Health effects of tea

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1912 advertisement for tea in the Sydney Morning Herald, describing its supposed health benefits Robur tea advert 1912.png
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]

Contents

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

Aluminum, iron and other metals

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

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

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

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

Cognitive effects

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

Cancer

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

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

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

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

Related Research Articles

<span class="mw-page-title-main">Caffeine</span> Central nervous system stimulant

Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class and is the most commonly consumed psychoactive substance globally. It is mainly used for its eugeroic, ergogenic, or nootropic (cognitive-enhancing) properties. Caffeine acts by blocking binding of adenosine at a number of adenosine receptor types, inhibiting the centrally depressant effects of adenosine and enhancing the release of acetylcholine. Caffeine has a three-dimensional structure similar to that of adenosine, which allows it to bind and block its receptors. Caffeine also increases cyclic AMP levels through nonselective inhibition of phosphodiesterase, increases calcium release from intracellular stores, and antagonizes GABA receptors, although these mechanisms typically occur at concentrations beyond usual human consumption.

Fluoride is an inorganic, monatomic anion of fluorine, with the chemical formula F−
, whose salts are typically white or colorless. Fluoride salts typically have distinctive bitter tastes, and are odorless. Its salts and minerals are important chemical reagents and industrial chemicals, mainly used in the production of hydrogen fluoride for fluorocarbons. Fluoride is classified as a weak base since it only partially associates in solution, but concentrated fluoride is corrosive and can attack the skin.

<span class="mw-page-title-main">Green tea</span> Unoxidized tea

Green tea is a type of tea that is made from Camellia sinensis leaves and buds that have not undergone the same withering and oxidation process which is used to make oolong teas and black teas. Green tea originated in China, and since then its production and manufacture has spread to other countries in East Asia.

Caffeinism is a state of intoxication caused by excessive consumption of caffeine. This intoxication covers a variety of unpleasant physical and mental symptoms associated with the consumption of excessive amounts of caffeine.

<span class="mw-page-title-main">Diet soda</span> Soft drink with little or no sugar or calories

Diet sodas are soft drinks which contain little or no sugar or calories. First introduced onto the market in 1949, diet sodas are typically marketed for those with diabetes or who wish to reduce their sugar or caloric intake.

<span class="mw-page-title-main">Caffeinated drink</span> Type of drink

A caffeinated drink, or caffeinated beverage, is a drink that contains caffeine, a stimulant that is legal practically all over the world. Some are naturally caffeinated while others have caffeine added as an ingredient.

Decaffeination is the removal of caffeine from coffee beans, cocoa, tea leaves, and other caffeine-containing materials. Decaffeinated products are commonly termed by the abbreviation decaf. Decaffeinated drinks contain typically 1–2% of the original caffeine content, but sometimes as much as 20%.

Recurrent miscarriage or recurrent pregnancy loss (RPL) is the spontaneous loss of 2-3 pregnancies that is estimated to affect up to 5% of women. The exact number of pregnancy losses and gestational weeks used to define RPL differs among medical societies. In the majority of cases, the exact cause of pregnancy loss is unexplained despite genetic testing and a thorough evaluation. When a cause for RPL is identified, almost half are attributed to a chromosomal abnormality. RPL has been associated with several risk factors including parental and genetic factors, congenital and acquired anatomical conditions, lifestyle factors, endocrine disorders, thrombophila, immunological factors, and infections. The American Society of Reproductive Medicine recommends a thorough evaluation after 2 consecutive pregnancy losses; however, this can differ from recommendations by other medical societies. RPL evaluation can be evaluated by numerous tests and imaging studies depending on the risk factors. These range from cytogenetic studies, blood tests for clotting disorders, hormone levels, diabetes screening, thyroid function tests, sperm analysis, antibody testing, and imaging studies. Treatment is typically tailored to the relevant risk factors and test findings. RPL can have a significant impact on the psychological well-being of couples and has been associated with higher levels of depression, anxiety, and stress. Therefore, it is recommended that appropriate screening and management be considered by medical providers.  

<span class="mw-page-title-main">Caffeine dependence</span> Medical condition

Caffeine dependence is a condition characterized by a set of criteria, including tolerance, withdrawal symptoms, persistent desire or unsuccessful efforts to control use, and continued use despite knowledge of adverse consequences attributed to caffeine. It can appear in physical dependence or psychological dependence, or both. Caffeine is one of the most common additives in many consumer products, including pills and beverages such as caffeinated alcoholic beverages, energy drinks, pain reliever medications, and colas. Caffeine is found naturally in various plants such as coffee and tea. Studies have found that 89 percent of adults in the U.S. consume on average 200 mg of caffeine daily. One area of concern that has been presented is the relationship between pregnancy and caffeine consumption. Repeated caffeine doses of 100 mg appeared to result in smaller size at birth in newborns. When looking at birth weight however, caffeine consumption did not appear to make an impact.

<span class="mw-page-title-main">Milk tea</span> Beverage mainly composed of tea and milk

Milk tea refers to several forms of beverage found in many cultures, consisting of some combination of tea and milk. The term milk tea is used for both hot and cold drinks that can be combined with various kinds of milks and a variety of spices. This is a popular way to serve tea in many countries, and is the default type of tea in many South Asian countries. Beverages vary based on the amount of each of these key ingredients, the method of preparation, and the inclusion of other ingredients Milk tea is the default type of tea in India and Pakistan and referred to as chai.

<span class="mw-page-title-main">Theanine</span> Amino acid

Theanine, commonly known as L-theanine and sometimes L-gamma-glutamylethylamide or N5-ethyl-L-glutamine, is an amino acid analogue of the proteinogenic amino acids L-glutamate and L-glutamine and is found primarily in particular plant and fungal species. It was discovered as a constituent of green tea in 1949 and isolated from gyokuro leaves in 1950, thus rendering it a natural product. It constitutes about 1–2% of the dry weight of green tea leaves.

<span class="mw-page-title-main">Alcohol and cancer</span> Relationship between cancer and the consumption of alcohol

Alcohol and cancer have a complex relationship. Alcohol causes cancers of the oesophagus, liver, breast, colon, oral cavity, rectum, pharynx, and larynx, and probably causes cancers of the pancreas. Cancer risk can occur even with light to moderate drinking. The more alcohol is consumed, the higher the cancer risk, and no amount can be considered completely safe.

<span class="mw-page-title-main">Nutrition and pregnancy</span> Nutrient intake and dietary planning undertaken before, during and after pregnancy

Nutrition and pregnancy refers to the nutrient intake, and dietary planning that is undertaken before, during and after pregnancy. Nutrition of the fetus begins at conception. For this reason, the nutrition of the mother is important from before conception as well as throughout pregnancy and breastfeeding. An ever-increasing number of studies have shown that the nutrition of the mother will have an effect on the child, up to and including the risk for cancer, cardiovascular disease, hypertension and diabetes throughout life.

The health effects of coffee include various possible health benefits and health risks.

<span class="mw-page-title-main">Diet and cancer</span> Connections between dietary habits and cancer

Many dietary recommendations have been proposed to reduce the risk of cancer, few have significant supporting scientific evidence. Obesity and drinking alcohol have been correlated with the incidence and progression of some cancers. Lowering the consumption of sweetened beverages is recommended as a measure to address obesity.

<span class="mw-page-title-main">Epigallocatechin gallate</span> Catechin (polyphenol) in tea

Epigallocatechin gallate (EGCG), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid, and is a type of catechin.

<span class="mw-page-title-main">Weight management</span> Techniques for maintaining body weight

Weight management comprises behaviors, techniques, and physiological processes that contribute to a person's ability to attain and maintain a healthy weight. Most weight management techniques encompass long-term lifestyle strategies that promote healthy eating and daily physical activity. Weight management generally includes tracking weight over time and identifying an individual's ideal body weight.

<span class="mw-page-title-main">Phenolic content in tea</span> Natural plant compounds

The phenolic content in tea refers to the phenols and polyphenols, natural plant compounds which are found in tea. These chemical compounds affect the flavor and mouthfeel of tea. Polyphenols in tea include catechins, theaflavins, tannins, and flavonoids.

<span class="mw-page-title-main">Sweetened beverage</span> Type of beverage

Sugar-sweetened beverages (SSB) are beverages with added sugar. They have been described as "liquid candy". Added sugars include brown sugar, corn sweetener, corn syrup, dextrose, fructose, high fructose corn syrup, honey, invert sugar, lactose, malt syrup, maltose, molasses, raw sugar, sucrose, trehalose, and turbinado sugar. Naturally occurring sugars, such as those in fruit or milk, are not considered to be added sugars. Free sugars include monosaccharides and disaccharides added to foods and beverages by the manufacturer, cook or consumer, and sugars naturally present in honey, syrups, fruit juices and fruit juice concentrates.

<span class="mw-page-title-main">Black tea</span> Type of tea

Black tea is a type of tea that is more oxidized than oolong, yellow, white, and green teas. Black tea is generally stronger in flavour than other teas. All five types are made from leaves of the shrub Camellia sinensis, though Camellia taliensis is also rarely used.

References

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