Tea is one of the most widely consumed beverages worldwide, and is the second-most consumed drink after water, thanks also to afternoon tea sessions. Tea, from the plant Camellia sinensis, is consumed in different parts of the world as green, black, or Oolong tea. Among all of these, however, the most significant effects on human health have been observed with the consumption of green tea.
To produce green tea, freshly harvested leaves are immediately steamed to prevent fermentation, yielding a dry, stable product. This steaming process destroys the enzymes responsible for breaking down the colour pigments in the leaves and allows the tea to maintain its green colour during the subsequent rolling and drying processes. These processes preserve natural polyphenols with respect to the health-promoting properties.
According to the European Society of Cardiology, drinking tea at least 3 times a week is linked with a longer and healthier life with “the favourable health effects are the most robust for green tea and for long-term habitual tea drinkers”.
Green tea contains polyphenols, which include flavanols, flavandiols, flavonoids, and phenolic acids. The major flavonoids of green tea are various catechins, which are found in greater amounts in green tea than in black or Oolong tea. There are four kinds of catechins mainly find in green tea: epicatechin, epigallocatechin, epicatechin-3-gallate, and EGCG.
Green tea consumption has also been linked to the prevention of many types of cancer, including lung, colon, esophagus, mouth, stomach, small intestine, kidney, pancreas, and mammary glands. Several epidemiological studies and clinical trials showed that green tea (and black and Oolong teas to a lesser extent) may reduce the risk of many chronic diseases. This beneficial effect has been attributed to the presence of high amounts of polyphenols, which are potent antioxidants. In particular, green tea may lower blood pressure and thus reduce the risk of stroke and coronary heart disease. The effectiveness of green tea in treating any type of diarrhea and typhoid has been known in Asia since ancient times. Green tea catechins have an inhibitory effect on Helicobacter pylori infection. Furthermore, Green tea is considered to be useful for insect stings due mainly to its anti-inflammatory effects and its capacity to stop bleeding.
Research showed that tea catechins can affect iron absorption, particularly in groups at risk of iron deficiency but their effects on other ions are poorly understood. Green tea ingestion over a long period does not affect the apparent absorption of copper, whereas it decreases that of zinc and increases that of manganese. However, catechin intake does not affect the plasma concentration of these ions. Green tea catechins have the potential to affect absorption and metabolism of ions because flavonoids interact with a variety of metal ions.
Adverse effects of green tea
The effects of green tea and its constituents may be beneficial up to a certain dose yet higher doses may cause some unknown adverse effects. Moreover, the effects of green tea catechins may not be similar in all individuals. EGCG (a type of catechin) of green tea extract is cytotoxic, and higher consumption of green tea can exert acute cytotoxicity in liver cells, a major metabolic organ in the body.
Harmful effects of tea overconsumption (black or green) are due to three main factors: (1) its caffeine content, (2) the presence of aluminum, and (3) the effects of tea polyphenols on iron bioavailability. For these reasons, green tea should not be taken by patients suffering from heart conditions or major cardiovascular problems.