Until a few years ago, tea might have seemed more appropriately addressed in a cookbook than in a book about herbs. This beverage is probably the most frequently consumed in the world after plain water.
To be sure, it has the requisite ancient history, with its use going back more than three thousand years in China.
Along with many other Chinese botanical medicines, it was discussed in The Herbal Classic of the Divine Plowman.
A few centuries later, Chinese Buddhists espoused tea because it could help the devout stay awake during long meditations, and Taoists adopted it because it was believed to promote health and longevity. But in the West, tea was more often thought of as simply a soothing hot drink, and it was far more likely to be black tea than green.
Recent research has shown that the Buddhists and the Taoists may have been right. Green tea in particular appears to have certain health benefits.
Green and black tea come from the same plant, but the fermentation process required to turn tea black appears to alter the chemistry of the leaf somewhat. Oolong tea is produced by means of a partial fermentation process.
Although green, oolong, and black are the principal categories for tea, the Chinese classify it in a much more complex fashion, with as many as 330 kinds of tea (cha) recognized. In a typical store offering tea in China, the inventory might include one hundred types of cha.
The methylxanthine alkaloids caffeine, theophylline, and theobromine comprise between 1 and 5 percent of tea.
These compounds have similar but not identical actions; caffeine is usually the dominant one.
Depending on the variety of tea and the way it was prepared, a cup (a proper six-ounce teacup) may contain from 10 to 50 mg of caffeine.
Low doses of caffeine may actually slow heart rate slightly, while at higher doses this stimulant can speed heart rate or even contribute to mild rhythm abnormalities.
Green tea, like black tea, is rich in tannins. Tannic acids make up 9 to 20 percent of the leaves.
Flavonoids including apigenin, kaempferol, myricetin, and quercetin have also been identified, but at low concentrations. Some flavonoids unique to black tea are theaflavins, theasinensins, thearubigens, and theacitrins.
Another set of flavonoids has been intensively studied recently. These polyphenols (catechins) are not vitamins, but they seem to have strong antioxidant properties.
Epigallocatechin-3-gallate (EGCG) in particular has been identified as capable of protecting experimental animals from radiation damage and possibly reducing the risk of cancer.
Green tea is especially rich in EGCG. With fermentation, many of the polyphenols are oxidized. This alters the composition of black tea in comparison to green tea (which is very similar in composition to the fresh leaves).
Tea appears to concentrate certain minerals in its leaves. Both fluoride and aluminum can be detected.
Current interest in green tea in the United States has focused mainly on its cancer preventive properties rather than on its flavor, which may be an acquired taste.
Japanese researchers were the first to report that people living in an area where green tea was an important crop were only half or even a fifth as likely to develop cancer as those in an area that did not grow tea.
There seems to be an inverse relationship between drinking tea and developing cancer of the digestive tract or urinary system. One recent epidemiological study in Japan showed that men who drank ten or more cups of green tea daily had a significantly lower risk of lung, liver, colon, or stomach cancer.
EGCG seems to be responsible in large measure for inhibiting the growth of cancer cells and mopping up free radicals that can damage healthy cells.
It also appears to work on the heterocyclic amines that form when meat, poultry, or fish is grilled and keeps them from initiating cancerous changes.
One Chinese study suggests that green tea can counteract the cancer-promoting effects of female hormones on breast tissue. Further research on this possibility is needed.
The polyphenols in green tea are thought to be responsible for its chemopreventive activity. Curcumin seems to act synergistically with green tea in preventing mutations and tumor development.
EGCG also has antibacterial and antiviral activity and stimulates the immune system to produce interleukin-1 and tumor necrosis factor. These and possibly other actions may explain the capacity of EGCG to reduce periodontitis, and this in turn may explain how green tea helps minimize bad breath.
The fluoride found in tea leaves may help explain why tea drinkers are less prone to dental caries. Tea can stain dental enamel, however.
Green tea drinkers are reported to have lower total cholesterol than those who do not imbibe. A special type of tea, Tao Cha, is associated with this benefit and is used in China to treat elevated cholesterol.
More recent research has cast some doubt on this finding, however. In Japan, where the first studies were undertaken, men who drink green tea are more likely to eat a traditional Japanese diet low in fat, while in Norway and Israel, people who choose tea are not drinking unfiltered coffee, a beverage that can elevate cholesterol levels.
Prospective trials have not shown that tea lowers blood lipids. Because tea discourages oxidation of low-density lipoprotein, however, it may help protect against atherosclerosis.
Both green tea and Earl Grey counteracted platelet clumping and prevented coronary blood clots in a dog experiment, but scientists have not been able to demonstrate any anti-platelet effect in humans.
Topical application of EGCG from green tea in animal experiments stopped the development of skin cancer after exposure to a carcinogenic chemical.
Polyphenols from green tea can also protect skin from ultraviolet radiation (UV-B) damage, acting essentially as a natural sunscreen.
Because of its high tannin content, tea has traditionally been used to treat mild diarrhea.
The caffeine content in tea has a stimulant effect on the brain and can increase alertness.
The theophylline content can aid bronchodilation in mild cases of asthma, and the astringent properties of the beverage, applied topically, have been used in China for skin problems.
There is very little information on a desirable dose of green tea. Approximately 2 g of tea is used with 250 ml of boiling water.
To treat diarrhea, the solution steeps for ten minutes to extract the most tannins. Two to three cups daily are appropriate.
Caffeine dissolves rather quickly in hot water, within approximately two minutes, and one cup can have a stimulant effect.
Large doses of green tea, up to nine or ten cups daily, were associated with cardiovascular benefit in the early epidemiological studies. These results have not been confirmed.
Daily use of green tea by much of the population in China and Japan suggests that no strict time limits on administration need be observed.
You really have to drink a lot of green tea to get into trouble.
High doses of caffeine have been linked to infertility and birth defects. Large amounts of green tea are therefore not recommended for pregnant women or those attempting to conceive. Caffeine is detectable in breast milk after the mother consumes a caffeine-containing beverage.
The diuretic effects of caffeine and theophylline may put a strain on kidneys with pre-existing problems.
People with ulcers, heart rhythm problems, and clinical anxiety disorders should minimize their intake of caffeine.
People who drink excessive amounts of green tea may get too much caffeine. High concentrations of caffeine (hard to achieve with moderate green tea intake) can result in rapid heart rate or altered heart rhythm (PVC), excess fluid elimination, jitteriness, and insomnia.
Chronic use of caffeine can lead to symptoms of headache, sluggishness, and irritability upon withdrawal.
Withdrawal has been reported in people who stop drinking as little as two or three cups of coffee daily and therefore might be anticipated in people who suddenly stop drinking many cups of green tea (five or six daily).
The tannins in tea can interfere with the absorption of non-heme iron (iron supplements, for example) taken at the same time. Milk added to black tea can reduce the binding capacity of tannins. (Milk is rarely if ever added to green tea.)
Caffeine (65 mg) can increase the analgesic effects of aspirin or acetaminophen.
Antibiotics such as Cipro, Noroxin, or Penetrex, and the ulcer drug Tagamet (cimetidine) can increase the stimulant effects of caffeine. Combining tea with a medication containing theophylline or caffeine could result in too much caffeine and resulting nervousness or insomnia.