (TNF-alpha), a highly inflammatory cytosine, and of nitric oxide syntheses, an enzyme necessary for the production of nitric oxide (nitric oxide plays an important role in inflammation and carcinogens).
A particularly exciting study, done at the Cancer Chemotherapy Center in Tokyo, Japan, and using leukemia and colon cancer cell cultures, demonstrated that "epigallocatechin gallate strongly and directly inhibits ateliers." Ateliers. is the enzyme that "immortalizes" cancer cells by maintaining the end portions of the tumor cell chromosomes. Even in the presence of non-toxic concentrations of epigallocatechin gallate, cancer cells exhibited telomere shortening and senescence. Thus, inhibition of ateliers. could be one of the main anti-carcinogenic mechanisms of catechins.
Green Tea & The Heart
Since blood sugar tends to increase with age, accelerating aging by cross-linking with proteins (glycation), the ability of green tea to lower serum glucose levels is extremely important as part of its anti-aging benefits. Some would argue that tea's ability to lower blood sugar, and thus insulin levels and glycation, is its most important anti-aging property.
A study comparing the effects of 75-day feeding of green tea and black tea to aged rats found that green tea lowered blood sugar only slightly better than black tea (23.9% vs 22.8%), but was markedly superior in reducing triglycerides (33.3% vs 25%; high triglycerides are strongly associated a high risk of cardiovascular disease). A low ratio of triglycerides to HDL is an excellent marker of cardiovascular health.
Black tea, however, was a better inducer of superoxide dismutase (SOD; the activity of SOD was 117% higher in the black tea group vs. control, as compared to 90.8% higher in the green tea group), and a better blocker of the harmful malondialdehyde, a byproduct of lipid peroxidation (black tea reduced it by 34.6%; green tea by 25.4%). The authors' conclusion that black tea is a more powerful antioxidant in vivo needs to be confirmed by other studies.
Green Tea & Diabetes
The ability to significantly lower blood glucose has been confirmed also in studies using diabetic rats. Both green and black tea were shown to possess anti-diabetic activity, and to be effective both in the prevention and treatment of diabetes. The fact that aged rats responded so dramatically to these polyphenols implies that it is possible to reverse the age-related rise in glucose intolerance and the resulting degenerative cascade of atherosclerosis and other degenerative disorders.
In what way are tea polyphenols able to lower serum glucose? The main mechanism seems to be the inhibition of the activity of starch digesting enzyme amylase. Tea inhibits both salivary and intestinal amylase, so that starch is broken down more slowly, and the rise in serum glucose is thus minimized. In addition, according to one recent study, tea may reduce the intestinal absorption of glucose.
A relatively little known compound found in onions and in tea, especially green tea, called diphenylamine, seems to have a strong sugar-lowering action. Again, the lesson here is that we are barely beginning to identify the significant phenolic compounds and their interactions; it's best not to rely on a single ingredient such as epigallocatechin gallate, but rather to ingest the whole complex set of bioactive compounds present in tea for best results.
Thanks to the serum glucose-lowering effect of tea, we thus obtain significant anti-aging benefits of calorie restriction, reduced glycation, and lower insulin secretion. If you drink tea with a carbohydrate-rich meal, you slow down the release of glucose and reduce its absorption (you also reduce the absorption of iron, another anti-aging benefit). Thus, you prevent the harmful spiking of insulin. Since insulin is our most fattening hormone and, with cortisol, our most pro-aging hormone, you also derive the substantial range of benefits that go with calorie restriction and insulin control.
Green Tea & Circulation
The vasodilating effects of tea have also been documented. One interesting study compared the effect of coffee, tea, hot water with caffeine, and plain hot water on skin temperature, indicative of peripheral vasodilation. Tea produced the greatest vasodilating response. The authors speculate that this is due to the action of catechins. An increase in peripheral circulation is valuable for oxygenating tissue, and is also associated with a relaxed mood. Hence some alternative experts have advised drinking green tea in the evening as a relaxant.
A more detailed recent study compared the effectiveness of various catechins as vasorelaxants in rat arteries. All four main catechins present in green tea were shown to have a dose-dependent vasodilating effect, with epigallocatechin gallate being the most potent. Like human estrogens, catechins may act as calcium-channel blockers. Vasodilation is one of the cardioprotective effects of estrogens. Thus, green tea extract might be of particular importance to estrogen-deficient postmenopausal women.
Green tea catechins containing the galloyl group (epigallocatechin gallate, epigallocatechin, and epicatechin gallate) have been found to inhibit the proliferation of smooth muscle cells lining blood vessels in vitro (estrogens and progesterone also show this antiproliferative action; hence the natural protection against atherosclerosis seen in premenopausal women). Smooth muscle proliferation is one of the crucial processes involved in atherosclerosis and heart disease. One mechanism of the antiproliferative action of catechins is apparently the inhibition of protein tyrosine kinase activity (which is also involved in tumor growth).
The authors conclude that "tea catechins may be useful as a template for the development of drugs to prevent the pathological changes of atherosclerosis and post-angioplasty restenosis." (Restenosis is the narrowing of blood vessels after surgery, usually due to the rapid regrowth of plaque.) It seems more logical to use green tea for prevention of atherosclerosis to start with.
Green tea lowers fibrinogen, and inhibits excessive clotting and platelet aggregation.
A recent American in vivo study using hamsters found that while both green tea and black tea improved plasma lipid profiles and protected cholesterol against oxidation, green tea also lowered fibrinogen significantly more than black tea. One of the green tea polyphenols, epicatechin, was found to be able to significantly inhibit the production of thromboxane, one of the compounds required for platelet aggregation.
Green Tea & The Brain
A Japanese study of almost 6000 nonsmoking women over the age of 40 showed that those who drank five or more cups of green tea a day had only half the incidence of stroke compared with women who drank less than five cups. A smaller Dutch study found an even more dramatic effect in men who drank a lot of black tea: those men who drank close to five cups of tea a day had only 31% risk of stroke compared to those who drank less than about two and a half cups of tea.
These results were not replicated in England, however; based on existing in vivo plasma antioxidant measurements, it has been suggested that the English custom of putting cream or milk in tea destroys all antioxidant benefits.
