Green Tea a natural source of antioxidants

Catechins to protect your health

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Green tea is a natural source of powerful antioxidants called catechins, and drinking green tea has significant health benefits, such as cardiovascular disease and cancer prevention, improved bone density, lower cholesterol, antimicrobial, antiviral and antifungal properties.

However excessive consumption of green tea extracts or taking it as a supplement in high concentrations can impair iron absorption and cause severe liver damage -infusions are generally considered safe.

In this article we will look into the extraordinay health benefits and the potential adverse effects of green tea, backed by hard facts and scientific studies.

Antioxidant rich green tea. A. Whittall

Green Tea

What is it?

Tea is produced from the leaves of the plant Camellia sinensis, which is native to eastern Asia.

It is an evergreen bush that can grow to a height of 27 feet (9 m).

Its leaves are picked and processed to produce the four known varieties of tea: white, green, black and Oolong.

Roughly 2.5 million tons of tea leaves are harvested each year of which roughly 20% become green tea (78% is black tea and around 2% is white and Oolong tea).

White Tea

This is a rare variety produced only in China from new growth buds and fresh unopened tea leaves which are covered with soft white "hair". This gives it its name: "White tea".

After being harvested it is allowed to wither without letting it oxidize, and it is steamed dry. As it undergoes the least processing of all tea leaves it has the highest amount of antioxidants -they aren't destroyed during processing. ( 3 ) ( 2 )

Green Tea

Fresh leaves are immediately steamed or pan-dried after being harvested. This process inactivates enzymes and preserves most of the leaves' antioxidants. (3) it also keeps them from turning brown due to oxidation -just like an apple turns brown, so does tea. That is why green tea keeps its characteristic green color during the subsequent rolling and drying processes. ( 1 )

Black Tea

The leaves are allowed to wither and are then crushed so that the enzymes oxidize them, a process known as "fermentation".

The process oxidizes the antioxidant catechins and creates theaflavins and thearubbigins.

Oolong Tea

Is an intermediate step between green and black teas: it is partially fermented and therefore contains more antioxidants (catechins) than black tea, but less than green tea.

Take home point

The way green tea is processed ensures that most of its healthy antioxidants are preserved.

basket with freshly picked green tea leaves
Green tea leaves

Green Tea's antioxidants

Green tea is a complex mixture of chemicals, below is the typical composition expressed as dry weight:

  • 30%. Antioxidants such as polyphenols (flavanols, flavandiols, flavonoids, and phenolic acids).
  • 15 - 20%. Proteins
  • 5 - 7%. Carbohydrates (cellulose, glucose, fructose, sucrose and pectins).
  • 1 - 4%. Amino acids (theanine, typtophan, glycine, glutamic acid, serine, tyrosine, leucine, lysine, arginine, threonine, valine and aspartic acid).
  • 5%. Minerals: calcium, magnesium, chromium, manganese, iron, sodium, zinc, copper, molybdenum, phosphorous, selenium, cobalt, strontium, nickel, potassium, fluorine, and aluminum (this last metal can cause some health issues).
  • <1%. Trace amounts of lipids -oils- such as linoleic and a-linolenic acids and sterols (stigmasterol). Vitamins (B, C, E), pigments (carotenoids and chlorophyll), alkaloids such as caffeine, and some volatile compounds (aldehydes, alcohols, esters, etc).


The polyphenol content of tea (30% of fresh leaf dry weight) is the reason for its health-promoting effects and among them flavonols and flavanols stand out.

Flavanols (with an "A") include flavan-3-ols such as catechins which display potent antioxidant activity:

The four main catechins found in tea are:( 4 )

  • 60% epigallocatechin-3-gallate (EGCG)
  • 20% epigallocatechin (EGC)
  • 14% epicatechin-3-gallate (ECG)
  • 6% epicatechin (EC)

Green tea has the highest content of catechins: 27% (dry leaf weight) followed by Oolong tea (23%) and black tea (4%).

How many antioxidants are in your cup of Green Tea?

Brewing tea extracts the catechins from the tea leaves making them available in the green tea infusion. Depending on the brewing conditions (temperature and steep time) more or less catechins will be available. This was studied by a group of Turkish scientists ( 5 ), who determined the optimal brewing conditions for green tea: water at 185°F (85°C) and a 3-minute brewing time.

Under these ocnditions catechin content was maximized, especially that of the epistructured catechins (EGCG, EGC, ECG and EC).

One cup of green tea brewed under those conditions, contained (one US legal cup = 240 ml):

  • 121.7 mg of EGCG
  • 64.5 mg of EGC
  • 9.2 mg of ECG
  • 16.18 mg of EC
  • 234.5 mg of total catechins*

* Note: total catechins also includes non-epistructured catechins such as C, GC, GCG.

Also, the best taste, aroma, color and caffeine content were obtained under these conditions.

Tea epicatechins are very stable when exposed to heat especially if the water is slightly acidic (a few drops of lemon juice can help) (3).

Milk should be avoided (3) because milk protein bind to flavonoids, forming compounds that resist breakdown in the stomach effectively keeping the polyphenols unavailable for absortion in the intestines.

Health Benefits of Green Tea

Several scientific studies have found that consuming green tea can have potent protective effects in preventing cancer and coronary & heart disease.

It also has antibacterial and anti-inflammatory effects, it improves oral health, it lowers blood pressure and "bad" LDL cholesterol levels. It can also impact positively on weight loss, improve bone density and protect against depression and neurodegenerative diseases. (1)

Let's look into each of these health benefit claims:

Prevention of Cancer

Tea's polyphenols have a potent antioxidant effect which may explain why green tea consumption has been identified as an agent that may prevent many types of cancer such as colon, mouth, stomach, esophagus, kidney, pancreas, breast, small intestine and colon. (1)

Tea components possess antioxidant, antimutagenic, and anticarcinogenic effects and could protect humans against the risk of cancer by environmental agents. Chacko, Thambi, Kuttan, Nishigaki, (2010). (1)

According to a recent study (Fujiki, 2018) ( 6 ) green tea's main antioxidant, EGCG has cancer preventive effects:

  • It delays the onset of cancer: drinking the equivalent of 5 cups of green tea per day, delayed cancer onset 7.3 years in Japanese female patients (3.2 years in men), compared to those who drank less than 1.5 cups ⁄ day.
  • It prevents recurrence of cancer: Breast cancer patients that drank more than 5 cups per day had a lower recurrence rate (16.7%) and were free from the disease for a longer period (3.6 years), than those consuming less than 4 cups per day (24.3% and 2.8 years).
  • Colorectal adenoma recurrence halved in two studies (15% vs. 31% in Japan and 27.8% vs. 60.6% in S. Korea) when the patients drank Green Tea and consumed a Green Tea Extract supplement which were equivalent to a total of 8 cups per day.
  • EGCG maintained cancer cells in place (they became stiffer), and this kept them from moving to other parts of the body (metastases) forming new tumors. This metastases inhibitory effect was observed in melanoma cells and in lung cancer cells in mice.

Stomach cancer risk decreases with an increase in the quantity of tea consumed (perhaps due to its antimicrobial activity on Helicobacter pylori, the microbe that causes ulcers and stomach cancer. (3)

Green -and black- tea apparently act upon the expression of certain genes that play a role in causing pancreatic cancer in humans and therefore act as chemopreventive agents against this type of cancer in humans. (3)

Possible anti-cancer mechanisms of Green Tea

Polyphenols seem to inhibit cell proliferation and in mice also increase the activity of antioxidants in specific organs, which means that green tea catechins have a chemoprotective effect in those organs.

Green tea also promotes the death of cancer cells and blocks their growth.

Another possible factor involves gap junction communication between cells: catechins enhance this communication which may protect from tumor development (the loss of direct intercellular communication is linked to cancer onset and progression).

Antimicrobial & antiviral, antifungal activity of Green Tea

Catechins found in green tea have antimicrobial effects on microorganisms such as viruses, fungi, parasites, bacteria and even prions (distorted proteins that cause the "mad cow disease").

These catechins reinforce the activity of conventional antibiotics and their anti-inflammatory and antioxidant effects contribute to their antimicrobial potential. (4)

Among microorganisms inhibited by tea are: Vibrio cholerae, Salmonella typhi, Campylobacter jejuni, Campylobacter coli, Helicobacter pylori, Shigella, Salmonella, Clostridium pseudomonas, Candida and Mycoplasma. (3)

This antimicrobial activity of green tea can also improve oral health issues:

Green tea and oral health

Green tea protects against dental caries and plaque by controling bacteria this can also help prevent periodontal diseases.

Bad breath or halitosis is neutralized by EGCG and other catechins which deodorizes foul smelling methyl mercaptan, a sulfur compound formed by anaerobic bacteria that live in the mouth. ( 7 )

Green Tea's Anti-inflammatory activity

Lupus was effectively treated in a randomized study involving 80 women, which were given an oral supplement twice a day with green tea extract (a total dose of 1 g per day with 121.8 mg of EGCG).

This is equivalent to drinking one cup of infused green tea every day.

After 12 weeks the treated group had better scores in quality of life, general health, vitality than the control (placebo) group. They also had significantly lower auto-antibodies levels. ( 8 )

Reduced risk of coronary and heart disease

Green tea, and to a lesser extent, black tea, have a protective effect against cardiovascular disease. The mechanisms involve lower blood pressure, reduced cholesterol and weight-loss effects:

Lower Blood Pressure

Epidemiological studies and clinical trials reveal that green tea (and also black tea) lowers blood pressure through their antioxidant properties; this in turn significantly reduces the risk of stroke and coronary heart disease. (1)(3)

Reduced "bad" LDL Cholesterol

A meta-analysis (a statistical procedure that combines data from multiple studies) of 17 clinical trials involving 1,356 subjects revealed that green tea's catechin EGCG lowers the blood levels of LDL cholesterol: " consumption of green tea EGCG resulted in a significant reduction of LDL-C". ( 9 )

The drop averaged 9.29 mg⁄dl, which is significant (the optimal value for LDL cholesterol is 100 mg⁄dl).

Its effect on weight loss (more on this below) may also play an important role in its heart disease prevention effects.

Weight Loss and Green Tea

A meta-analysis involving 11 separate studies on found that catechins and EGCG have "a small positive effect on weight loss and weigh maintenance... Catechins significantly decreased body weight and significantly maintained body weight after a period of weight loss".

Weight loss averaged 2.88 lbs (1.31 kg). ( 10 )

Although caffeine has an impact on energy expenditure, the green tea polyphenols may also influence the amount of energy that the body burns by elevating metabolic rate and increasing fat oxidation. (3)

Improved bone density

Green tea lowers the risk of osteoporosis and the risk of hip fractures in subjects aged 50 and over. Women (ages 65 to 76) that consumed tea had higher bone mineral density.(3)

The green tea effect is independent of other factors such as smoking, hormone replacement therapy, coffee drinking or adding milk to tea. These positive effects are due to the catechins activity upon bone cells.(1).

Other beneficial health effects of green tea

Green tea has many more positive effects on our health, below we list some of them:

Chronic liver diseases

If not consumed in excess (see its risks and side effects below) green tea can help control de proliferation of hepatic stellate cells, which are linked to fibrosis in chronic liver disease. The antioxidant effects of EGCG inhibits their growth. (1)

Neurodegenerative diseases

Despite the fact that the aluminium content in tea may affect the central nervous system and have some link to Alzheimer's disease (more about this here), studies have shown that its antioxidants protect against neurodegenerative diseases, including Parkinson's and Alzheimer's (1).

Improves deppressive symptoms

A study ( 14 ) involving 1058 senior citizens (aged 70) found that "A more frequent consumption of green tea was associated with a lower prevalence of depressive symptoms". The risk of suffering from mild or severe depression fell by 44% for those drinking 4 cups of green tea per day.

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Adverse effects of Green Tea

The side effects and risks

As with many things in life, too much of something can be bad for you, and in this case, too much green tea could have some negative consequences for your health:

Liver toxicity

Prompted by several cases of liver damage, the European Food Safety Authority (whose role is the same as that of the Food and Drug Administration or FDA in the US) evaluated the risks posed by green tea as a supplement and as an infusion and found that: ( 11 )

  • Catechins from green tea infusions and similar drinks are generally safe
  • Catechins, when taken as food supplements in doses at or above 800 mg/day may pose health concerns.

So the EFSA found "No concerns for infusions... there is generally no indication of liver damage even after high consumption", so catechins from green tea brewed with hot water and ready-to-drink green tea beverages are generally safe.

Suppplements containing green tea catechins are problematic because they are far more concentrated, supplements are less subject to strict quality controls than regular medicines and they can have variable quantities of catechins from 5 to 1000 mg of EGCG (while regular green tea infusions have between 90 and 300 mg), furthermore drinks are spread throughout the day while supplements are taken as one pill usually on an empty stomach.

A recent study (Hu, 2018) ( 12 ) reached a similar conclusion and proposed a safe daily dosage of 704 mg EGCG for brewed tea preparations and 338 mg EGCG ⁄ day for adults for tea supplements.

This study also found that large supplement doses and fasting could provoke hepatic adverse events "but not when consumed as brewed tea..."

Liver damage can be severe enough to cause death or chronic hepatic illness (See this article as an example: The food supplement that ruined my liver - BBC, 25 Oct. 2018)

Iron deficiency

Black tea inhibits aborption of non-heme iron by up to 94% if consumed together. The reason for this is that the polyphenols in tea form insoluble iron compounds which keep it from being absorbed in the gut. (1)(3)

For people suffering from iron deficiency this can be a serious concern.

Green tea also decreases absorption of zinc and increases that of manganese. (1)

Aluminium in tea

The presence of aluminium in tea can have potentially harmful effects. (1)( 13 )

Tea is a plant that accumulates aluminium (Al), taking it from the soil. Poor quality tea from older branches and leaves will contain more of it.

Aluminim can be toxic and affect the central nervous system, the skeleton and may contribute to some neurodegenerative diseases, including Alzheime's disease.

The average American has a daily intake of 5 to 10 mg, which is well below the 20 mg per day limit.

Tea infusions can contain an average of 0.25 to 1.5 mg per cup, so for people drinking 4 cups per day, tea can become a major source of dietary aluminium.

Green Tea, conclusions

Drinking green tea as an infusion is an interesting way to add antioxidants in the form of catechins to your diet. Taken without sugar it is a zero calorie option which could also help you control your body weight.

Supplements due to their very nature (variable amount of active catechins, less stringent controls of raw materials, etc.) and how they are taken (one daily dose, very concentrated, fasting) entail the risk of hepatic disease.

As with all good things, too much can be bad for you, it can affect absorption of non-heme iron and zinc, and increase your intake of aluminium which can have some side-effects on your health.

Prepared as an infusion in "normal" quantities it is a natural source of potent antioxidants, minerals and it will also keep you hydrated.

Cite this article:

. ©2019. Green Tea Health Benefits. Patagonia Wellness, 22 Jan. 2019.

Tags: green tea, black tea, Oolong tea, white tea, phytochemicals, flavonoids, phenolic compounds, antioxidants, polyphenols, catechins, epigallocatechin-3-gallate, EGCG, health, inflammation, oxidative stress, chronic diseases, nutrition,cancer, chemoprevention

Subject: The health benefits of green tea: the high content of catechins (potent antioxidants) in green tea account for its anti-inflammatory, antimicrobial, antiviral and antifungal properties. Antioxidants such as EGCG found in green tea have cancer, cardiovascular disease and neurodegenerative disease preventing effects. Weight loss, bone density, depression and oral health also benefit from green tea infusion intake. Supplements entail the risk of hepatic damage and should be consumed in moderation.


References and Further Reading

(1) Sabu M Chacko, Priya T Thambi, Ramadasan Kuttan, Ikuo Nishigaki, (2010). Beneficial effects of green tea: A literature review. Chin Med. 2010; 5: 13. doi: 10.1186/1749-8546-5-13

(2) Kavita Singhal, Neerja Raj, Khushboo Gupta, and Saurabh Singh, (2017). Probable benefits of green tea with genetic implications. J Oral Maxillofac Pathol. 2017 Jan-Apr; 21(1): 107-114. doi: 10.4103/0973-029X.203758

(3) M.G. Sajilata, Poonam R. Bajaj, and R.S. Singhal, (2008). Tea Polyphenols as Nutraceuticals. Comprehensive Reviews in Food Science and food safety. Institute of Food Technologists, Vol. 7:3 2008 229-254.

(4) Wanda C. Reygaert, (2018). Green Tea Catechins: Their Use in Treating and Preventing Infectious Diseases. Biomed Res Int. 2018; 2018: 9105261. doi: 10.1155/2018/9105261

(5) Sena Saklar, Erdal Ertas, Ibrahim S. Ozdemir, and Bulent Karadeniz, (2015). Effects of different brewing conditions on catechin content and sensory acceptance in Turkish green tea infusions. J Food Sci Technol. 2015 Oct; 52(10): 6639-6646. doi: 10.1007/s13197-015-1746-y

(6) Hirota Fujiki, Tatsuro Watanabe, Eisaburo Sueoka, Anchalee Rawangkan,Masami Suganuma, (2018). Cancer Prevention with Green Tea and Its Principal Constituent, EGCG: from Early Investigations to Current Focus on Human Cancer Stem Cells. Mol Cells. 2018 Feb 28; 41(2): 73-82. doi: 10.14348/molcells.2018.2227

(7) Kavita Singhal, Neerja Raj, Khushboo Gupta, and Saurabh Singh, (2017). Probable benefits of green tea with genetic implications. J Oral Maxillofac Pathol. 2017 Jan-Apr; 21(1): 107-114. doi: 10.4103/0973-029X.203758

(8) Shamekhi Z, Amani R, Habibagahi Z, Namjoyan F, Ghadiri A, Saki Malehi A, (2017). A randomized, double-blind, placebo-controlled clinical trial examining the effects of green tea extract on systemic lupus erythematosus disease activity and quality of life. . Phytother Res. 2017;31(7):1063-1071.28585735

(9) Momose Y, Maeda-Yamamoto M, Nabetani H, (2016). Systematic review of green tea epigallocatechin gallate in reducing low-density lipoprotein cholesterol levels of humans. Int J Food Sci Nutr. 2016 Sep;67(6):606-13. doi: 10.1080/09637486.2016.1196655. Epub 2016 Jun 20

(10) Hursel R, Viechtbauer W, Westerterp-Plantenga MS. (2009). The effects of green tea on weight loss and weight maintenance: a meta-analysis. Int J Obes (Lond). 2009 Sep;33(9):956-61. doi: 10.1038/ijo.2009.135. Epub 2009 Jul 14

(11) European Food Safety Authority, (2018). EFSA assesses safety of green tea catechins. Published 18 April 2018

(12) Hu J, Webster D, Cao J, Shao A, (2018). The safety of green tea and green tea extract consumption in adults - Results of a systematic review. Regul Toxicol Pharmacol. 2018 Jun;95:412-433. doi: 10.1016/j.yrtph.2018.03.019. Epub 2018 Mar 24

(13) Robert A. Yokela,b and Rebecca L. Florence, (2008). Aluminum bioavailability from tea infusion. Food Chem Toxicol. 2008 Dec; 46(12): 3659-3663. 2008 Sep 21. doi: 10.1016/j.fct.2008.09.041

(14) Kaijun Niu et al., (2009). Green tea consumption is associated with depressive symptoms in the elderly. The American Journal of Clinical Nutrition, Vol 90:6, 1 December 2009, 1615-1622,