According to legend, the health effects of black 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.
By constituents or substances
Aluminum, iron and other metals
Further information: Aluminum § Health concerns
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.
In 2011, the US Food and Drug Administration (FDA) reported that there was very little evidence to support the claim that green tea consumption may reduce the risk of breast and prostate 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.” Though there is some positive evidence for risk reduction of breast, prostate, ovarian, and endometrial cancers with green tea, it is weak and inconclusive.
Meta-analyses of observational studies have concluded that black tea consumption does not appear to protect against the development of oral cancers in Asian or Caucasian populations, the development of esophageal cancer or prostate cancer in Asian populations, or the development of lung cancer.
Black tea consumption may be associated with a reduced risk of stroke. Green tea and black tea both have a beneficial effect on endothelial function (and thus arterial health). However, the addition of milk to the tea completely blunts the tea’s artery-relaxing effects.
A 2013 Cochrane review of randomized controlled trials (RCT) greater than 3 months duration concluded that long-term consumption of black tea slightly lowers systolic and diastolic blood pressures (about 1–2 mmHg). This conclusion was based on limited evidence. Another meta-analysis of RCTs reached a similar conclusion.
Tea may have a protective effect against age-related cognitive impairment/decline and dementia later in life, based on correlations found in epidemiological studies; if there is a protective effect it is not dependent on dose and there appears to be a stronger effect for women than men. However, the association is not found in all cognitive domains investigated.
Tea consumption does not appear to affect the risk of bone fracture including hip fractures or fractures of the humerus in men or women.
A 2013 Cochrane review concluded that long-term black tea consumption lowers the blood concentration of LDL cholesterol by 0.43 mmol/L (or 7.74 mg/dL).
Green tea is commonly believed to be a weight loss aid, but there is no good evidence that its consumption has any meaningful benefit in helping overweight or obese people to lose weight, or that it helps them to maintain a healthy body weight.
The fluoride content of a tea leaf depends on the leaf picking method used and the fluoride content of the soil from which it has been 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.
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. Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations, which results in a cup of brewed tea containing significantly less caffeine than a cup of coffee of the same size.
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]”.
Sources from Wikipedia