Interest in tea is growing. Scientists now report that tea is good food because it may help prevent some chronic diseases. This article reviews the history of tea as a beverage; the botany and cultivation of the plant; and the chemistry, pharmacology, and health benefits of its active ingredients. Tea polyphenols are described in detail to reflect their potential role in maintaining good health.

A BRIEF HISTORY OF TEA

Chinese legend has it that tea was the accidental discovery of King Shen Nong in about 2700 B.C. (Shouyi, 1982). A clever man, he is also credited with the invention of plowing tools and the use of other herbs in traditional Chinese medicine. The earliest (780 A.D.) written record on the subject, Cha Ching (tea book), pays homage to Shen Nong's discovery which it says he made when a gust of wind blew some tea leaves into a kettle of boiling water (Yu, 1974).

A competing legend started in India and followed the spread of Buddhism from India to China, Japan, and Southeast Asia. The British East India Company later advanced this legend as a part of its marketing strategy to weaken China's competitive advantage in global tea markets. It claims that tea was first grown in India and that Prince Siddhartha Gautama, Buddhism's founder, brought it to China. Tea was supposed to be a divine creation of the Buddha. The Prince was said to have torn off his eyelids and thrown them to the ground because he fell asleep despite his vow to remain awake during his pilgrimage through China. (There is no evidence he ever went to China). Supposedly, the eyelids took root and germinated into tea plants that sprouted leaves with an eyelid shape. All Siddhartha's fatigue was said to have vanished when he chewed the leaves of this plant.

In fact, the habit of chewing tea leaves was long established by the time of this alleged discovery (Evans, 1992). Evans also speculates, based on archeological evidence presented by Jelinek (1978), that the prehistoric people, Homo erectus pekinensis, were consumers of both boiling water and leaves, including tea leaves from wild forest plants, more than 500,000 years before tea legends became available for public consumption (Jelinek, 1978).

Today, both China and India (and possibly Burma and Thailand) share the distinction of tea's birthplace. Botanic evidence for this assertion has been found in the forests of these countries. Only there have tea plants been found that appear to be the wild type source of present-day cultivars. Although genuine wild type plants may no longer exist in these regions, previous explorations have determined that wild tea plants once grew from Nepal north and eastward to Formosa, the Liu-Kiu Islands, and southern Japan. Semi-wild species have been sighted from time to time in areas near the Assam-Burma border and in Indonesia, but they are believed to be tea left by migratory tea-drinking people (Kingdon-Ward, 1950).

Tea was introduced in the West by Turkish traders of the 6th century who apparently bartered for it at the Mongolian border. Then the Chinese of the Song Dynasty (960-1127 A.D.) realized tea's commercial value and actively exported it to Tibet. The tea traveled five miles a day by mule, yak, and on the backs of porters along 5,000-foot-high mountain passes. (One could compare 5,000-foot-high passes to stacks of 4 Empire State Buildings.) It is said that each man had to put opium behind his ears to deaden the pain of the 300-pound load strapped to his back (Weatherstone, 1992).

First the Dutch, then the British, began, then established the tea trade between China and Europe. Later, the British pioneered the cultivation and manufacture of tea in India and Ceylon where they introduced the plantation system. The Dutch did the same on Java and Sumatra. China's monopoly on this trade ended in the mid-1850s when its exports were surpassed by those of India and Ceylon. China's methods of cultivation on small plots could not compete with the output of Indian and Ceylonese plantations (Weatherstone, 1992).

Today tea is also grown in several African countries, Malaysia, Taiwan, Iran, Turkey (Turkish tea is no longer exported because it contains unacceptable amounts of radioactive contamination contributed by fallout from the Chernobyl nuclear power accident in 1986, although shipments are being made within Russia where contamination is considered to be at "acceptable" levels), Georgia and south Russia, Argentina, and even Brazil.

BOTANY

The first botanical classification of cultivated tea, made by Linnaeus in 1752, divided it into two species, Thea sinensis and Thea bohea. Later, when the number of petals was invalidated as a basis for this classification, Thea sinensis became the designation for the small-leaved China variety and Thea assamica the large-leaved Assam plant (Masters, 1844).

For a long time Thea and Camellia were considered to be separate genera, with cultivated tea plants included in the genus Thea and the non-tea Camellias in the genus Camellia. Nevertheless, confusion remained because major characteristics such as patina, leaf pose and pigmentation were, for all practical purposes, identical in both genera (Sharma & Venkataramani, 1974). Today, Thea and Camellia are considered to be synonymous (Wight, 1962) and the genus Camellia now belongs to the family Theaceae.

The most reliable diagnostic criteria for distinguishing the three recognized varieties of cultivated tea are provided by floral morphology: variation in the number of styles and the degree of their fusion; disposition of the stylar arm; and globular or pubescent ovary. For example, the styles are free for most of their length in the China variety (C. sinensis), fused for most of their length in the Assam variety (C. assamica), and remain free for about half their length in the Cambod race (C. assamica ssp. lasiocalyx). Growth habitat and leaf features continue to be used to distinguish between C. sinensis and C. assamica. Generally speaking the former is a small slow-growing shrub with small, narrow, serrate, dark green leaves, while the latter is a tall, quick-growing tree with large, horizontal, broad, light green leaves (Kitamura, 1950; Sealy, 1958).

Two other taxa, C. irrawadiensis and C. taliensis, are of interest because of their potential contribution to the genetic pool of tea. They, however, produce a liquor that lacks the quality of tea and are therefore not grown commercially. Other non-tea Camellia species, e. g. C. reticulata, C. sasanqua, and C. japonica, may have importance in understanding the evolution and interrelationships of Camellia species.

Because tea is easily cross-pollinated, all taxa freely interbreed to produce a cline (a gradual and more or less continual change between two extremes of a plant character occurring within the geographical range of a species) ranging from the China type plant to the Assam variety. Hybridization occurs so readily that the legitimacy of the three main tea taxa has been questioned (Visser, 1969). The possible contribution of other taxa to the tea genetic pool cannot be ruled out. Vegetative characters, e.g., internodal length, girth of bud, and leaf pose, are commonly used to group hybrids and relate them to the main taxa. Other anatomical differences, e.g., distribution and morphology of leaf sclereids (Barua, 1958) and leaf hair size and lumen length (Amma, 1986) have been shown to be useful. The potential for biochemical features such as polyphenol oxidase activity, individual polyphenols, amino acids, and chlorophyll contents has been realized but never fully utilized.

PHARMACOLOGY OF TEA CONSTITUENTS POLYPHENOLS

Upon ingestion, concentrations of tea polyphenols can be easily detected in blood, urine, and feces. Hence, polyphenols are absorbed and spread throughout the human or animal body. They probably exert their actions directly at the tissue and cellular level rather than have indirect intestinal effects (He & Kies, 1994).

The benefits associated with tea polyphenols, especially EGCG, are generally attributed to their antioxidant activity and their ability to scavenge free radical oxygen. But several other actions are suggested by studies of their role as chemopreventers.

Polyphenols are known to increase antioxidant and phase II enzyme activities in a variety of mouse organs, thus enhancing the overall chemopreventive effect of antioxidants in these organs (Khan, et al. 1992). One important phase II enzyme (UDP-glucuronosyl transferase), elevated in rat livers after treatment with green tea, may contribute to its anticancer effect by inactivating carcinogens and converting them into excretable form (Bu-Abbas, et al., 1995). Polyphenols also bind to cytochrome P450 in rat livers and indirectly block the activity of cytochrome P450-dependent enzymes. The cytochrome P450 superfamily contains sixty to several hundred oxidative enzymes that normally metabolize and detoxify numerous foreign chemicals in the liver. However, it has become apparent that genetic variants of these enzymes may sometimes function in an ambivalent manner, generating toxic or carcinogenic intermediates from the substances that they are supposedly detoxifying. Some of these en zymes convert procarcinogens to carcinogenic metabolites (Wang, et al., 1988; Mukhtar, et al., 1992). But green tea also stimulates the synthesis of certain forms of the P450 complex and could conceivably increase carcinogenic metabolites (Bu-Abbas, et al., 1994a).

In the test tube, the catechin gallates also selectively inhibit 5-à reductase, an enzyme responsible for the conversion of testosterone to 5-à dihydrotestosterone (Shutsung & Hiipakka, 1995). High levels of 5-à dihydrotestosterone are associated with benign prostate hyperplasia, prostate cancer, and male pattern baldness.

Many cancer-causing agents inhibit cell communication which may be an important mechanism of tumor development. The polyphenols, particularly the catechin gallates, may protect cells by enhancing gap junctional communication (Ruch, et al., 1989: Sigler & Ruch, 1993).

Another possible mechanism of action for tea polyphenols is suggested by the inhibition of tumor promoter binding to mouse skin. GTE (green tea extract) compounds might block by sealing receptors (Komori,et al., 1993). This mechanism might also explain why GTE blocks the uptake of nucleosides by tumors (a receptor-dependent mechanism) without affecting the uptake of receptor-independent cancer agents AraC and MTX (Zhen, et al., 1991). The report that tea extract inhibits the intestinal absorption of glucose and sodium in rats could be similarly explained (Kreydiyyeh, et al., 1994).

A different mode of preventive action by EGCG against carcinogenesis is suggested by Hayatsu, et al., (1992) who detects direct binding to certain carcinogens.

One study (Hu, et al., 1992) reports that EGCG stimulates B but not T cell proliferation and that the galloyl group is responsible for this enhancement. Confirmation of this important in vitro effect has not as yet appeared in the scientific literature.

METHYLXANTHINES

The pharmacological effects of caffeine are an important reason for the popularity of tea Caffeine by far the most abundant methylxanthine in tea, may exert its behavioral and other effects through competitive antagonism at the adenosine-binding receptor. Caffeine levels, produced in humans by ingestion of a few cups of tea, are sufficient to antagonize adenosine's sympathetic nervous stimulation of the vascular system, heart, kidney, and adipose tissue (Rail, 1980). Although acute studies seem to suggest that caffeine is harmful to the cardiovascular system and worsens hypertension, the results of prolonged administration of caffeine do not support this contention (Robertson, et al., 1981). Caffeine does increase serum levels of non-esterified free fatty acids. Since these substances may cause cardiac arrhythmias, a mechanism is suggested for the rare association of caffeine ingestion and arrythmia.

Tea given to children (especially in Britain) is suspected of causing hyperactivity but no confirmation of that exists. In fact, 200-300 mg of caffeine per day has been used to treat hyperactivity (Marks, 1992).

Tea drinking has also been accused of promoting the formation of calcium oxalate-containing kidney stones because the caffeine induces calcium excretion and the tea itself is a source of oxalate. But no association has been found with either kidney or bladder stones (Marks, 1992).

Methylxanthines, particularly theophylline, are used in the treatment of asthma and bronchitis to relax the smooth muscle of the bronchi. However, the amounts of theophylline and theobromine ingested by drinking even large amounts of tea are too small to be pharmacologically significant. Their effects will not be considered here.

CONCLUSION

Tea has a long history as a medicinal plant and an even longer one as a beverage that ranks second only to water in worldwide popularity. In the West, this appeal stems from the stimulating and relaxing effects tea has on human physiology. In the East, that appeal is also based on cherished traditional beliefs in the health benefits of tea. Now, East meets West. The scientific basis for this belief is growing.

The animal and most recent epidemiological studies suggest that tea contains dietary factors which protect the consumer from the development of certain cancers, cardiovascular disease, some infectious diseases, and dental caries.

On the other hand, certain studies have concluded that tea may also have a negative or no effect on the growth of certain human, cancers. Why doesn't the animal data, which regularly shows health benefits, always agree with the human situation?

The problem may lie with inconsistencies between study designs. For example, studies have not always excluded or even identified the confounding variables that are linked to the tea drinking habit, the strength of that linkage to tea drinking, and its positive or negative impact on health independent of drinking tea. Clearly, better designed and carefully controlled epidemiological studies are needed; and they have been recommended (Mukhtar, et al., 1994). The results of the animal experiments are too compelling to be ignored.

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