Tea is after water the most widely consumed drink in the World.
By definition it is an aromatic beverage commonly prepared by pouring hot or boiling water over cured leaves of the Camellia sinensis an evergreen shrub native to Asia .
Tea originated in southwestern China, where it was used as a medicinal drink and became popular as a recreational drink during the Chinese Tang dynasty, and then tea drinking spread to other East Asian countries .
Tea appeared in Europe thanks to Portuguese priests and merchants in 16th century .
During the 17th century tea became fashionable among Britons, who started large-scale production and commercialization of the plant in India to bypass a Chinese monopoly at that time .
Tea is quite often consumed with additions such as a milk, sugar and lemon. Tea is generally divided into six categories :
- white (wilted and unoxidized)
- yellow (unwilted and unoxidized, but allowed to yellow)
- green (unwilted and unoxidized
- oolong (wilted, bruised, and partially oxidized)
- black (wilted, sometimes crushed, and fully oxidized called 'red tea' in China)
- post-fermented (green tea that has been allowed to ferment/compost, called 'black tea' in China)
The most common are white (contains up to 3 times more antioxidants than green one), green, oolong, and black, however we will focus on green and black tea in terms of potential health benefits for humans, but let's first get to know about it’s chemical composition.
What is tea made up of?
Approximately 3% of tea dry weight is caffeine, which can provide between 30-80mg of caffeine per cup (250 ml) depending on type, brand and brewing method.
The difference between green and black caffeine content is significant as 1g of black tea provides 22-28mg and 1g of green tea provides 11-20mg of caffeine .
Tea also contains small amounts of theobromine (similarily like in cocoa), theophylline which are stimulants and xanthines whic are similar to caffeine.
Because of the environmental pollution, fluoride and alluminium also occurs in tea (especially tea bricks or compressed tea) .
Tea (especially black tea) contains tannins, class of compounds which tend ot have a bitter flavor and astringent properties. They are also found in red wine and make dark color of tea.
Antioxidants are the most the most valuable ingredient that tea provides. Polyphenols are the main group found in tea, especially flavonoids, epogallocatechin gallate (EGCG) and other catechins .
How to brew a tea properly?
Black tea – should be brewed in temperature over 90°C to allow the active substances to develop completely. Usually the water near its boiling point (99°C) is the best. Usually brewing takes around 4 minutes, in general, between 30 seconds and 5 minutes. According to ISO when prepared for testing (ISO 3103 – Tea Preparation of liquor for use in sensory test) for comparison and rating purposes – brewing time is 6 minutes .
Green tea – should be steeped in water around 80-85°C, with the rule – the higher quality of the leaves the lower temperature. The timing varies from 1-3 minutes, however steeping for longer can results in more bitterness and less balanced collor. Some regions (North Africa, Centra Asia) prefer a bitter tea and hotter water, and boiling time can be even 15 minutes. High quality green tea can be brewed with new water many times (5 or more) depending on variety with increasing temperature of added water .
White tea – brew in 70-77°C for 1 to 3 minutes, however some sources recommend 7 to 10 minutes. Bear in mind that similarily to green tea , it can be steeped many times 
Oolong tea – should be brewed around 82-96°C for 1-5 minutes, similarily to green one multiple times, and unlikely to green one, seeming to improve with reusing .
Health benefits of drinking tea:
The risk of depression is amendable, and some of the studies suggest that tea may decrease the risk of depression.
Meta analysis of 11 studies has suggested that comparing to individuals with lower tea consumption, those with higher tea consumption (3 cups/day) had 37% lower relative risk of depressia .
Results from another meta-analysis which examined coffee/tea/caffeine intake have suggested, that caffeine consumption was associated with lower relative risk of depressia .
Cardiovascular disease and stroke
Cardiovascular disease (CVD) is the single biggest cause of the in the World. The risk of developing CVD is amendable by diet and physical exercise.
Peters et al performed meta-analysis which has shown that consumption of 3 cups of tea per day was associated with lower risk of myocardial infarction by 11% .
Another results suggested that daily consumption of 3 cups either green or black tea could prevent the onset of ischemic stroke (21% lower relative risk) .
Large chinese prospective study has shown that regular green tea consumption was associated with significantly reduced risk of cardiovascular disease .
Other sciencists have also examined the impact of green on the risk of CVD. The findings have suggested that those who did not consume green tea had higher risk of CVD, intracerebral hemorrhage, cerebral infarction compared to
On the other hand those who drank 1-3 cups and >4 cups per day had reduced risk of myocardial infarction, stroke compared to those who drank < 1cup/day [11,12].
Glaucoma is a leading cause of irreversible blindness worldwide.
The main symptom of this disorder is the loss of the visual field in peripheral to central pattern.
Antioxidants, particulary polyphenols found in green tea, red wine in cocoa have neuroprotecive characteristics and has been suggested as useful in the context of glaucoma .
Meta analysis conducted by Patel et al has shown that flavonoids have a promising role in improving visual function in patients with glaucoma and ocular hypertension .
Diabetes, weight loss and blood pressure
Diabetes mellitus, commonly referred to as diabetes, results in impaired blood glucose lever over the prolonged period.
Liu et al have proven that green tea consumption significantly reduced fasting glucose and hemoglobin A1c (Hb A1c) concentrations .
Jurgens et al have examined 14 studies and suggested that green tea intake induce only small non-significant weight loss in overweight or obese adults.
Green tea had no significant effect on the maintenance of weight loss .
Two meta-analysis have shown that long-term green tea intake (>12 weeks) could result in a significant reduction in systolic and diasystolic BP [16,17].
Cancer is after CVD the most common cause of death worldwide.
It has been suggested that oxidative processes and free-radicals can contribute to carcenogenesis.
Antioxidants, found in tea appears to have protective effect against different forms of cancer .
- prostate cancer - according to the studies consumption of green tea was not associated with localized prostate cancer, however consumption was associated with dose-dependent decrease in the risk of advanced prostate cancer (5 or more cups/day vs
- breast cancer – the results of some studies have shown that green tea can lower the risk of breast cancer 
- gallblader cancer – the researches have shown that tea consumption may reduce the risk of gallbladder cancer in women, but not in men 
- endometrial cancer – some studies have shown that green tea, but not black, may be related to a reduction of endometrial cancer 
- liver cancer – Huang et al have indicated in their meta-analysis that green tea intake can be protective against liver cancer in female Asian population (but not in men) 
- colorectal cancer – regular tea intake was linked with lower risk for both colon and rectal cancer in women 
- thyroid cancer – studies have shown that higher tea consumption can have protective effect on thyroid cancer especially in Europe and America 
Diet plays a role in the onset and progrssion of many metabolic disorders, including fatty liver disease (NAFLD).
Studies have shown that tea ingestion could protect against fibrosis.
Also there were suggestions that it is linked with less likelihood of having metabolic syndrome, however further research are required to confirm those findings .
Another meta-analysis has also confirmed that green tea intake may reduces the risk of liver disease .
A study found that green tea was very effective in reducing oral malodor temporarily because of its disinfectant and deodorant acitivities .
Studies has shown that green tea can inhibit the growth of Streptococcus mutants which is the main harmful bacteria contributing to cavities and tooth decay .
Gargling with tea and flu
Influenza viruses can easily spread across large group of people and every year becoming serious public hazard.
Experimental studies on catechnins (present in tea) appear to prevent influenza infections.
Meta-analysis of 5 studies have shown that participants who gargled with tea of its ingredients showed a lower risk of influenza infection vs those who gargled with placebo or did not gargle at all .
Is drinking tea safe?
Too much of tannins present especially in black tea can impair the absorbtion of non-haem iron, which contributes to the formation of insoluble iron tannate complexes.
The excess of tea, when meals consist of mainly plant food (vegan and vegetarian diets) can contributes to iron deficiency (anemia) .
Tea plant and leaves can absorb significant amount of aluminium, however with moderate intake and using water with normal pH it is not an issue.
According to the research, aluminium from tea can be absorbed only in 4.96-9.13%, so normally drinking higher amount of tea is not an issue once kidney function and normal aluminium excretion are not impaired .
However drinking lemon tea bags or adding lemon to tea can modify the conditions of infusion.
Studies have shown that tea infusions made from lemon tea bags contained even 10-70 fold higher content of elements Al and heavy metals Cd and Pb .
The exposition on Al have been linked with Alzheimer's disease and neurotoxicity .
Similarily drinking beverages (also a tea) out of aluminium cans, especially the damaged one can lead to exceeding exposure on aluminum .
If you take any medications, talk to your doctor to see if drugs you take may interfere with tea costituents .
Limit your tea intake when you pregnant
According to the National Health Services (UK) everyday consumption of caffeine should be limited to 200mg .
High caffeine intake was associated with a significant increase in risk of low body weight (LBW) and this risk appears to increase linearly as caffeine intake increases .
Better without milk
Most of the beneficial effects of drinking tea come from antioxidants and caffeine intake, but addition of milk (10, 15, and 20 ml of whole, semiskimmed and skimmed bovine milk in 200-ml tea infusion) seems to decrease the antioxidant capacity of tea .
Milk counteracts the favourable health effects of tea on vascular function.
The study has found that black tea significantly improved flow-mediated dilation (vascular function parameter), however addition of the milk completely blunted the effects of tea .
Tea is after water the most popular beverage worldwide.
It has originated in China, however in 16th century became trending in Europe.
Tea health benefits are suggested to be caused by the compounds: polyphenols and caffeine.
The intake of tea can have favorable effects on disorders like: depression, oral health, glaucoma, cardiovascular diseases, high blood pressure, non-alcoholic fatty liver disease, diabetes and different form of cancers.
Appropriate tea preparation: timing, tea quality, not adding lemon/infusing lemon tea bag, not adding milk ensure (antioxidant capacity) ensure good impact on health.
Adults and children with kidneys impairments, iron deficient/borderline, vegan/vegetarian (who mainly eat plant based diet), pregnant women should limit their tea consumption.
Patients taking mediciations, should inform a doctor, once their take medications and drink a lot of tea.
According to the doses recommended in the above studies it seems that intake around 1-3 cups/day for adults seems to be reasonable addition to a diet and do not cause side effects.
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