Health effects of tea

This article addresses effects only of tea made from the plant Camellia sinensis, including black tea, oolong tea, green tea, and white tea. It does not address effects of other plant infusions that might be called "tea", including what are sometimes called "herbal tea".
Information about the different varieties of tea, its impact on people, Australia, 1912

According to legend, the health effects of tea have been examined ever since the first infusions of Camellia sinensis about 4700 years ago in China. Emperor Shennong claimed in The Divine Farmer's Herb-Root Classic that Camellia sinensis infusions were useful for treating a variety of disease conditions.[1]

Historically as well as today, in regions without access to safe drinking water, the boiling of water to make tea has been effective in reducing waterborne diseases by destroying pathogenic microorganisms. Recently, concerns have been raised about the traditional method of over-boiling tea to produce a decoction, which may increase the amount of pesticides and other harmful contaminants released and consumed.[2]

Black tea has been studied extensively for its potential to lower the risk of human diseases, but none of this research is conclusive as of 2015.[3]

By constituents or substances

Aluminum, iron and other metals

Further information: Aluminum § Health concerns

Tea drinking accounts for a high proportion of aluminum in the human diet.[4] 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).[5] There is still insufficient evidence to draw firm conclusions on this subject.[6]

Most studies have found no association between tea intake and iron absorption.[7] However, drinking excessive amounts of black tea may inhibit the absorption of iron, and may harm people with anaemia.[8]

Fluoride exposure

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.[9][10][11][12][13][14] 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) can cause fluorosis in humans.[15][16]

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.[17] 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.[18]

Oxalates

Tea contains oxalate, overconsumption of which can cause kidney stones, as well as binding with free calcium in the body. The bioavailability of oxalate from tea is low, thus a possible negative effect requires a large intake of tea.[19] Massive black tea consumption has been linked to kidney failure due to its high oxalate content (acute oxalate nephropathy).[20][21]

Theanine and caffeine

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[22] and brewing method.[23] Tea also contains small amounts of theobromine and theophylline.[24] Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations,[25] which results in a cup of brewed tea containing significantly less caffeine than a cup of coffee of the same size.

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]".[26]

By conditions

Cancer

In 2011, the US Food and Drug Administration (FDA) reported that there was very little evidence to support the claim that green tea consumption may reduce the risk of breast and prostate cancer.[27]

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."[28] Though there is some positive evidence for risk reduction of breast, prostate, ovarian and endometrial cancers with green tea, it is weak and inconclusive.[29]

Meta-analyses of observational studies have concluded that black tea consumption does not appear to protect against the development of oral cancers in Asian or Caucasian populations, the development of esophageal cancer or prostate cancer in Asian populations, or the development of lung cancer.[30][31][32][33][34]

Cardiovascular disease

Black tea consumption may be associated with a reduced risk of stroke.[35][36] Green tea and black tea both have a beneficial effect on endothelial function (and thus arterial health).[37][38][39] However, the addition of milk to the tea completely blunts the tea's artery-relaxing effects.[40]

A 2013 Cochrane review of randomized controlled trials (RCT) greater than 3 months duration concluded that long-term consumption of black tea slightly lowers systolic and diastolic blood pressures (about 1–2 mmHg).[41] This conclusion was based on limited evidence.[41] Another meta-analysis of RCTs reached a similar conclusion.[42]

Cognitive decline

Tea may have a protective effect against age-related cognitive impairment/decline and dementia later in life, based on correlations found in epidemiological studies; if there is a protective effect it is not dependent on dose and there appears to be a stronger effect for women than men. However, the association is not found in all cognitive domains investigated.[43]

Fracture risk

Tea consumption does not appear to affect the risk of bone fracture including hip fractures or fractures of the humerus in men or women.[44]

Hyperlipidemia

A 2013 Cochrane review concluded that long-term black tea consumption lowers the blood concentration of LDL cholesterol by 0.43 mmol/L (or 7.74 mg/dL).[41]

Weight loss

Green tea is commonly believed to be a weight loss aid, but there is no good evidence that its consumption has any meaningful benefit in helping overweight or obese people to lose weight, or that it helps them to maintain a healthy body weight.[45] Use of green tea for attempted weight loss carries some risk: in rare cases adverse effects can occur – these are mostly mild.[45]

See also

References

  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. ↑ Abd El-Aty AM, Choi JH, Rahman MM, Kim SW, Tosun A, Shim JH (2014). "Residues and contaminants in tea and tea infusions: a review". Food Addit Contam Part A Chem Anal Control Expo Risk Assess. 31 (11): 1794–804. doi:10.1080/19440049.2014.958575. PMID 25164107.
  3. ↑ "Black tea". MedlinePlus, US National Library of Medicine. 2015. Retrieved 15 March 2015.
  4. ↑ 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.
  5. ↑ "The Benefits and Risks of Consuming Brewed Tea: Beware of Toxic Element Contamination".
  6. ↑ 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.
  7. ↑ Beck KL, Conlon CA, Kruger R, Coad J (2014). "Dietary determinants of and possible solutions to iron deficiency for young women living in industrialized countries: a review". Nutrients. 6 (9): 3747–76. doi:10.3390/nu6093747. PMC 4179187Freely accessible. PMID 25244367.
  8. ↑ Wierzejska R (2014). "Tea and health--a review of the current state of knowledge". Przegl Epidemiol (Review). 68 (3): 501–6, 595–9. PMID 25391016.
  9. ↑ M. H. Wong; K. F. Fung; 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.
  10. ↑ Yi Lu; Wen-Fei Guo; 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. PMID 15237954.
  11. ↑ 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.
  12. ↑ 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.
  13. ↑ 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.
  14. ↑ "Calls for FDA to introduce tea fluoride safety standard". Foodnavigator-usa.com. Retrieved 2011-08-04.
  15. ↑ 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
  16. ↑ Fawell, J (2006). 92 4 156319 2. WHO. ISBN 92 4 156319 2.
  17. ↑ Jianyun Ruan; 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.
  18. ↑ (25 July 2013) Do fluoride levels in cheap tea pose a health risk? British National Health Service "Choices, Retrieved 26 July 2013
  19. ↑ Michael Liebman; Shawnna Murphy (2007). "Low oxalate bioavailability from black tea". Nutrition Research. 27 (5): 273–278h. doi:10.1016/j.nutres.2007.04.004.
  20. ↑ Gene Emery, reuters.com Massive tea consumption linked to kidney failure Apr 1, 2015
  21. ↑ Elahe Izadi washingtonpost.com Why drinking too much iced tea caused this man’s kidneys to fail April 3, 2015
  22. ↑ Bennett Alan Weinberg; 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.
  23. ↑ 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)
  24. ↑ Graham H. N.; Green tea composition, consumption, and polyphenol chemistry; Preventive Medicine 21(3):334-50 (1992)
  25. ↑ "Caffeine and Tea Information". Stash Tea. Retrieved 2009-07-15.
  26. ↑ BDA Supports Dehydration Awareness Week with some Handy Tips (PDF). British Dietetic Association, June 2011.
  27. ↑ Food and Drug Administration (24 February 2011). "Summary of Qualified Health Claims Subject to Enforcement Discretion". Retrieved 9 October 2014.
  28. ↑ National Cancer Institute (5 April 2016). "Tea and Cancer Prevention: Strengths and Limits of the Evidence". Retrieved 20 March 2012.
  29. ↑ Johnson R, Bryant S, Huntley AL (December 2012). "Green tea and green tea catechin extracts: an overview of the clinical evidence". Maturitas (Review). 73 (4): 280–7. doi:10.1016/j.maturitas.2012.08.008. PMID 22986087. Green tea consumption does help reduce body weight and aid weight management as shown in short term RCTs (12 weeks) but not to a clinically relevant level.
  30. ↑ Wang W, Yang Y, Zhang W, Wu W (April 2014). "Association of tea consumption and the risk of oral cancer: a meta-analysis". Oral Oncol (Meta-Analysis). 50 (4): 276–81. doi:10.1016/j.oraloncology.2013.12.014. PMID 24389399.
  31. ↑ Wang Y, Yu X, Wu Y, Zhang D (November 2012). "Coffee and tea consumption and risk of lung cancer: a dose-response analysis of observational studies". Lung Cancer (Meta-Analysis). 78 (2): 169–70. doi:10.1016/j.lungcan.2012.08.009. PMID 22964413.
  32. ↑ Zheng J, Yang B, Huang T, Yu Y, Yang J, Li D (June 2011). "Green tea and black tea consumption and prostate cancer risk: an exploratory meta-analysis of observational studies". Nutr Cancer (Meta-Analysis). 63 (5): 663–72. doi:10.1080/01635581.2011.570895. PMID 21667398.
  33. ↑ Lin YW, Hu ZH, Wang X, Mao QQ, Qin J, Zheng XY, Xie LP (February 2014). "Tea consumption and prostate cancer: an updated meta-analysis". World J Surg Oncol (Meta-Analysis). 12: 38. doi:10.1186/1477-7819-12-38. PMC 3925323Freely accessible. PMID 24528523.
  34. ↑ Zheng JS, Yang J, Fu YQ, Huang T, Huang YJ, Li D (January 2013). "Effects of green tea, black tea, and coffee consumption on the risk of esophageal cancer: a systematic review and meta-analysis of observational studies". Nutr Cancer (Systematic Review and Meta-Analysis). 65 (1): 1–16. doi:10.1080/01635581.2013.741762. PMID 23368908.
  35. ↑ Shen L, Song LG, Ma H, Jin CN, Wang JA, Xiang MX (August 2012). "Tea consumption and risk of stroke: a dose-response meta-analysis of prospective studies". J Zhejiang Univ Sci B (Review). 13 (8): 652–62. doi:10.1631/jzus.B1201001. PMC 3411099Freely accessible. PMID 22843186.
  36. ↑ Larsson SC (January 2014). "Coffee, tea, and cocoa and risk of stroke". Stroke (Review). 45 (1): 309–14. doi:10.1161/STROKEAHA.113.003131. PMID 24326448.
  37. ↑ Munir, KM; Chandrasekaran, S; Gao, F; Quon, MJ (September 2013). "Mechanisms for food polyphenols to ameliorate insulin resistance and endothelial dysfunction: therapeutic implications for diabetes and its cardiovascular complications". American Journal of Physiology, Endocrinology, and Metabolism (Review). 305 (6): E679-86. doi:10.1152/ajpendo.00377.2013. PMC 4073986Freely accessible. PMID 23900418.
  38. ↑ Grassi, D; Desideri, G; Di Giosia, P; De Feo, M; Fellini, E; Cheli, P; Ferri, L; Ferri, C (December 2013). "Tea, flavonoids, and cardiovascular health: endothelial protection.". American Journal of Clinical Nutrition (Review). 98 (6 (supplement)): 1660S-1666S. doi:10.3945/ajcn.113.058313. PMID 24172308.
  39. ↑ Wang, Ze-Mu; Zhou, Bo; Wang, Yong-Sheng; Gong, Qing-Yue; Wang, Qi-Ming; Yan, Jian-Jun; Gao, Wei; Wang, Lian-Sheng (2011-03-01). "Black and green tea consumption and the risk of coronary artery disease: a meta-analysis". The American Journal of Clinical Nutrition. 93 (3): 506–515. doi:10.3945/ajcn.110.005363. ISSN 1938-3207. PMID 21248184.
  40. ↑ Lorenz, Mario; Jochmann, Nicoline; von Krosigk, Amélie; Martus, Peter; Baumann, Gert; Stangl, Karl; Stangl, Verena (2007-01-01). "Addition of milk prevents vascular protective effects of tea". European Heart Journal. 28 (2): 219–223. doi:10.1093/eurheartj/ehl442. ISSN 0195-668X. PMID 17213230.
  41. 1 2 3 Hartley L, Flowers N, Holmes J, Clarke A, Stranges S, Hooper L, Rees K (June 2013). "Green and black tea for the primary prevention of cardiovascular disease". Cochrane Database Syst Rev (Systematic Review and Meta-Analysis). 6: CD009934. doi:10.1002/14651858.CD009934.pub2. PMID 23780706.
  42. ↑ Liu G, Mi XN, Zheng XX, Xu YL, Lu J, Huang XH (October 2014). "Effects of tea intake on blood pressure: a meta-analysis of randomised controlled trials". Br J Nutr (Meta-Analysis). 112 (7): 1043–54. doi:10.1017/S0007114514001731. PMID 25137341.
  43. ↑ Panza, Francesco; Solfrizzi, V.; Barulli, M. R.; Bonfiglio, C.; Guerra, V.; Osella, A.; Seripa, D.; Sabbà, C.; Pilotto, A.; Logroscino, G. (2014). "Coffee, tea, and caffeine consumption and prevention of late-life cognitive decline and dementia: A systematic review". The journal of nutrition, health & aging. 19 (3): 313–328. doi:10.1007/s12603-014-0563-8. ISSN 1279-7707. PMID 25732217.
  44. ↑ Chen B, Shi HF, Wu SC (March 2014). "Tea consumption didn't modify the risk of fracture: a dose-response meta-analysis of observational studies". Diagn Pathol. 9: 44. doi:10.1186/1746-1596-9-44. PMC 4017777Freely accessible. PMID 24588938.
  45. 1 2 Jurgens TM, Whelan AM, Killian L, Doucette S, Kirk S, Foy E (2012). "Green tea for weight loss and weight maintenance in overweight or obese adults". Cochrane Database Syst Rev (Systematic review). 12: CD008650. doi:10.1002/14651858.CD008650.pub2. PMID 23235664.
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