Health Benefits of Green Tea

In this Article (Index)

• What is Green Tea?
• Types of tea
• Green Tea's antioxidants
• Health Benefits of Green Tea

What is Green Tea?

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, and around 20% become green tea (78% is black tea, and 2% are white and Oolong tea).

Types of tea

Let's look into each type of tea:

• White Tea
• Green Tea
• Black Tea
• Oolong Tea

White Tea

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

After being harvested, it is allowed to wither without letting it oxidize. Then it is steamed dry. As it is the least processed of all tea leaves, it has the highest antioxidant content because they aren't destroyed during processing. (2, 3).

Green Tea

To make 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).

This process 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 (4).

Black Tea

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

This oxidizes the antioxidant catechins and creates theaflavins and thearubigins.

Oolong Tea

Oolong 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.

Green Tea's antioxidants

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

• 30%. Antioxidants such as polyphenols (flavonols, flavanols, flavandiols, flavonoids, and phenolic acids).
• 15 - 20%. Proteins
• 5 - 7%. Carbohydrates (cellulose, glucose, fructose, sucrose, and pectins).
• 1 - 4%. Amino acids (theanine, tryptophan, 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.).

Let's take a closer look at the antioxidant polyphenols.

Polyphenols (Tea's antioxidants)

The polyphenol content of green tea (30% of fresh leaf dry weight) is the reason for its health-promoting effects.

Flavanols and flavonols stand out as the main antioxidant compounds.

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

The four main catechins found in tea, and their proportions, are shown below (5):

• 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 fewer catechins will be available.

A group of Turkish scientists (6) determined the optimal brewing conditions for green tea: water at 185°F (85°C) and a 3-minute brewing time.

These conditions maximized catechin content, 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):

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

• 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.

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 absorption in the intestines.

Health Benefits of Green Tea

Several scientific studies have found that consuming green tea can have potent protective effects like:

• Preventing cancer.
• Preventing coronary & heart disease.
• Antimicrobial, antiviral,& antifungal effects.
• Anti-inflammatory effects.
• Oral health benefits.
• It lowers blood pressure.
• Reduced "bad" LDL cholesterol levels.
• Weight loss.
• Improved bone density.
• Protection against depression and neurodegenerative diseases.
• Gastrointestinal diseases.
• Slows aging.

Let's look into the evidence that backs each of these health benefit claims.

Prevention of Cancer

Tea's polyphenols have a potent antioxidant effect that may explain why green tea consumption acts as an agent that may prevent many types of cancer. These cancers include the mouth, stomach, esophagus, kidney, pancreas, breast, small intestine, and colon cancers (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). (4)

According to a recent study (Fujiki, 2018) (7):

• 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 per 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 that was 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 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 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; in mice, they also increase the activity of antioxidants in specific organs providing a chemoprotective effect.

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 type of communication and may protect cells from tumor development. The loss of direct intercellular communication is linked to cancer onset and progression.

Antimicrobial & antiviral, antifungal activity of Green Tea

Green tea's catechins 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. (5)

Among the germs 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 controlling bacteria. It can also help prevent periodontal diseases.

Bad breath or halitosis is neutralized by EGCG and other catechins. These compounds deodorize foul-smelling methyl mercaptan, a sulfur compound formed by anaerobic bacteria that live in the mouth (8).

Green Tea's Anti-inflammatory activity

Menegazzi et al. (2020) (9) reported the protective anti-inflammatory properties of green tea against a wide range of diseases including covid-19: autoimmune diseases such as rheumatoid arthritis, Sjogren’s Syndrome, multiple sclerosis, inflammatory bowel diseases, and lung fibrosis.

Reduced risk of coronary and heart disease

Green tea, and to a lesser extent, black tea, have a protective effect against cardiovascular disease.

The protective 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 (3)(4).

Reduction of "bad" LDL Cholesterol levels

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" (10).

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

Its effect on weight loss may also play an important role in its heart disease prevention effects.

Weight Loss and Green Tea

A meta-analysis involving 11 separate studies found that catechins and EGCG have "a small positive effect on weight loss and weight 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). (11)

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

Other beneficial health effects of green tea

Gastrointestinal diseases

Reserch conducted in 2024 found that a "heightened consumption of green tea could lead to a reduced risk of gastrointestinal diseases," and suggest that the protective effect of green tea might come from the gut microbiome. (16)

Slows aging

A 2024 study involving data from almost 14,000 subjects found evidence that tea consumption could delay the biological aging process (BA). Not only was drinking tea associated with a slower aging, those who moved from not drinking tea to drinking it showed an attenuated aging acceleration compared to those who didn't drink tea. The suggested intake is: "consuminmg around 3 cups of tea or 6–8 g of tea leaves per day."(17)

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's 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 (4).

Green tea also has many more positive effects on your 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 the proliferation of hepatic stellate cells, which are linked to fibrosis in chronic liver disease. The antioxidant effects of EGCG inhibit their growth (4).
However, as you will see below, it supplements can cause liver damage.

Neurodegenerative diseases

Although the aluminum 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 (4).

Green tea improves depressive symptoms

A study (12) 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.

Adverse effects of Green Tea

Green tea's 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 (13):

• 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.

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

Supplements 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.

Hu (2018) (14) reached a similar conclusion and proposed a safe daily dosage of 704 mg of EGCG for brewed tea preparations and 338 mg EGCG ⁄ day for adults for tea supplements.

Liver damage

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

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

Iron deficiency

Black tea inhibits the absorption 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 (3)(4).

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

Green tea also decreases the uptake of zinc and increases that of manganese (4).

Aluminum in tea

The presence of aluminum in tea can have potentially harmful effects. (4, 15).

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

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

The average American has a daily intake of 5 to 10 mg of aluminum, 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 aluminum.

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 that could also help you control your body weight.

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

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

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