The legendary emperor Shennong claimed in The Divine Farmer's Herb-Root Classic that Camellia sinensis infusions were useful for treating conditions including tumors, abscesses, bladder ailments, and lethargy. The possible beneficial health effects of tea consumption have been suggested and supported by some studies, but others have found no beneficial effects. The studies contrast other claims, including antinutritional effects such as preventing absorption of iron and protein, usually attributed to tannin. The vast majority of studies have been of green tea; however, some studies have been made of the other types of tea derived from Camellia sinensis, such as white, oolong, and black tea. Green tea has been claimed to be helpful for atherosclerosis, LDL cholesterol, cancer, inflammatory bowel disease, diabetes, liver disease, weight loss, neurodegenerative diseases, and even halitosis.

Anti-cancer properties

An article in New Scientist magazine mentions that numerous studies suggest that green tea protects against a range of cancers, including lung, prostate and breast cancer. The reason cited is the antioxidant epigallocatechin gallate (EGCG), according to Hirofumi Tachibana's team at Kyushu University. Their research showed that growth of human lung cancer cells with a cell receptor called 67 LR is slowed significantly after drinking just two or three cups of green tea, which contains EGCG. The research also showed that 67 LR is involved in the propagation of prion diseases such as human Creutzfeldt-Jakob (related to mad cow disease in animals). This is not direct evidence of tea's effect on prion diseases, but a hint that EGCG's effect on 67 LR is an interesting lead in the search for treatments.

White tea has been claimed to be even more effective, based upon preliminary work by Santana-Rios et al.

Another study from the Life Science journal Carcinogenesis demonstrated that green tea, in combination with tamoxifen, is effective in suppressing breast cancer growth in vitro human breast cancer tumors and in vivo animal experiments in mice.

The anticarcinogenic effect of green tea on gastric cancer was refuted by a large-scale, population-based, prospective cohort study in Japan that involved more than 26,000 residents. Several case control studies suggest an inverse relation between green tea consumption and gastric cancer. Further evaluation is needed to assess the role of green tea and gastric cancer reduction.

Topical applications of green tea extracts (EGCG) have protective effects on UVA- and UVB-induced skin damage (photoaging and carcinogenesis).

In a July 2005 review of claims made about the health benefits of green tea, the U.S. Food and Drug Administration concluded that it was highly unlikely that green tea reduces the risk of breast and prostate cancer. The FDA believes that the evidence does not support qualified health claims for green tea consumption and a reduced risk of cancer.

Increases metabolic rate

Clinical trials conducted by the University of Geneva and the University of Birmingham indicate that green tea raises metabolic rates, speeds up fat oxidation and improves insulin sensitivity and glucose tolerance.In addition to caffeine, green tea contains catechin polyphenols that raise thermogenesis (the rate at which calories are burned), and hence increases energy expenditure.

There is also a suggestion that it can increase endurance in exercise by improving fat metabolism.

Possible anti-diabetes effect

There is also epidemiological evidence that drinking green tea and black tea may help prevent diabetes, although it is worth noting that this is evidence of an association, and that future studies are needed to confirm the effect.

Boosts mental alertness

The amino acid L-theanine, found almost exclusively in the tea plant, actively alters the attention networks of the brain, according to results of human trials announced in September 2007. It has been proposed that theanine is absorbed by the small intestine and crosses the blood-brain barrier, where it affects the brain's neurotransmitters and increases alpha brain-wave activity. The result is a calmer, yet more alert, state of mind.

Boosts immune system

On 21 April 2003 the Brigham and Women's Hospital released details of a research project which indicated that theanine may help the body's immune system response when fighting infection, by boosting the disease-fighting capacity of gamma delta T cells. The study included a four-week trial with 11 coffee drinkers and 10 tea drinkers, who consumed 600ml of coffee or black tea daily. Blood sample analysis found that the production of anti-bacterial proteins was up to five times higher in the tea-drinkers, an indicator of a stronger immune response.

Lowers chances of cognitive impairment

A 2006 study showed that elderly Japanese people who consumed more than 2 cups of green tea a day had a 50 percent lower chance of having cognitive impairment, in comparison to those who drank fewer than 2 cups a day, or who consumed other tested beverages.[25][26] This is probably due to the effect of EGCG, which passes through the blood-brain barrier.

Lowers stress hormone levels

According to a study by UCL researchers, drinking black tea lowers stress hormone levels.Just 50 minutes after a high stress event, tea drinkers, who had been drinking 4 cups of black tea daily for a month, had a 20% greater drop in cortisol than the placebo group. Blood platelet activation, which is linked to blood clotting and the risk of heart attacks was also lower for tea drinkers.

Effects on HIV

A recent study appearing in the Journal of Allergy and Clinical Immunology was the subject of an article on BBC News. It stated that epigallocatechin gallate (EGCG) found in green tea can lead to the inhibition of HIV virus binding and may be used as a complementary therapy for HIV patients, but qualified it by noting that "It is not a cure, and nor is it a safe way to avoid infection, however, we suggest that it should be used in combination with conventional medicines to improve quality of life for those infected." It was an in vitro (test tube) study, not an in vivo study, which only tested effects of a chemical in green tea. "Many substances shown to prevent HIV infection in the test tube turn out to have little or no effect in real life, so I think there's a long way to go before anyone should rely on green tea to protect against HIV infection."

Effects on inflammatory bowel disease (IBD)

The polyphenols in green tea have been shown to reduce intestinal inflammation in mouse models of IBD. This effect seems to be related to tea’s ability to interrupt the cascade of inflammatory reactions that are the cause of IBD.

Effects on bad breath

Researchers at the University of Illinois at Chicago stated that polyphenols help inhibit the growth of bacteria that cause bad breath.[32]

Iron overload disorders

Researchers in Germany have found that a daily cup of black tea can help stop excess iron damaging the bodies of people who suffer from hemochromatosis due to its high content of flavonoids (commonly mistaken for tannins), which limit iron absorption.

Effects associated with caffeine

A cup of green tea contains between 15 and 50 mg of caffeine. Certain cognitive benefits are associated with caffeine consumption, such as a reduction in the likelihood of Parkinson's disease and a temporary increase in short term memory. Further, caffeine consumption has been linked with greater athletic performance, healthy weight loss, reduction in duration and severity of headaches and is effective in treating the symptoms of asthma.

Effects on obstructive sleep apnea-related brain deficits

University of Louisville researchers report that green tea polyphenols may stave off the cognitive deficits that occur with obstructive sleep apnea, in the second issue for May, 2008 of the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine.[35] Researchers examined the effects of green tea polyphenols administered through drinking water, on rats that were intermittently deprived of oxygen during 12-hour “night” cycles, mimicking the intermittent hypoxia that humans with OSA experience.

Effects on bacterial and fungal infections

A study at Pace University reported in American Society For Microbiology (May 2008) found white tea extracts effective at treating bacterial infections, such as Staphylococcus, Streptococcus, pneumonia and dental caries.[36] White tea was also found to be effective in treating fungal infections from Penicillium chrysogenum and Saccharomyces cerevisiae. Researchers also reported that white tea extracts showed a greater effect than green tea extracts.

Anti-venom effects

Antivenin activity of melanin extracted from black tea (MEBT) was reported for the first time in 2004. Low toxicity of MEBT in combination with its antagonistic activity against different venoms may allow effective life-saving treatment against snakebites. Such application of MEBT is important when identification of the snake is impossible or if specific treatment is unavailable.

Stroke

Research presented at the International Stroke Conference in February 2009 found that drinking three or more cups of tea per day can reduce the risk of suffering a stroke by as much as 21%. The research, conducted at the University of California, Los Angeles (UCLA), found that drinking green and black varieties of teas has a significant impact on the risk of stroke.

Cardiovascular health

Research published in April 2009 by the University of L'Aquila suggests that drinking just one cup of regular, black tea per day may help to protect against cardiovascular disease.The research showed that black tea consumption does - depending on dose - improve blood vessel reactivity, reduce both blood pressure and arterial stiffness, indicating a notably better cardiovascular health profile.[39]

Anogenital warts

Sinecatechin, an extract from green tea, was shown to be effective in treating anogenital warts in a double-blinded, randomized controlled trial of greater than 500 subjects. The subjects applied a topical ointment containing either sinecatechin or placebo to the affected area for up to 4 months, and were followed for 3 months after treatment. More than half of the subjects in the treatment group (57%) experienced a complete resolution of their warts, compared with a third (34%) in the control group. 78% of the patients in the treatment group experienced at least 50% improvement in their warts. The number needed to treat was 4-5 patients. The green tea extract treatment was well-tolerated, with relatively few side-effects.