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Health effects of tea

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Health effects of 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 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]

Contents

  • Aluminum, iron and other metals 1
  • Cancer 2
  • Cardiovascular disease 3
  • Fluoride exposure 4
  • Fracture risk 5
  • Hyperlipidemia 6
  • Oxalates 7
  • Theanine and caffeine 8
  • Weight loss 9
  • See also 10
  • References 11
  • External links 12

Aluminum, iron and other metals

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]

Cancer

In 2011, the 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.[9]

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."[10] Though there is some positive evidence for risk reduction of breast, prostate, ovarian and endometrial cancers with green tea, it is weak and inconclusive.[11]

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.[12][13][14][15][16]

Cardiovascular disease

Black tea consumption may be associated with a reduced risk of stroke.[17][18]

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).[19] This conclusion was based on limited evidence.[19] Another meta-analysis of RCTs reached a similar conclusion.[20]

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.[21][22][23][24][25][26] 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.[27][28]

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.[29] 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.[30]

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.[31]

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).[19]

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.[32] Massive black tea consumption has been linked to kidney failure due to its high oxalate content (acute oxalate nephropathy).[33][34]

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[35] and brewing method.[36] Tea also contains small amounts of theobromine and theophylline.[37] Fluoride has also been found to occur in tea, with certain types of brick tea made from old leaves and stems having the highest levels[38] Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations,[39] 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]".[40]

Weight loss

Green tea consumption has no meaningful effect in aiding weight loss.[11]

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 23 (September): 1–11.  
  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.  
  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.  
  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.  
  8. ^ Wierzejska R (2014). "Tea and health--a review of the current state of knowledge". Przegl Epidemiol (Review) 68 (3): 501–6, 595–9.  
  9. ^  
  10. ^ National Cancer Institute (Reviewed 17 November 2010). "Tea and Cancer Prevention: Strengths and Limits of the Evidence". Retrieved 20 March 2012. 
  11. ^ a b 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.  
  12. ^ 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.  
  13. ^ 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.  
  14. ^ 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.  
  15. ^ 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.  
  16. ^ 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.  
  17. ^ 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.  
  18. ^ Larsson SC (January 2014). "Coffee, tea, and cocoa and risk of stroke". Stroke (Review) 45 (1): 309–14.  
  19. ^ a b c 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) 6: CD009934.  
  20. ^ 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.  
  21. ^ 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".  
  22. ^ 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.  
  23. ^ 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.  
  24. ^ 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.  
  25. ^ 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.  
  26. ^ "Calls for FDA to introduce tea fluoride safety standard". Foodnavigator-usa.com. Retrieved 2011-08-04. 
  27. ^ 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
  28. ^ Fawell, J (2006). 92 4 156319 2. WHO.  
  29. ^ 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.  
  30. ^ (25 July 2013) Do fluoride levels in cheap tea pose a health risk? British National Health Service "Choices, Retrieved 26 July 2013
  31. ^ 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.  
  32. ^ Michael Liebman and Shawnna Murphy (2007). "Low oxalate bioavailability from black tea". Nutrition Research 27 (5): 273–278h.  
  33. ^ Gene Emery, reuters.com Massive tea consumption linked to kidney failure Apr 1, 2015
  34. ^ Elahe Izadi washingtonpost.com Why drinking too much iced tea caused this man’s kidneys to fail April 3, 2015
  35. ^ 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.  
  36. ^ 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)
  37. ^ Graham H. N.; Green tea composition, consumption, and polyphenol chemistry; Preventive Medicine 21(3):334-50 (1992)
  38. ^ Fluoride contents in tea and soil from tea plantations and the release of fluoride into tea liquor during infusion ScienceDirect 2015-08-05
  39. ^ "Caffeine and Tea Information". Stash Tea. Retrieved 2009-07-15. 
  40. ^ BDA Supports Dehydration Awareness Week with some Handy Tips (PDF). British Dietetic Association, June 2011.

External links

  • Tea and Cancer Prevention - National Cancer Institute press release
  • Green Tea (an overview from Drugs.com)
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