Health effects of tea

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Information about the different varieties of tea, its impact on people, Australia, 1912

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.[1]

Historically, it was probably beneficial in many situations before modern safe drinking water that tea is normally made with boiled water, with at least a reduced load of microrganisms. In addition tea itself has significant antimicrobial properties, against for example vibrio cholerae, the bacterium responsible for cholera.[2]

Content[edit]

Tea contains catechins, a type of antioxidant. In a freshly picked tea leaf, catechins can compose up to 30% of the dry weight. Catechins are highest in concentration in white tea and green tea, while black tea has substantially fewer due to its oxidative preparation.[3][4] Research by the U.S. Department of Agriculture has suggested that levels of antioxidants in green and black tea do not differ greatly, with green tea having an oxygen radical absorbance capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in μmolTE/100g).[5] The amounts of carbohydrates, fat, and protein found in tea are negligible. Although tea contains various types of phenolics and tannin, tea does not contain tannic acid.[6] Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.[7]

Theanine and caffeine[edit]

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[8] and brewing method.[9] Tea also contains small amounts of theobromine and theophylline.[10] Due in part to modern-day environmental pollution fluoride and aluminium have also been found to occur in tea, with certain types of brick tea made from old leaves and stems having the highest levels. This occurs due to the tea plant's high sensitivity to and absorption of environmental pollutants.[11] Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations[12]—which results in a cup of brewed tea containing significantly less caffeine than a cup of coffee of the same size.

Potential benefits[edit]

Green tea may lower blood low-density lipoprotein and total cholesterol levels, though the studies were of short duration and it is unknown whether these effects result in fewer deaths; moreover, the evidence does not indicate that green tea reduces the risk of coronary artery disease. Several randomized controlled trials suggest green tea can reduce body fat by a small amount for a short time, though it is uncertain whether the reduction would be meaningful for most people.[13]

Anti-cancer properties[edit]

The Food and Drug Administration (FDA) in 2005 reports that green tea consumption has no known beneficial effect on breast or prostate cancer. "Two studies do not show that drinking green tea reduces the risk of breast cancer in women, but one weaker, more limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of breast cancer."

The FDA also finds that existing evidence does not support qualified health claims for green tea consumption and a reduced risk of any other type of cancer.[14]

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."[15]

Though there is some positive evidence for risk reduction of breast, prostate, ovarian and endometrial cancers with green tea, it is weak and inconclusive.[13]

Potential drawbacks[edit]

Fluoride[edit]

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.[16][17][18][19][20][21] Although low concentrations of fluoride are maintained in many public water supplies for dental health, very high fluoride intake (over 2 mg per day for children, 4 mg adults) increases the risk of osteofluorosis and fractures. There is evidence that over-intake of teas produced using mature leaves (e.g. brick tea) or a combination of mature and young (e.g. through inefficient mechanical harvesting) can cause fluorosis in humans.[22] [23]

The fluoride content of made tea depends on the picking method and fluoride content of the soil in which it is 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.[24] 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.[25]

Aluminum and heavy metals[edit]

Tea drinking accounts for a high proportion of aluminum in the human diet.[26] 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).[27] There is still insufficient evidence to draw firm conclusions on this subject.[28]

Caffeine[edit]

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]".[29]

Oxalates[edit]

Tea contains oxalate, overconsumption of which can cause kidney stones, as well as binding with free calcium in the body; other minerals may be bound as well. The bioavailability of oxalate from tea is low, thus negative effect requires a large intake of tea.[30]

Hot drinking temperature[edit]

The limited available evidence suggests that drinking a beverage at very high temperature increases the risk of esophageal cancer.[31]

Prostate cancer[edit]

A study, published in 2012, suggested that men who drink large quantities of black tea – more than seven cups per day – increase their risk of prostate cancer by 50%.[32] The story was relayed in the media, but according to the National Health Service, "this study had many limitations that call into question the reliability of its results".[33][34]

See also[edit]

References[edit]

  1. ^ N. H. Woodward, Teas of the World (1980), as cited in D. A. Balentine, M. E. Harbowy, H. N. Graham, Tea: The Plant and Its Manufacture; Chemistry and Consumption of the Beverage in Caffeine ed G. Spiller (1998)
  2. ^ Lund, Barbara; Baird-Parker, Baird C.; Warwick Gould, Grahame, Microbiological Safety and Quality of Food, pp. 960-964, 960 on antimicrobial activity, 2000, Springer, ISBN 0834213230, 9780834213234, google books
  3. ^ Hamza Nassrallah (2009-01-02). "Which Tea is Healthiest?". Wonders of Tea. Retrieved 2009-07-15. 
  4. ^ "Journal of Nutrition - Antioxidant Activity of Black Tea vs. Green Tea". Jn.nutrition.org. Retrieved 2009-07-15. 
  5. ^ "Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods – 2007". Webcitation.org. 2009-05-23. Archived from the original on 2009-05-23. Retrieved 2009-07-15. 
  6. ^ "Science Magazine letters". Sciencemag.org. 1979-04-06. Retrieved 2011-01-24. 
  7. ^ "Tannin Chemistry" (PDF). Retrieved 2011-01-24. 
  8. ^ Bennett Alan Weinberg and Bonnie K. Bealer (2001). The World of Caffeine: The Science and Culture of the World's Most Popular Drug. Routledge. p. 228. ISBN 0-415-92722-6. Retrieved 2008-09-20. 
  9. ^ M. B. Hicks, Y-H. P. Hsieh, L. N. Bell, Tea preparation and its influence on methylxanthine concentration, Food Research International 29(3-4) 325-330 (1996)
  10. ^ Graham H. N.; Green tea composition, consumption, and polyphenol chemistry; Preventive Medicine 21(3):334-50 (1992)
  11. ^ "Environmental Pollution: Fluoride contents in tea and soil from tea plantations and the release of fluoride into tea liquor during infusion". ScienceDirect. Retrieved 2009-07-15. 
  12. ^ "Caffeine and Tea Information". Stash Tea. Retrieved 2009-07-15. 
  13. ^ a b Johnson, R.; Bryant, S.; Huntley, A. L. (2012). "Green tea and green tea catechin extracts: An overview of the clinical evidence". Maturitas 73 (4): 280. doi:10.1016/j.maturitas.2012.08.008. PMID 22986087
  14. ^ Food and Drug Administration (30 June 2005). "FDA announcement". Retrieved 20 March 2012. 
  15. ^ National Cancer Institute (Reviewed 17 November 2010). "Tea and Cancer Prevention: Strengths and Limits of the Evidence". Retrieved 20 March 2012. 
  16. ^ M. H. Wong, K. F. Fung and H. P. Carr (2003). "Aluminium and fluoride contents of tea, with emphasis on brick tea and their health implications". Toxicology Letters 137 (12): 111–120. doi:10.1016/S0378-4274(02)00385-5. PMID 12505437. 
  17. ^ Yi Lu, Wen-Fei Guo, and Xian-Qiang Yang (2004). "Fluoride Content in Tea and Its Relationship with Tea Quality". J. Agric. Food Chem. 52 (14): 4472–4476. doi:10.1021/jf0308354+S0021-8561(03)00835-5. PMID 15237954. 
  18. ^ K. F. Fung, Z. Q. Zhang1, J. W. C. Wong and M. H. Wong (1999). "Fluoride contents in tea and soil from tea plantations and the release of fluoride into tea liquor during infusion". Environmental Pollution 104 (2): 197–205. doi:10.1016/S0269-7491(98)00187-0. 
  19. ^ Lung SC, Cheng HW, Fu CB (2008). "Potential exposure and risk of fluoride intakes from tea drinks produced in Taiwan". J Expo Sci Environ Epidemiol 18 (2): 158–66. doi:10.1038/sj.jes.7500574. PMID 17410113. 
  20. ^ Malinowska E, Inkielewicz I, Czarnowski W, Szefer P (2008). "Assessment of fluoride concentration and daily intake by human from tea and herbal infusions". Food Chem. Toxicol. 46 (3): 1055–61. doi:10.1016/j.fct.2007.10.039. PMID 18078704. 
  21. ^ "Calls for FDA to introduce tea fluoride safety standard". Foodnavigator-usa.com. Retrieved 2011-08-04. 
  22. ^ Fung KF and MH Wong (2004) Application of different forms of calcium to tea soil to prevent aluminium and fluorine accumulation. J Sci Food Agric 84:1469–1477
  23. ^ Fawell, J (2006). 92 4 156319 2. WHO. ISBN 92 4 156319 2. 
  24. ^ Jianyun Ruan and Ming H. Wong (2001). "Accumulation of Fluoride and Aluminium Related to Different Varieties of Tea Plant". Environmental Geochemistry and Health 23 (1): 53–63. doi:10.1023/A:1011082608631. 
  25. ^ (25 July 2013) Do fluoride levels in cheap tea pose a health risk? British National Health Service "Choices, Retrieved 26 July 2013
  26. ^ Streeta R, Drábeka O, Szákováb J, Mládkováa L (2007). "Total content and speciation of aluminium in tea leaves and tea infusions". Food Chemistry 104 (4): 1662–1669. doi:10.1016/j.foodchem.2007.03.019. 
  27. ^ "The Benefits and Risks of Consuming Brewed Tea: Beware of Toxic Element Contamination". 
  28. ^ Karak T, Bhagat RM (2010). "Trace elements in tea leaves, made tea and tea infusion: A review". Food Research International (Review) 43 (9): 2234–2252. doi:10.1016/j.foodres.2010.08.010. 
  29. ^ BDA Supports Dehydration Awareness Week with some Handy Tips. British Dietetic Association press release, June 2011.
  30. ^ Michael Liebman and Shawnna Murphy (2007). "Low oxalate bioavailability from black tea". Nutrition Research 27 (5): 273–278h. doi:10.1016/j.nutres.2007.04.004. 
  31. ^ Islami F, Boffetta P, Ren JS, Pedoeim L, Khatib D, Kamangar F (August 2009). "High-temperature beverages and foods and esophageal cancer risk--a systematic review". Int. J. Cancer 125 (3): 491–524. doi:10.1002/ijc.24445. PMC 2773211. PMID 19415743. 
  32. ^ Kashif Shafiquea, Philip McLooneb, Khaver Qureshic, Hing Leungd, Carole Harta & David S. Morrisonb. Tea Consumption and the Risk of Overall and Grade Specific Prostate Cancer: A Large Prospective Cohort Study of Scottish Men. Nutrition and Cancer Volume 64, Issue 6, 2012. pp. 790–797. doi:10.1080/01635581.2012.690063. 
  33. ^ Brazian. "Seven cups of tea a day 'ups prostate cancer risk'". NHS Choices. Retrieved 1 Dec 2012. 
  34. ^ Anekew, Lilian. "Drinking tea linked to prostate cancer - but study is flawed". BMJ. Retrieved 1 December 2012. 

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