Introduction
Three billion kilograms of tea are produced each year worldwide making tea the second most commonly consumed beverage (after water). Tea consumption is currently growing at a rate of 2.1% per year.
The high and growing rates of tea consumption globally makes tea hugely important from a public health perspective as even small health effects (positive or negative) in humans could have large implications (and costs). Green tea represents about 20% of the total tea market (20% of 3 billion kg is a whole lot of green tea).
Green tea is considered to offer multiple health benefits over and above its role as a refreshing drink. Instagrammers have posted almost 6,000,000 posts for the #green tea and claim that it helps with heart disease, strokes, Alzheimer’s, osteoporosis, macular degeneration. Best of all, it is supposed to help extend our lifespan and reduce our waistlines. Who wouldn't love that?
What Actually Is Green Tea?
Tea, a beverage made from the leaves of the evergreen shrub Camellia sinensis, originates from China. Archeologic findings suggest that infusions of leaves from the tea plant, may date back 500,000 years. Legends say that the Chinese Emperor Shen Nung (aka Divine Healer) discovered the tea beverage in 2737 BC when dried leaves accidentally blew into hot boiling water. Hey presto, tea was discovered.
The Zen priest Eisai is believed to have brought tea from China to Japan. There are three main types of tea which are determined by the manufacturing process: green, black and oolong.
Green tea is produced by lightly steaming tea leaves prior to drying. The steaming inactivates the enzymes that are responsible for fermentation of tea leaves. Put another way, green tea is derived from unfermented leaves, oolong tea is partially fermented and back tea is fully fermented.
Fermentation reduces the polyphenol content and increases the caffeine content, hence the rationale for the supposed superior health benefits of green tea. Green tea is very revered in some cultures and is described in the ancient writings from Asian healers for its medicinal use.
There are a total of 60,000 green tea related products for sale on Amazon. Available forms of green tea products include leaves, bags, extract, powder and even ‘green tea Kit Kat bars’. (Have a break, have a ‘green tea Kit Kat’).
In exchange for $1125, Amazon will happily send you 25kg of green tea extract.
What’s In It?
Green tea is rich in polyphenols and antioxidants. The main flavanol in green tea is epigallocatechin gallate (EGCG). This represents 40% of the total flavonoid content of green tea extract. Green tea also contains high levels of antioxidants, perhaps even more than most vegetables and fruits.
Researchers at Tufts University, Boston tested both green and black teas by placing one tea bag (1.95 g) in 150 ml (5 oz.) of boiling water (1). They used the accepted methodology of the Trolox Equivalent score to measure antioxidant activity. Green and black teas had a mean antioxidant capacity of 761.1 +/- 85.3 micromol Trolox Equivalents (TE) per g dry matter.
In the first brewed cup, approximately 84% of the total antioxidant activity was solubilized within the first 5 min of brewing and contained 8. 31 micromol TE per ml. This suggests that tea can be an important source of antioxidants when green tea is benchmarked against drinks from fruits and vegetables that had antioxidant capacity values ranging from 1.6 to 15 micromol TE/ml.
Similar work from the UK shows that 2 cups of tea (non-specified)is equivalent to 1 glass (150 ml) red wine equivalent to 12 glasses white wine equivalent to 4 apples equivalent to 5 portions of onion equivalent to 5.5 portions egg plant equivalent to 3.5 glasses of blackcurrant juice equivalent to 3.5 (500 ml) glasses of beer equivalent to 7 glasses of orange juice equivalent to 20 glasses of apple juice (long life) (2) .
A typical cup of green tea contains (per 100gm):
Flavan-3-ols (-)
(-) - Epicatechin 7.36 mg
(-) -Epicatechin 3-gallate 16.39 mg
(-) -Epigallocatechin 22.27 mg
(-) -Epigallocatechin 3-gallate 64.15 mg
(+) -Catechin 3.28 mg
(+) -Gallocatechin 1.54 mg
Theaflavin 0.05 mg
Theaflavin-3, 3'-digallate 0.01 mg
Theaflavin-3'-gallate 0.01 mg
Thearubigins 1.08 mg
Flavones
Apigenin 0.17 mg
Luteolin 0.13 mg
Flavonols
Kaempferol 1.31 mg
Myricetin 1.02 mg
Quercetin 2.49 mg
It also contains amino acids (theanine), methylxanthines (caffeine, theobromine, theophylline), organic acids (caffeic acid, quincic acid, gallic acid) and volatiles (linalool). A daily total of 2 to 3 cups of green tea per day (a total of 240 -320 mg of polyphenols) is the usual recommended dose.
However it should be noted that the individual ingredients differ depending on factors such as the type of tea, cultivation, processing of the raw tea and preparation of the tea. As an example, iced tea contains less active ingredients than freshly boiled teas.
Decaffeination of tea removes significant quantities of the health promoting polyphenols. This is especially the case of the solvent ethyl acetate is used to remove the caffeine. The preferred method is effervescence which uses just water and carbon dioxide and retains up to 95% of the phenols in the parent compound.
What About Green Tea Extract?
Green tea extract is a concentrated extract from green tea. Technically it is possible to extract and concentrate any or all of the natural constituents of green tea. Again as an example, it is possible to avail of the polyphonic components of green tea but without the caffeine in extract form.
Is There Any Research?
This is a little confusing. Usually when doing a Pub Med search, we look at the papers relating to the both the common everyday name and the botanical name.
From a purist perspective, the botanical name is generally considered to be best. Take echinacea for example. Botanically speaking it is classified as purpurea, angustifolia and pallida. Unless we specify which exact botanical species we are researching we cannot compare across studies in a meaningful way. Additionally, the research is often better in studies that go to the bother of using the botanical classification - just my elitist opinion.
Camellia sinesis is the name of the parent plant. As mentioned above, the differences between green, oolong and black tea relate to the manufacturing process involved and not inherent differences among species of tea from a botanical perspective.
There are over 28,000 studies on tea which include over 1000 clinical studies. There are 2582 published studies on Camellia sinesis including 129 clinical studies. There are 7907 publications and 337 trials on ‘green tea’. All a bit confusing.
Does Green Tea Prevent Alzheimers or Memory Loss?
Wistar rats received green, rooibos or black tea for 8 weeks prior to intra-hippocampal injection to induce Alzheimer’s Disease (3) . Green tea prevented memory loss in memory tests in the live rats (obviously the rats were alive if their memory was being tested).
Following euthanasia (now we have dead rats) green tea was the only one of the three teas tested that avoided oxidative stress and hippocampal damage.
A series of in vitro experiments from investigators in Michigan found that green tea exhibited anti-amyloidogenic acitivity (4)
Aged beagles (98-115 months) were treated with a medical cocktail containing curcumin, pipeline, N acetyl cysteine, alpha lips acid and green tea for three months (5)
Placebo treated dogs had statistically significantly higher errors scores and loss of accuracy across distances suggesting that the medical cocktail improved spatial attention.
Measurements of visual discrimination, executive function, spatial memory, brain and cerebrospinal fluid amyloid levels were unaffected by the medical cocktail. The study suggested that the medical cocktail (or components thereof) can improve some (but not all) cognitive defects associated with Alzheimer’s Disease.
Bottom Line
There is no human data to support a role for green tea in Alzheimer’s Disease.
Does It Reduce Free Radical Damage?
A mentioned above, green tea is rich in antioxidants (1). Hence we could assume that green tea acts only as an anti-oxidant. Ah, beware of assumptions in science.
Here we come across the first paradox of green tea. A study in 2014 showed that both a single dose and regular intake of green tea resulted in statistically significant geno-protective effects as measured by lymphocyte DNA (6) . However, catechins are a key component of green tea and show pro-oxidant effects in vitro. This has been described as the two sides of the coin of the green tea redox effect (7)
The investigators in the 2014 study believed that this paradox (pro-oxidant in vitro and anti-oxidant in vivo) can be explained by the fact that there fact that catechins may have different mechanisms of action in vivo versus in vitro.
Indian investigators studied the effect of tea without milk, tea with milk and tea with lemon on serum lipid peroxidation levels (8) Lipid peroxidation was used as a surrogate marker for free radical generation. The study found that tea without milk (and especially tea with lemon) is a good source of antioxidants.
Italian investigators studied the antioxidant effect of 2 cups of green tea to a controlled diet versus no green tea with the controlled diet (9)
Following 42 days of green tea intake there was a statistically significant increase in the total antioxidant activity alongside significant decreases in plasma peroxidation levels.
Botton Line
Green tea is rich in anti-oxidants but appears to have differential effects in vitro versus in vivo making it difficult to make any assumptions or generalizations about the clinical relevance of the anti-oxidant content of green tea.
Does It Improve Heart Health?
Overall, Japanese people enjoy one of the lowest rates of heart disease globally. However, tobacco consumption is relatively high in Japan. This is the so-called ‘Asian Paradox’ (the second paradox for green tea)(10) .
Consumption of green tea (which contains a host of health promoting ingredients) is high in Japan and is believed to explain this apparent paradox. The good effects of tea outweighs the bad effects of tobacco.
Researchers from Connecticut carried out a meta-analysis of studies looking at green tea and plasma lipids (11). Twenty trials (1415 study participants) were reviewed They found that green tea catechins were associated with a statistically significant reductions in total and LDL cholesterol levels; however, there was no significant effect on HDL cholesterol or triglyceride levels.
The Ohsaki National Health Insurance Cohort Study is a population-based, prospective cohort study initiated in 1994 among 40,530 Japanese adults aged 40 to 79 years (12). Within this region, 80% of the population drinks green tea and more than half of them consume 3 or more cups per day.
The study participants were followed for 11 years. Green tea consumption was inversely associated with mortality due to all causes and due to cardiovascular disease. The inverse association with cardiovascular disease mortality was stronger than that with all-cause mortality. In practical terms, participants who consumed 5 or more cups of tea per day had a 16 percent lower risk of all-cause mortality and a 26 percent lower risk of heart disease than participants who consumed less than one cup of tea per day.
The strongest cardiovascular inverse association was observed for stroke mortality. There was a non-statistically significant trend towards a greater effect in women.There was also a non-statistical trend in favour of non-smokers benefiting more from green tea.
The main criticism of observational studies such as this is the fact that green tea consumption may be part of a generally healthy lifestyle. This could represent a major confounder in any study. Green tea consumption is very common in the Miyagi Prefecture in northeastern Japan where this study was carried out which reduces the risk that green tea consumption was driven by health concerns alone.
We know from the Blue project that certain aspects of Japanese lifestyle can prolong life-expectancy and there certainly could be confounding by other inherently healthy aspects of Japanese lifestyle. For this reason, a commentary on the paper from Germany noted that the results of a Japanese study are difficult to extrapolate to European patients (13).
Bottom Line
Green tea has been shown to reduce total LDL cholesterol without impacting on triglycerides in a meta-analysis but the clinical impact of this was not evaluated.
Green tea has been shown in an epidemiological study to reduce heart disease in Japanese subjects but it remains unclear if these data can be extrapolated to non-Japanese populations.
Does It Help Prevent Diabetes or Insulin Resistance?
A 2013 meta-analysis of randomized controlled trials on the effects of green tea on insulin sensitivity and glycemic control in populations at risk of type 2 diabetes showed that green tea consumption did not decrease levels of fasting glucose, insulin or HBA1C ( a long term measure of glucose control) (14) . This conclusion was based on data from 7 randomised controlled trials involving 510 patients.
More bad news for green tea comes from a 2017 meta-analysis. British investigators carried out a systematic review and evaluated the effect of green tea on insulin resistance and glycemic control in people with pre-diabetes/type 2 diabetes mellitus (15) . Six studies and 382 patients were included in this review.
They found no evidence to support the fact that consumption of green tea or green tea extract could reduce the levels of HbA1c, fasting insulin, or fasting glucose in people with pre-diabetes/type 2 diabetes.
Bottom Line
Two independent reviews failed to show any benefit of green tea in diabetes.
Does It Prevent Cancer.
A Cochrane review in 2016 of 51 studies involving 1.6 million participants found that the information was insufficient and conflicting in terms of cancer and green tea (16).
How can data from 1.6 million people be inconclusive? Because of terrible research design, a lack of actual benefit or a combination of both probably.
Bottom Line
Green tea has not been shown to prevent cancer.
Does It Promote Bone Health?
Epidemiological data suggests that habitual tea intake is associated with higher bone mineral density especially in post-menopausal women (17).
Canadian investigators compared green, black and rooibos tea on cell culture. They found that all three teas improved osteoblastic activity which could be seen as lending some support to the epidemiological data.
And the plot thickens. The Minnesota Green Tea Trial was a 12 month randomized double blind study in 937 post-menopausal women who were randomized to receive either epigallocatechin 843 mg or placebo (18).
A sub-study conducted in 121 overweight women showed that the green tea extract was not associated with improvements in bone mineral density.
Bottom Line
There is no proof that green tea helps with bone mineral density.
Does It Prevent Eye Disease Or Protect Vision?
The benefits of tea for ocular health was described in the Chinese pharmacopoeia about 2000 years ago. We have had 2000 years to formally prove this, but alack this is not the case. There are no human clinical trials on green tea and ophthalmology.
That means we have to move from clinical evidence (which is of course superior) to scientific evidence (which ranks lower on the totem pole of value). Fatty acids in tea shoots helped with cell growth and anti-oxidation in retinal cell lines (19).
A group of Indian pharmacologists studied the effect of green tea on cataract formation using ex vivo (they enucleated rats eyes) and in vivo rat experiments (20). The two experimental designs showed that green tea possesses significant anti-cataract potential and acts primarily by preserving the antioxidant defense system.
Chinese investigators found that green tea protected against UVB damage in retinal cell lines (21).
Russian investigators found that green tea protects cultured human lens epithelial cells under conditions of hyperglycemia (22).
Finally Korean investigators found that epigalloccatechin-3-gallate inhibits ocular neovascularization and vascular permeability in human retinal pigment epithelial and human retinal microvascular endothelial cells (23).
Bottom Line
Despite interesting (gruesome) animal studies, green tea remains unproven for eye health.
Does It Reduce Weight?
A study from Seattle compared the satiating effects of a beverage containing fibre versus the same beverage fortified with caffeine and catechins from green tea versus no beverage (24). The fortified beverage drink resulted in the lowest hunger score, the highest fullness score and the lowest energy intake at the next meal .
Green tea is widely believed (according to Instagram) to promote weight loss. Proposed mechanisms include:
- increased satiety
- increased thermogenesis via the sympathetic nervous system
- increased energy expenditure and/or
- upregulation of enzymes involved in hepatic fat oxidation (25)
The catechins found in green tea are also believed to reduce absorption of glucose and lipids in the gut wall and other tissues. A 2013 study looked at co-formulation of green tea with polyethylene glycol to prolong the activity of green tea at the luminal level in mice (26).
It is proposed that this novel formulation could play a role in the prevention and treatment of obesity induced diabetes. It is all very interesting to upgrade green tea to get extra benefits (pimp my green tea) if it has been proven to have beneficial effects on obesity in humans in the first place. Alas this is not the case.
A Cochrane review looked at 15 studies involving 1945 participants who were followed over 12 -3 weeks (27). Weight loss in adults who took green tea was small, statistically insignificant and unlikely to be clinically relevant. The results of the review were consistent regardless of which method was used to assess weight (body mass index or waist circumference).
Bottom Line
Sorry all you instagrammers out there, green tea has not been proven to reduce weight.
Does it Slow The Effects Of Ageing?
As mentioned above, the Blue Zones project which studied the lifestyle habits of the longest lived people in the world found that people in Okinawa, Japan regularly consumed green tea and lived long healthy lives. This can either be taken as an epidemiological signal that green tea promotes longevity or that it is just a surrogate marker for a healthy lifestyle in general (ie perhaps it is the omega 3 rich diet that helps Okinawans live longer).
Certainly green tea consumption was inversely associated with mortality in the Ohsaki study but it is unclear if we can extrapolate this to other demographics (12)
As we have seen green tea does not have any significant effect on other key diseases of ageing such as diabetes, Alzheimer’s, bone density, cataracts and cancer.
Botton Line
The jury is out on the the antiageing effects of green tea outside of Japan.
Is Green Tea Supplementation Safe?
There is a case report from Florida of a man with a prosthetic heart valve who had an unexpectedly low INR (the test used to measure the activity of the oral anticoagulant warfarin) (28) . The man had (inexplicably) been drinking one and a half gallons of green tea in the weeks before the unexpected labroratory result. Maybe he is believes what he reads on Instagram? On cessation of the excessive green tea consumption, the blood test returned to normal. This has led to concerns that green tea might antagonise the effects of some oral anticoagulants.
Even decaffeinated green tea products can contain substantial amounts of caffeine and care should be taken in people who are sensitive to the effects of caffeine.
There are inconclusive findings regarding green tea and liver toxicity. As a precaution, in view of the uncertainty, it is recommended that green tea be avoided by people with liver disease and that people report any side effects that could be liver related e.g. skin itch, yellow eyes or skin and dark urine.
There are possible interactions between green tea and numerous medications. Some interactions may be beneficial eg green tea may enhance the activity of beta-lactam and quinoline antibiotics (but this also increases the risk toxicity). Some interactions can be deleterious eg reduced effectiveness of lithium, hypertensive crisis in people on MAOI inhibitors, beta-blockers.
Conclusion
The paradoxes associated with green tea remind us of the inherent risk of extrapolating scientific data to the clinical arena. Science rarely plays out in the clinical arena as just a carbon copy of the laboratory. The Cochrane review on cancer prevention had a very sensible conclusion.
Having reviewed data from 1.6 million people they advised that the evidence was conflicting and it was questionable whether any of it could be generalized. That sounds just right to me. They also had a very practical bottom line 'if not exceeding the daily recommended allowance, those who enjoy a cup of green tea should continue its consumption’.
How could I improve on the musings of people who reviewed such a wealth of data? So feel free to enjoy your green tea without deluding yourself about its health benefits.
I especially love Japanese green tea ceremonies and even though I am addicted to science, I don't need a full Cochrane review to know that the slow, graceful, mindful Japanese green tea ceremony has to be better for my health than inhaling a Starbucks eggnog latte on the run.
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