The health effects of tea have been examined ever since the first infusions of Camellia sinensis about 4700 years ago in China. 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.
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.
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. A study at Taiwan's Chung Shan Medical University found that people who drank at least one cup of green tea per day were five times less likely to develop lung cancer than those who did not.
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.
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.
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.
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.
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.
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.
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. This is probably due to the effect of EGCG, which passes through the blood-brain barrier.
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.
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."
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.
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.
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.
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. 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.
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. 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.
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.
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.
Research published in April 2009 by the University of L'Aquila and funded by the Unilever-owned Lipton Institute of Tea 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.
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.
In a Japanese study Green tea consumption was inversely associated with psychological distress even after adjustment for possible confounding factors.
A more frequent consumption of green tea was associated with a lower prevalence of depressive symptoms in another Japanese study . Researchers conducted a cross-sectional study in 1,058 community-dwelling elderly Japanese individuals 70 years of age. The prevalence of mild and severe and severe depressive symptoms was 34.1 percent and 20.2 percent, respectively. After adjustment for confounding factors, the odds ratios for mild and severe depressive symptoms when higher green tea consumption was compared with green tea consumption of 1 cup/d were: 2 to 3 cups green tea/d and 4 cups green tea/d. Similar relations were also observed in the case of severe depressive symptoms.
Some tea bags are made using a wet paper strength reinforcing coating using epichlorohydrin, which is known to be carcinogenic. Uses are not limited to tea bags, as coffee filters and sausage/salami casings can have the same issues. The problem can be avoided by using loose-leaf tea or tea bags which do not use the coating.
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. White tea contains less fluoride than green tea and black tea, because it is made of buds and young leaves only.
The fluoride content of tea depends directly on the 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.
Caffeine is an addictive drug and overuse of tea can result in harmful side effects, such as an increased likelihood of certain sleep disorders. Decaffeination reduces total catechins in both black and green dry teas by about 15 times and 3 times respectively.
One consideration to take into account when investigating the relationship between caffeine and diuresis is the dose size of caffeine ingested. Where the dose relationship has been systematically investigated it is only at a high dose of 360 mg that a diuretic action is found. A recent systematic review of the accumulated evidence has shown that acute diuretic effects are observed generally in cases where at least 300 mg of caffeine is ingested. This finding suggests that tea does not have a diuretic effect unless the amount of tea consumed at one sitting contains more than 250–300 mg of caffeine, equivalent to between 5 and 6 cups of tea.
Tea contains oxalate, overconsumption of which can cause kidney damage, as well as soak up free calcium in the body; other minerals could be soaked up as well. The bioavailability of oxalate from tea is low and because of this a negative effect requires large amounts of tea.
It has been suggested that chemicals known as tannins may increase one's risk of esophageal cancer, with some studies having found that tea drinking may in fact be negatively associated with risk of esophageal cancer.
Hot tea consumption has been linked to a higher risk for esophageal cancer: "In the case-control study, risk for esophageal cancer was increased for drinking hot tea...or very hot tea...vs lukewarm or warm tea. Risk was also significantly increased for drinking tea 2 to 3 minutes after pouring...or less than 2 minutes after pouring...vs drinking tea at least 4 minutes after being poured."
A study at the Charité Hospital of the Berlin Universities showed that adding milk to tea will block the normal, healthful effects that tea has in protecting against cardiovascular disease. This occurs because casein from the milk binds to the molecules in tea that cause the arteries to relax, especially EGCG. Milk may also block tea's effect on other things, such as cancer. Other studies have found little to no effect from milk on the observed increase in total plasma antioxidant activity. Teas with high EGCG content, such as green tea, are not typically consumed with milk. Previous studies have observed a beneficial effect from black tea which was not attributable to the catechin content. Plant-based "milks", such as soy milk, do not contain casein and are not known to have similar effects on tea.
Drinking tea, particularly green tea, with citrus such as lemon juice is common. Studies, including a study from Purdue University in 2007, found that most of the antioxidant catechins are not absorbed into the bloodstream when tea is drunk by itself. The study, however, found that adding citrus to the tea lowers the pH in the small intestine and causes more of the catechins to be absorbed, making the tea healthier.