Health effects of tea
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In clinical research conducted during the early 21st century, the health effects of tea has been widely studied.
Black tea is considered likely effective for improving alertness and possibly effective for certain conditions such as low blood pressure, but evidence does not support its effectiveness for preventing several types of cancer or diabetes; there is currently limited high-quality scientific evidence supporting most specific therapeutic uses of black tea.<ref name="mlp">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The United States Food and Drug Administration has approved a topical ointment formulated with a specific green tea extract for the treatment of external genital and perianal warts; although green tea and its extracts have been widely studied, the evidence remains inconclusive overall, with only modest or inconsistent benefits observed.<ref name="nccih">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref name="Hartley2013">Template:Cite journal</ref><ref name="Liu2014">Template:Cite journal</ref> A 2013 Cochrane review found some evidence of benefit from tea consumption on cardiovascular markers (total and LDL cholesterol).<ref name="Hartley2013"/>
In regions without access to safe drinking water, boiling water is effective for reducing waterborne diseases by destroying pathogenic microorganisms, and this water can be used to make tea.
By constituents or substancesEdit
Aluminum, iron and other metalsEdit
Template:Further Tea drinking accounts for a high proportion of aluminum in the human diet.<ref>Template:Cite journal</ref> 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).<ref>Template:Cite journal</ref> There is still insufficient evidence to draw firm conclusions on this subject.<ref>Template:Cite journal</ref>
Most studies have found no association between tea intake and iron absorption.<ref name="beck">Template:Cite journal</ref> However, drinking excessive amounts of black tea may inhibit the absorption of iron, and may harm people with anaemia.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
Fluoride exposureEdit
Template:See also 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.<ref name="pmid17410113">Template:Cite journal</ref><ref name="pmid18078704">Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref> 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.<ref>(25 July 2013) Do fluoride levels in cheap tea pose a health risk? British National Health Service "Choices, Retrieved 26 July 2013</ref> Brick tea, made from fallen leaves, old leaves and stems has the highest levels.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
OxalatesEdit
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.<ref>Template:Cite journal</ref> Massive black tea consumption has been linked to kidney failure due to its high oxalate content (acute oxalate nephropathy).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Elahe Izadi washingtonpost.com Why drinking too much iced tea caused this man’s kidneys to fail 3 April 2015</ref>
Theanine and caffeineEdit
Template:See also Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per Template:Convert depending on type, brand<ref>Template:Cite book</ref> and brewing method.<ref>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)</ref> Tea also contains small amounts of theobromine and theophylline.<ref>Graham H. N.; Green tea composition, consumption, and polyphenol chemistry; Preventive Medicine 21(3):334-50 (1992)</ref> Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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]".<ref>BDA Supports Dehydration Awareness Week with some Handy Tips (PDF). British Dietetic Association, June 2011.</ref>
By conditionsEdit
Cognitive effectsEdit
Drinking caffeinated tea may improve mental alertness due to the effects of caffeine.<ref name=mlp/>
CancerEdit
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.<ref name="FDA2011">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref name=NCI/> 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."<ref name=NCI>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Though there is some positive evidence for risk reduction of breast, prostate, ovarian, and endometrial cancers with green tea, it is weak and inconclusive.<ref name=Johnson>Template:Cite journal</ref>
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.<ref name="OralCancer2014">Template:Cite journal</ref><ref name="LungCancer2012">Template:Cite journal</ref><ref name="ProstateCancer2011">Template:Cite journal</ref><ref name="ProstateCancer2014">Template:Cite journal</ref><ref name="EsophagealCancer2013">Template:Cite journal</ref> 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).<ref>Template:Cite journal</ref>
The consumption of very hot tea could increase the risk of esophageal cancer.<ref>Template:Cite journal</ref>
Cardiovascular diseaseEdit
In preliminary long-term clinical studies, black tea consumption showed evidence for providing a small reduction in the risk of stroke,<ref name="Stroke2012">Template:Cite journal</ref><ref name="Stroke2014">Template:Cite journal</ref> whereas, in another review, green tea and black tea did not have significant effects on the risk of coronary heart disease.<ref>Template:Cite journal</ref> 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.<ref name="Hartley2013">Template:Cite journal</ref><ref name="Liu2014">Template:Cite journal</ref> A 2013 Cochrane review found some evidence of benefit from tea consumption on cardiovascular markers (total and LDL cholesterol), though more research is needed.<ref name="Hartley2013"/>
Fracture riskEdit
Tea consumption does not appear to affect the risk of bone fracture including hip fractures or fractures of the humerus in men or women.<ref name="Chen2014">Template:Cite journal</ref>
Weight lossEdit
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.<ref name=coch-gt>Template:Cite journal</ref><ref>Template:Cite journal</ref> Use of green tea for attempted weight loss carries a small risk of adverse effects, such as nausea, constipation, and stomach discomfort.<ref name=coch-gt/>
See alsoEdit
- Coffee
- Decaffeination
- Detox tea
- gamma-Glutamylmethylamide
- Health effects of coffee
- Phenolic content in tea
- Release of microplastics by tea bags
- Traditional medicine
ReferencesEdit
External linksEdit
- Tea and Cancer Prevention – National Cancer Institute press release
- Green Tea (an overview from Drugs.com)