Health effects of tea
According to legend, the health effects of tea have been examined ever since the first infusions of Camellia sinensis about 4700 years ago in China. Emperor Shennong claimed in The Divine Farmer's Herb-Root Classic that Camellia sinensis infusions were useful for treating a variety of disease conditions.
Historically as well as today, in regions without access to safe drinking water, the boiling of water to make tea has been effective in reducing waterborne diseases by destroying pathogenic microorganisms. Recently, concerns have been raised about the traditional method of over-boiling tea to produce a decoction, which may increase the amount of pesticides and other harmful contaminants released and consumed.
Black tea has been studied extensively for its potential to lower the risk of human diseases, but none of this research is conclusive as of 2015.
Tea drinking accounts for a high proportion of aluminum in the human diet. The levels are safe, but there has been some concern that aluminum traces may be associated with Alzheimer's disease. A recent study additionally indicated that some teas contained possibly risky amounts of lead (mostly Chinese) and aluminum (Indian/Sri Lanka blends, China). There is still insufficient evidence to draw firm conclusions on this subject.
Most studies have found no association between tea intake and iron absorption. However, drinking excessive amounts of black tea may inhibit the absorption of iron, and may harm people with anaemia.
All tea leaves contain fluoride; however, mature leaves contain as much as 10 to 20 times the fluoride levels of young leaves from the same plant.
The fluoride content of made tea depends on the picking method and fluoride content of the soil in which it is grown; tea plants absorb this element at a greater rate than other plants. Care in the choice of the location where the plant is grown may reduce the risk. It is speculated that hand-picked tea would contain less fluoride than machine-harvested tea, because there is a much lower chance of harvesting older leaves during the harvest process. A 2013 British study of 38 teas found that cheaper UK supermarket tea blends had the highest levels of fluoride with about 580 mg per kilogram, green teas averaged about 397 mg per kg and pure blends about 132 mg per kg. The researchers suggested that economy teas may use older leaves which contain more fluoride. They calculated a person drinking a litre of economy tea per day would consume about 4 mg of fluoride, the maximum recommended amount of fluoride per day but below the maximum tolerable amount of 10 mg fluoride per day.
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