Health effects of tea-Benefits and Contents of Tea
Benefits and Contents of Tea
Tea contains catechins, a type of antioxidant. In a freshly picked tea leaf, catechins can compose up to 30% of the dry weight. Catechins are highest in concentration in white and green teas, while black tea has substantially fewer due to its oxidative preparation. Research by the U.S. Department of Agriculture has suggested that levels of antioxidants in green and black tea do not differ greatly, with green tea having anoxygen radical absorbance capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in μmolTE/100g).The amounts ofcarbohydrates, fat, and protein found in tea are negligible. Although tea contains various types of phenolics and tannin, tea does not containtannic acid. Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.
Theanine and caffeine
Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type, brand and brewing method.Tea also contains small amounts of theobromine and theophylline. Due to modern day environmental pollution fluoride and aluminium have also been found to occur in tea, with certain types of brick tea made from old leaves and stems having the highest levels. This occurs due to the tea plant's high sensitivity to and absorption of environmental pollutants.Dry tea has more caffeine by weight than coffee; nevertheless, more dried coffee is used than dry tea in preparing the beverage,which means that a cup of brewed tea contains significantly less caffeine than a cup of coffee of the same size.
Potential benefits
Anti-cancer properties
The FDA in 2005 reports that green tea consumption has no known beneficial effect on breast or prostate cancer. "Two studies do not show that drinking green tea reduces the risk of breast cancer in women, but one weaker, more limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of breast cancer." "One weak and limited study does not show that drinking green tea reduces the risk of prostate cancer, but another weak and limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of prostate cancer."
The FDA also finds that existing evidence does not support qualified health claims for green tea consumption and a reduced risk of any other type of cancer.
The US National Cancer Institute reports that in epidemiological studies and the few clinical trials of tea for the prevention of cancer, the results have been inconclusive. The institute "does not recommend for or against the use of tea to reduce the risk of any type of cancer." ... "Inconsistencies in study findings regarding tea and cancer risk may be due to variability in tea preparation, tea consumption, the bioavailability of tea compounds (the amounts that can be absorbed by the body), lifestyle differences, and individual genetic differences."
Potential drawbacks
Fluoride
The fluoride content of made tea depends on the picking method and fluoride content of the soil in which it is grown; tea plants absorbs 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.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. High fluoride intake (daily intakes over 2 mg for children, 4 mg adults) increases the risk of osteofluorosis and fractures. 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. There is some evidence that over-intake of teas produced using mature leaves (e.g. brick tea) or a combination of mature and young (e.g. through inefficient mechanical harvesting) has been found to cause fluorosis in humans
Aluminium
As with fluorine, the tea plant is a natural bioaccumulator of aluminium, particularly in older leaves, with aluminium concentrations of up to 30,000 ppm aluminium by dry weight . Again, machine-harvested tea leaves are thought to be higher in free aluminium than those harvested by hand, since harvesting machines are more likely to inadvertently harvest older leaves during harvest. While large doses of aluminium can cause toxicity in humans, which may even result in death, there is no evidence that consuming any quantity of made tea has had this result.
Caffeine
The caffeine in tea is a mild diuretic. However, the British Dietetic Association has suggested that tea can be used to supplement normal water consumption, and that "the style of tea and coffee and the amounts we drink in the UK are unlikely to have a negative effect [on hydration]".
Oxalates
Tea contains oxalate, overconsumption of which can cause kidney stones, as well as binding with free calcium in the body; other minerals may be bound as well. The bioavailability of oxalate from tea is low, thus negative effect requires a large intake of tea.
Hot drinking temperature
The limited available evidence points to high temperature beverage-drinking increasing the risk of esophageal cancer.
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