The health benefits of tea is a controversial topic with many proponents and detractors. An article from the Nutrition (1999, pp. 946–949) journal as related on PubMed states:
The possible beneficial effects of tea consumption in the prevention of cancer and cardiovascular diseases have been demonstrated in animal models and suggested by studies in vitro. Similar beneficial effects, however, have not been convincingly demonstrated in humans: beneficial effects have been demonstrated in some studies but not in others. If such beneficial effects do exist in humans, they are likely to be mild, depending on many other lifestyle-related factors, and could be masked by confounding factors in certain populations. Another concern is that the amounts of tea consumed by humans are lower than the doses required for demonstrating the disease-prevention effects in animal models. Caution should be applied, however, in the use of high concentrations of tea for disease prevention. Ingestion of large amounts of tea may cause nutritional and other problems because of the caffeine content and the strong binding activities of tea polyphenols, although there are no solid data on the harmful effects of tea consumption. More research is needed to elucidate the biologic activities of green and black tea and to determine the optimal amount of tea consumption for possible health-beneficial effects.
In 2010 researchers found that people who consumed tea had significantly less cognitive decline than non-tea drinkers. The study used data on more than 4,800 men and women aged 65 and older to examine change in cognitive function over time. Study participants were followed for up to 14 years for naturally-occurring cognitive decline. (AAICAD 2010; Lenore Arab, PhD; UCLA)
Several of the potential health benefits proposed for tea are outlined in this excerpt from Mondal (2007, pp. 519–520) as following:
Tea leaves contain more than 700 chemicals, among which the compounds closely related to human health are flavanoides, amino acids, vitamins (C, E and K), caffeine and polysaccharides. Moreover, tea drinking has recently proven to be associated with cell-mediated immune function of the human body. Tea plays an important role in improving beneficial intestinal microflora, as well as providing immunity against intestinal disorders and in protecting cell membranes from oxidative damage. Tea also prevents dental caries due to the presence of fluorine. The role of tea is well established in normalizing blood pressure, lipid depressing activity, prevention of coronary heart diseases and diabetes by reducing the blood-glucose activity. Tea also possesses germicidal and germistatic activities against various gram-positive and gram negative human pathogenic bacteria. Both green and black tea infusions contain a number of antioxidants, mainly catechins that have anti-carcinogenic, anti-mutagenic and anti-tumoric properties.
In a large study of over 11,000 Scottish men and women completed in 1993 and published in the 1999 Journal of Epidemiology and Community Health (1999, pp. 481–487), there was an increase in the risk of coronary disease with the regular consumption of tea, although it disappeared after adjustment for confounding factors (age and occupational status).
The IARC list teas as under Group 3 carcinogens since injection of black tea concentrates under the skins of mice showed some cancerous growths. However, it has not been possible to prove that tea affects humans in similar ways through consumption.
Consumption of some forms of tea has the potential to result in acute liver damage in some individuals. Several herbal & dietary supplements have been linked to liver damage, caused in part or completely by the presence of green tea extract in these supplements; the most notable cases include Hydroxycut (415 mg of a mixture of green, white, and oolong tea extracts, and several other herbal extracts, per dose); Exolise (360 mg of green tea extract per dose); and Tealine (250 mg of green tea extract per dose). These concerns resulted in withdrawals of the first two products and a class action lawsuit against the manufacturer of Hydroxycut. The risk is thought to be quite small: in case of Hydroxycut, 9 million bottles were sold in the U.S. over the lifetime of the product, resulting in 23 known severe cases, however, these included at least one fatality and at least three cases of liver failure resulting in a liver transplantation.In case of Exolise, the risk of an adverse effect was estimated as less than 1 per 100,000.