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Health effects of tea

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1912 advertisement for tea in the Sydney Morning Herald, describing its supposed health benefits

The health effects of tea have been studied throughout human history. In clinical research conducted over the early 21st century, tea has been studied extensively for its potential to lower the risk of human diseases, but there is no good scientific evidence to support any therapeutic uses other than possibly increasing alertness, an effect caused by caffeine in the tea leaves.[1][2]

A small number of studies suggests that both green and black tea might have beneficial effects on some cardiovascular disease risk factors, including blood pressure and cholesterol.[1] The research has limitations though, including how the data were evaluated and differences in study populations, leading to no certain conclusions about health effects.[1][2]

In regions without access to safe drinking water, boiling water to make tea is effective for reducing waterborne diseases by destroying pathogenic microorganisms.

By constituents or substances

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Aluminum, iron and other metals

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Further information: Aluminium § Toxicity

Tea drinking accounts for a high proportion of aluminum in the human diet.[3] The levels are safe, but there has been some concern that aluminum traces may be associated with Alzheimer's disease. A 2013 study additionally indicated that some teas contained lead (mostly Chinese) and aluminum (Indian/Sri Lanka blends, China).[4] There is still insufficient evidence to draw firm conclusions on this subject.[5]

Most studies have found no association between tea intake and iron absorption.[6] However, drinking excessive amounts of black tea may inhibit the absorption of iron, and may harm people with anaemia.[7]

Concerns have been raised about the traditional method of over-boiling tea to produce a decoction, which may increase the amount of environmental contaminants released and consumed.[8]

Fluoride exposure

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See also: Fluoride toxicity

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]

The fluoride content of a tea leaf depends on the leaf picking method used and the fluoride content of the soil from which it has been 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.[11] 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 6 mg of fluoride, above the recommended average dietary intake level of 3–4 mg of fluoride per day, but below the maximum tolerable amount of 10 mg of fluoride per day.[12] Brick tea, made from fallen leaves, old leaves and stems has the highest levels.[13]

One study indicated that green tea leaves have an average fluoride concentration of 52 mg/kg, and approximately 89% of the fluoride was released from the leaves into tea after brewing.[14]

Oxalates

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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.[15] Massive black tea consumption has been linked to kidney failure due to its high oxalate content (acute oxalate nephropathy).[16][17]

Theanine and caffeine

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See also: Health effects of 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 US fluid ounces (240 mL) depending on type, brand[18] and brewing method.[19] Tea also contains small amounts of theobromine and theophylline.[20] Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations,[21] 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]".[22]

By conditions

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Cognitive effects

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Drinking caffeinated tea may improve mental alertness due to the effects of caffeine.[1]

Cancer

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In 2011, the US Food and Drug Administration reported that there was little evidence to support the claim that green tea consumption may reduce the risk of breast and prostate cancer.[23]

A 2010 report by the US National Cancer Institute stated that epidemiological studies and the few clinical trials of tea for the prevention of cancer were inconclusive.[24] 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."[24] Though there is some positive evidence for risk reduction of breast, prostate, ovarian, and endometrial cancers with green tea, it is weak and inconclusive.[25]

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.[26][27][28][29][30] In 2018, a meta-analysis based on 14 case-control studies found that tea consumption appears protective of oral cancer with a greater decrease in risk occurring with a larger intake (except for black tea and American people).[31]

The consumption of very hot tea could increase the risk of esophageal cancer.[32]

Cardiovascular disease

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In preliminary long-term clinical studies, black tea consumption showed evidence for providing a small reduction in the risk of stroke,[33][34] whereas, in another review, green tea and black tea did not have significant effects on the risk of coronary heart disease.[35] Two reviews of randomized controlled trials concluded that long-term consumption of black tea slightly lowers systolic and diastolic blood pressures (about 1–2 mmHg), a finding based on limited evidence.[36][37] A 2013 Cochrane review found some evidence of benefit from tea consumption on cardiovascular markers (total and LDL cholesterol), though more research is needed.[36]

Fracture risk

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Tea consumption does not appear to affect the risk of bone fracture including hip fractures or fractures of the humerus in men or women.[38]

Weight loss

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Although green tea is commonly believed to be a weight loss aid, there is no good evidence that its long-term consumption has any meaningful benefit in helping overweight or obese people to lose weight, or that it helps to maintain a healthy body weight.[39][40] Use of green tea for attempted weight loss carries a small risk of adverse effects, such as nausea, constipation, and stomach discomfort.[39]

See also

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References

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  1. ^ a b c d "Black tea". MedlinePlus. US National Library of Medicine. 6 October 2022.
  2. ^ a b "Green tea". National Center for Complementary and Integrative Health, US National Institutes of Health. October 2020.
  3. ^ 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.
  4. ^ Schwalfenberg, Gerry; Genuis, Stephen J.; Rodushkin, Ilia (2013). "The Benefits and Risks of Consuming Brewed Tea: Beware of Toxic Element Contamination". Journal of Toxicology: 1–8. doi:10.1155/2013/370460. PMC 3821942. PMID 24260033.
  5. ^ 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.
  6. ^ 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 4179187. PMID 25244367.
  7. ^ 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.
  8. ^ 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. S2CID 21497059.
  9. ^ 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.
  10. ^ 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.
  11. ^ 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. S2CID 127931635.
  12. ^ (25 July 2013) Do fluoride levels in cheap tea pose a health risk? British National Health Service "Choices, Retrieved 26 July 2013
  13. ^ Fung KF, Zhang ZQ, Wong JW, Wong MH (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.
  14. ^ Maleki, Afshin; Daraei, Hiua; Mohammadi, Elham; Zandi, Shiva; Teymouri, Pari; Mahvi, Amir Hossien; Gharibi, Fardin (28 March 2016). "Daily Fluoride Intake from Iranian Green Tea: Evaluation of Various Flavorings on Fluoride Release". Environmental Health Insights. 10: 59–63. Bibcode:2016EnvHI..10S8511M. doi:10.4137/EHI.S38511. PMC 4811265. PMID 27042093.
  15. ^ 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.
  16. ^ Emery, Gene (1 April 2015). "Massive tea consumption linked to kidney failure". Reuters. Archived from the original on 7 March 2018. Retrieved 13 December 2020.
  17. ^ Elahe Izadi washingtonpost.com Why drinking too much iced tea caused this man’s kidneys to fail 3 April 2015
  18. ^ 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 978-0-415-92722-2. Retrieved 20 September 2008.
  19. ^ 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)
  20. ^ Graham H. N.; Green tea composition, consumption, and polyphenol chemistry; Preventive Medicine 21(3):334-50 (1992)
  21. ^ "Caffeine and Tea Information". Stash Tea. Archived from the original on 13 May 2011. Retrieved 15 July 2009.
  22. ^ BDA Supports Dehydration Awareness Week with some Handy Tips (PDF). British Dietetic Association, June 2011.
  23. ^ Food and Drug Administration (24 February 2011). "Summary of Qualified Health Claims Subject to Enforcement Discretion". Food and Drug Administration. Retrieved 9 October 2014.
  24. ^ a b National Cancer Institute (17 November 2010). "Tea and Cancer Prevention: Strengths and Limits of the Evidence". Retrieved 7 December 2022.
  25. ^ 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.
  26. ^ 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.
  27. ^ 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.
  28. ^ 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. S2CID 21567675.
  29. ^ 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 3925323. PMID 24528523.
  30. ^ 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. S2CID 8612872.
  31. ^ Zhou, Hao; Wu, Weiwei; Wang, Fengqin; Qi, Huizhong; Cheng, Zhigang (21 December 2018). "Tea consumption is associated with decreased risk of oral cancer". Medicine. 97 (51): e13611. doi:10.1097/MD.0000000000013611. ISSN 0025-7974. PMC 6320052. PMID 30572470.
  32. ^ Zhong, Y; Yang, C; Wang, N; Pan, D; Wang, S; Sun, G (2022). "Hot Tea Drinking and the Risk of Esophageal Cancer: A Systematic Review and Meta-Analysis". Nutrition and Cancer. 74 (7): 2384–2391. doi:10.1080/01635581.2021.2007963. PMID 34818954. S2CID 244682963.
  33. ^ 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 3411099. PMID 22843186.
  34. ^ 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.
  35. ^ Wang, Ze-Mu; Zhou, Bo; Wang, Yong-Sheng; Gong, Qing-Yue; Wang, Qi-Ming; Yan, Jian-Jun; Gao, Wei; Wang, Lian-Sheng (1 March 2011). "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.
  36. ^ a b 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" (PDF). Cochrane Database Syst Rev (Systematic Review and Meta-Analysis). 2013 (6): CD009934. doi:10.1002/14651858.CD009934.pub2. PMC 7433290. PMID 23780706.
  37. ^ 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.
  38. ^ 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 4017777. PMID 24588938.
  39. ^ a b 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). 2012 (12): CD008650. doi:10.1002/14651858.CD008650.pub2. PMC 8406948. PMID 23235664.
  40. ^ Kovacs EM (March 2004). "Effects of green tea on weight maintenance after body-weight loss". British Journal of Nutrition. 91 (3): 431–437. doi:10.1079/BJN20041061. PMID 15005829.
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