Potential effects of tea on health

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This article only deals with the effects of tea which is made from the plant Camellia sinensis (i.e. black tea, oolong tea, green tea and white tea). This page does not deal with the effects of other teas.

Bai Hao Yin Zhen white tea
Bai Hao Yin Zhen white tea

The potential effects of tea on health have been touted for infusions made from the plant Camellia sinensis for over 4700 years; ever since its discovery was attributed to the legendary emperor, Shennong. The Shen Nong Ben Cao Jing claimed its taste and stimulative properties were useful for treating tumors, abscesses, bladder ailments, lethargy, among other conditions.[1] 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[2] to be helpful for atherosclerosis, LDL cholesterol, cancer, inflammatory bowel disease, diabetes, liver disease, weight loss, neurodegenerative diseases, and even halitosis.

Contents

[edit] Potential benefits

[edit] Anti-cancer properties

An article in New Scientist magazine[3] 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 in Japan. Their research showed that growth of human lung cancer cells that have 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.[4]

According to the U.S. National Cancer Institute, in laboratory studies using animals, catechins inactivated oxidants before cell damage occurred, reduced the number and size of tumors, and inhibited the growth of cancer cells.[citation needed]

White tea has been claimed to be even more effective, based upon preliminary work by Santana-Rios et al.[5]

Another study from the Oxford Life Science journal Carcinogenesis showing a capacity of 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.[6]

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.[7] 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.[8]

Topical applications of green tea extracts (EGCG) have protective effects on UVA- and UVB-induced skin damage (photoaging and carcinogenesis).[9][10][11][12][13][14][15][16]

[edit] 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.[17] In addition to caffeine, green tea contains catechin polyphenols that raise thermogenesis (the rate at which calories are burned), and hence increases energy expenditure.[18]

There is also a suggestion that it can increase endurance in exercise by improving fat metabolism.[19]

[edit] Possible anti-diabetes effect

There is also epidemiological evidence that drinking green tea (but not black tea or oolong tea) may help prevent diabetes,[20] although it is worth noting that this is evidence of an association, and that future studies are needed to confirm the effect.

[edit] 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. John Foxe, Ph.D., professor of neuroscience, biology and psychology at City College of the City University of New York, found 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.[21][22]

[edit] 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.[23]

[edit] Lowers chances of cognitive impairment

A 2006 study[24][25] 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.

[edit] Lowers stress hormone levels

According to a study[26] 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.

[edit] Effects on HIV

A recent study appearing in the Journal of Allergy and Clinical Immunology 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.[27][28]

[edit] EGCG Specific Mechanism

EGCG does this by competing for gp120 binding sites with HIV, and also protects the brain, which modern anti-retrovirals medicines cannot do, since they can't pass the blood brain barrier. This is one reason why HIV cannot currently be eliminated as modern medicines cannot penetrate the testes,[29] brain, and kupffer cells of the liver,[30] where HIV regroups.

[edit] Oxalates

Oxalates, including those found in all teas, help with HIV and general infections by mopping up free iron, leaving one less thing for the immune system to do.[citation needed] Oxalates chelate zinc as well, a crucial nutrient HIV uses to hijack a cell, as it has zinc fingers. However, oxalates are a mixed bag - see the problems below.

[edit] Effects on Inflammatory Bowel Disease (IBD)

The polyphenols in green tea show an ability to inhibit intestinal inflammation in the IL-2 knock-out mouse model of IBD. This is an important possibility for treating IBD, which relate to the polyphenol fraction on NF-k B. EGCG interrupts the proinfammatory cascade by inhibiting the effects of inflammatory mediators on NF-k B by inhibiting the phosphorylation of Ik B by Ik B kinase (IKK). NF-k D cannot migrate to the nucleus and bind with cytokine promoter sequences when bound to Ik B. (Reference: Textbook of Gastroenterology, Volume one, 4th edition, 2003 Editor Tadataka Yamada, Lippincott Williams and Wilkins, pages 1141-1142).

[edit] 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.[31]

[edit] 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 haemochromatosis due to its high content of tannin which limits iron absorption.[32]

[edit] Effects associated with caffeine

Main article: 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.[33]

[edit] 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 [34]. 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.

[edit] Potential drawbacks

[edit] Effects of fluoride

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.[35][36][37][38][39][40] In general, the level of fluoride in tea is inversely related to the EGCG contents: the more EGCG, the less fluoride. 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 soil and air pollution; 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.[41]

According to Andreas Schuld of the Canadian "Parents of Fluoride Poisoned Children" tea is very high in fluoride content, much higher than the Maximum Contaminant Level (MCL) set for fluoride in drinking water.[42] Decaffeinated teas have an even higher fluoride content, which is thought to be due to high fluoride levels in the water used in decaffeination. According to Schuld, fluoride could reduce the anti-cancer properties of tea, or even possibly cause cancer at continued toxic levels of the mineral. For instance, Schuld references a 1998 study, which found positive correlation between colon cancer and tea intake. The high fluoride content could also cause neurological and renal damage, especially in the presence of aluminum. Additionally, the high fluoride content could cause osteoporosis, arthritis, skeletal fluorosis[43] and other bone disorders.

Cancers of the digestive tract, which have a higher incidence in Far East countries, have manifold environmental causes and cannot be blamed solely in those peoples' habit of consuming tea.[citation needed]

[edit] Effects associated with caffeine

Caffeine is an addictive substance 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.[44]

[edit] Oxalates

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.[45]

[edit] Tannin

It has been suggested that the chemical known as tannin may increase the risk of nasal and esophogeal cancer. [2] However, some studies have found tea drinking might be negatively associated with risk of esophageal cancer.[46][47]

[edit] United States FDA

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.[48]

[edit] Effect of milk on tea

A study[49] 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.[50] Other studies have found little to no effect from milk on the observed increase in total plasma antioxidant activity.[51] 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.[52] Plant-based "milks", such as soy milk, do not contain casein and are not known to have similar effects on tea.

Milk also binds the tannin in the tea, rendering it harmless.[53]

Tea
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Black tea | Blended and flavored teas | Green tea | Masala chai | Oolong tea | Post-fermented tea | White tea | Yellow tea
Ceremonies and methods Related to tea
China | India | Japan | Korea | Morocco | Turkey Tea house | Teapot | Tea and health

[edit] References

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  29. ^ "Men's testicles HIV hiding spot", BBC News. 
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  32. ^ BBC News | Health | Cuppa a day is a lifesaver
  33. ^ Health Benefits of Coffee - WebMD
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  40. ^ Calls for FDA to introduce tea fluoride safety standard
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  42. ^ Green Tea
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  48. ^ FDA Issues Information for Consumers about Claims for Green Tea and Certain Cancers
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[edit] External links