Physiological Effects

Soon after the introduction of tea into England, extreme positive and negative effects were attributed to it. A London teahouse advertisement stated that "Tea removeth lassitude, vanquisheth heavy dreams, easeth the frame, and strengtheneth the memory. It overcometh superfluous sleep, and prevents sleepiness in general, so that without trouble whole nights may be passed in study." Though some doctors extolled its curative powers, others attacked tea for causing "palsies, impotence, leanness, sterility, and nervousness" (83).

Tea is now considered to be a healthful beverage. It is a useful source of fluoride and potassium. Its vitamin content is too low to be significant except for heavy users (5 cups per day), who may obtain 5 to 10% of the RDA for riboflavin, niacin, folic acid, and pantothenic acid.

Catechins and their oxidation products have been shown to be effective antioxidants. They exhibit significant quenching of active oxygen species such as hydroxyl and peroxyl radicals as well as more stable free radicals. In lipophilic systems they inhibit the free radical-mediated oxidation of low-density lipoproteins (LDL). Active oxygen radicals are known to play a role in mutagenesis and chemical carcinogenesis. Oxidized LDL is believed to promote atherosclerotic lesions. Tea has therefore been studied for its possible usefulness in preventing these developments. It has exhibited protective effects on laboratory animals treated with mutagenic, carcinogenic, or atherogenic agents.

Prior administration of green-tea extracts inhibits the chromosome aberrations normally induced by aflatoxin Bx in rat bone marrow cells (84). Green-tea polyphenol consumption exhibits protective effects for rodents treated with carcinogens (85). Topical application of green-tea polyphenols is protective against the initiation of skin tumors in mice treated with 3-methylcholanthrene (86). Though most of these studies have been conducted with green tea or its components, black tea also exhibits protective effects in biological systems (87). Extension of these studies to humans (primarily with epidemiological approaches) has not produced definitive results, partly due to the confounding factors inherent in studies of this type. Epidemiological studies carried out in different parts of the world are difficult to compare and evaluate because of dietary differences. There is often a lack of quantification concerning the tea solids consumed. In a few such investigations, however, some evidence for its anticarcinogenic effect for humans has been obtained (88).

Green tea and green-tea polyphenols lower serum cholesterol levels of rats maintained on a cholesterogenic diet (89). A strong negative correlation has been observed between green-tea consumption and the onset of stroke in a large Japanese population (90).

Tea may be inhibitory to the absorption of iron. Borderline iron deficiencies may be accentuated by high levels of intake (91). A similar effect occurs with thiamine (92). Reports linking tea consumption with esophageal cancer have been based on faulty epidemiological studies: In some areas where the condition is rampant and where tea consumption is high, such as in parts of China, dietary contamination with carcinogenic molds and nitrosamines is widespread (93); in an area of Iran, beverages are commonly consumed scaldingly hot—a practice in itself conducive to esophageal cancer.

Confusion of herbal teas with Camellia sinensis and tannic acid with tea components has led to further misunderstanding concerning the effects of tea consumption.

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