This article was written for NHD magazine, and was published in the February 2020 edition.
Green tea is often touted as a ‘superfood’ which has numerous health-boosting qualities.
However, there is no such thing as a ‘superfood’, as all foods have positive and negative qualities in different contexts.
This article will examine the specific health claims related to consuming green tea.
All types of tea, including green tea, are made using the Camilla Sinius leaf. Green tea leaves are steamed and pressed soon after harvest, whereas black tea leaves are withered then fully oxidised before being dried, which changes the flavour and nutritional content of the leaves.1
There are many different types of green tea, which differ depending on the region they are from and how they have been produced:
- Gunpowder tea (China)
- Dragon’s Well (China)
- Bi Luo Chun (China)
- Ujeon (Korea)
- Sejak (Korea)
- Sencha (Japan)
- Kukicha (Japan)
- Matcha (Japan)
All types of tea contain polyphenols which are thought to confer health benefits by acting as antioxidants and preventing damage to our cells2.
Green tea tends to have higher levels of polyphenols than other types of tea, as the levels of polyphenols reduce during the oxidisation process and green tea leaves are not oxidised before being sold.3
It is estimated that polyphenols make up roughly 35% of the dry weight of green tea leaves
Most of these polyphenols belong to a group called catechins and the main catechin found in green tea is called epigallocatechin-3-gallate or EGCG4. Other types of polyphenols found in green tea include: theaflavins, theorubigins, quercetin, gallic acid and chlorogenic acid.3-4
Green tea also contains: B vitamins, folate, potassium, manganese, magnesium, caffeine, and an amino acid called l-theanine which is thought to make us feel relaxed.3-4
Brewing tea for longer releases more nutrients from the tea leaf.
For example, it has been found that in order to extract the most benefit from the active ingredients found in tea it should be brewed in boiling water for 30 seconds, followed by microwaving for one minute.5
Green Tea & Heart Disease
There is quite good evidence that consuming green tea (both as a drink and in supplement form) lowers total and LDL cholesterol for those with high cholesterol levels.6
However, green tea hasn’t been found to affect HDL cholesterol or triglycerides levels.6-7
Green tea has also been seen to help lower systolic blood pressure by 2.08 mmHg, and diastolic blood pressure by 1.71 mmHg.7 This impact was seen to be strongest in those who have high blood pressure (defined as systolic blood pressure ≥130 mmHg), and when green tea supplements were used rather than drinking green tea.
A meta-analysis from 2009 also found that consuming three cups of either green or black tea per day reduced the risk of an ischaemic stroke by 21%.8
However, these results were based on observational studies, and the authors highlight that “a randomized clinical trial would be necessary to confirm the effect”.
There is also a possible link between consuming green tea and a reduced risk of coronary artery disease, although more research is needed to investigate this.9
Green Tea & Diabetes
Green tea is sometimes promoted as a way to prevent and treat diabetes.
Some studies have found a link between drinking tea and lower fasting blood glucose levels.10-11
However, there isn’t enough evidence to say whether drinking green tea or taking green tea supplements is useful for the prevention or management of type 2 diabetes.10-11
Green Tea & Weight Management
The caffeine and catechins found in green tea may increase energy metabolism slightly, as well as reducing fat absorption, reducing appetite and increasing fat burning.4, 12
However, these mechanisms don’t always translate into significant weight loss.
For example, studies have found that the use of green tea extract results in an insignificant amount of weight loss, such as 400g to 1.3kg weight loss over three months.13 – 14
When weight loss does occur, this only seems to happen when green tea supplements also contain caffeine.15
Green Tea & Cancer
Research is conflicting when it comes to whether consuming green tea is associated with a reduced risk of cancer.
There is some evidence in Asian populations that drinking 3 to 5 cups of green tea per day is associated with a lower risk of liver, prostate and lung cancer.16-17
Whereas evidence is more conflicting in terms of oesophageal, colon, rectal, gastric and pancreatic cancer.16
A Cochrane review from 2009 concluded that “There is insufficient and conflicting evidence to give any firm recommendations regarding green tea consumption for cancer prevention. If not exceeding the daily recommended allowance [of 3-5 cups of green tea per day], those who enjoy a cup of green tea should continue its consumption. Drinking green tea appears to be safe at moderate, regular and habitual use”.16
Green Tea & Healthy Aging
There is a possible link between green tea consumption and improvements in cognitive health and bone health.
For example, a recent meta-analysis of observational studies found that drinking two cups of green tea per day was associated with a reduced risk of cognitive disorders, such as cognitive decline, dementia and Alzheimer’s disease18.
Epidemiological studies have also found an association between green tea consumption and a lower risk of osteoporosis and fractures in elderly women and men.19 This is thought to be related to the polyphenols and fluoride found in green tea which may improve bone mineral density and support the activity of osteoblast cells, which are involved in forming new bone. This is an interesting area, but more robust studies are needed.
Green Tea & Skin Health
Lab and animal studies have found that applying polyphenols found in green tea topically on to the skin can reduce and repair damage and inflammation in the skin caused by UVB rays.20
There is also some evidence that green tea polyphenols can reach the skin when they are consumed by humans.20 One controlled study found that taking green tea supplements was associated with improved skin outcomes (improved UV protection, skin elasticity, hydration, blood flow, oxygen saturation, skin density and reduced transepidermal water loss).21
However, other studies have found no positive impact from using green tea supplements.22-23.
Therefore, more robust research is needed to see whether consuming green tea, or using green tea supplements is beneficial for skin health.
Green Tea & Liver Disease
There is an association between consuming green tea and a lower risk of liver disease, including fatty liver disease, hepatitis, liver cirrhosis, hepatocellular carcinoma, and chronic liver disease.24
However, this research mainly applies to the Chinese population and there is a lack of randomised controlled trials in this area.
Furthermore, there have been some reports of severe liver damage related to taking specific green tea supplements (as discussed below).
The Safety of Green Tea
As green tea contains caffeine (which is a stimulant), consuming large amounts can contribute to issues such as trouble sleeping, anxiety, irritability and frequent urination.
Matcha green tea could potentially interfere with Warfarin medication as it is very high in Vitamin K, whereas this is much less likely to occur with brewed green tea as this is less concentrated than matcha25-26.
It has been found that consuming green tea extract supplements with meals can reduce the absorption of non-heme iron (i.e. iron from plant sources rather than meat), but more research is needed to see if this has an overall negative impact on health.27
There have been some reports of severe liver damage related to the use of green tea supplements.
For example, in October 2018 an American man needed an urgent liver transplant in order to save his life, and doctors believe that the cause of harm may have been the green tea supplements which he was taking.28
The risk of liver damage seems to be highest when taking multi-ingredient products or taking green tea supplements on an empty stomach, as this can lead to dangerously high levels of catechins in the blood which may have a toxic effect.29 It may also depend on genetics as to whether individuals are at risk of liver damage from these supplements.29
Drinks which are brewed from green tea leaves are not seen to have the same damaging effects as green tea extracts – as these extracts and supplements contain much more concentrated levels of catechins.29
The European Food Safety Authority (EFSA) recently reviewed the safety of green tea and green tea supplements and concluded that: “catechins from green tea infusions and similar drinks are generally safe. When taken as food supplements, however, catechin doses at or above 800 mg/day may pose health concerns”.30
Green tea is high in polyphenols which are associated with health benefits such as lowering total cholesterol, LDL cholesterol, and blood pressure.
However, some of the health benefits of green tea are overstated. For example, green tea should not be promoted as a treatment for diabetes or cancer. Similarly, in terms of weight management, there is no evidence that green tea promotes significant weight loss.
It is interesting that green tea may reduce the risk of developing certain diseases including stroke, coronary artery disease, cognitive disorders, liver disease, osteoporosis and certain types of cancer; but more research is needed to explore these possible associations.
Overall, green tea is a healthy drink for most adults to have, but high dose green tea extracts or supplements can be risky.
- Varnam & Sutherland ‘Beverages:Technology, Chemistry and Microbiology’, Springer (1994)
- Khan and Mukhtar (2007) “Tea polyphenols for health promotion” <https://www.sciencedirect.com/science/article/pii/S0024320507004717>
- NHS Choices: Green tea: the elixir of life or just hype? [accessed March 2018 via: https://www.nhs.uk/Livewell/superfoods/Pages/is-green-tea-a-superfood.aspx]
- Rains et al. (2011) “Antiobesity effects of green tea catechins: a mechanistic review” <https://www.ncbi.nlm.nih.gov/pubmed/21115335>
- Vuong (2012) “Improved extraction of green tea components from teabags using the microwave oven” <https://www.sciencedirect.com/science/article/pii/S0889157512000828>
- Zheng et al. (2012) “Green tea intake lowers fasting serum total and LDL cholesterol in adults: a meta-analysis of 14 randomized controlled trials” <https://www.ncbi.nlm.nih.gov/pubmed/21715508>
- Khalesi et al. (2014) “Green tea catechins and blood pressure: a systematic review and meta-analysis of randomised controlled trials” <https://link.springer.com/article/10.1007/s00394-014-0720-1>
- Arab et al. (2012) “Green and black tea consumption and risk of stroke: a meta-analysis” <https://www.ncbi.nlm.nih.gov/pubmed/19228856>
- Wang et al. (2012) “Black and green tea consumption and the risk of coronary artery disease: a meta-analysis” <https://www.ncbi.nlm.nih.gov/pubmed/21248184>
- Polychronopoulos et al. (2012) “Effects of black and green tea consumption on blood glucose levels in non-obese elderly men and women from Mediterranean Islands (MEDIS epidemiological study)” <https://www.ncbi.nlm.nih.gov/pubmed/18204918>
- Odegaard et al. (2008) “Coffee, tea, and incident type 2 diabetes: the Singapore Chinese Health Study” <https://www.ncbi.nlm.nih.gov/pubmed/18842784>
- Wolfram et al. (2006) “Anti-obesity effects of green tea: from bedside to bench” <https://www.ncbi.nlm.nih.gov/pubmed/16470636>
- Jurgens et al. (2012) “Green tea for weight loss and weight maintenance in overweight or obese adults” <https://www.ncbi.nlm.nih.gov/pubmed/23235664>
- Hursel et al. (2009) “The effects of green tea on weight loss and weight maintenance: a meta-analysis” <https://www.ncbi.nlm.nih.gov/pubmed/19597519>
- Phung et al. (2010) “Effect of green tea catechins with or without caffeine on anthropometric measures: a systematic review and meta-analysis” <https://www.ncbi.nlm.nih.gov/pubmed/19906797)>
- Boehm et al. (2009) “Green tea (Camellia sinensis) for the prevention of cancer” <http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD005004.pub2/abstract>
- Cancer Research UK: Green tea (Chinese tea) [accessed March 2018 via: http://www.cancerresearchuk.org/about-cancer/cancer-in-general/treatment/complementary-alternative-therapies/individual-therapies/green-tea]
- Liu et al. (2017) “Association between tea consumption and risk of cognitive disorders: A dose-response meta-analysis of observational studies” <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5522147/>
- Shen et al. (2009) “Green tea and bone metabolism” <https://www.ncbi.nlm.nih.gov/pubmed/19700031>
- Spiro and Lockyer (2018) “Nutraceuticals and skin appearance: Is there any evidence to support this growing trend?” <https://onlinelibrary.wiley.com/doi/full/10.1111/nbu.12304>
- Heinrich et al. (2011) “Green tea polyphenols provide photoprotection, increase microcirculation, and modulate skin properties of women”
- Janjua et al. (2009) “A two-year, doubleblind, randomized placebo-controlled trial of oral green tea polyphenols on the long-term clinical and histologic appearance of photoaging skin”
- Chiu et al. (2005) “Double-blinded, placebo controlled trial of green tea extracts in the clinical and histologic appearance of photoaging skin”
- Yin et al. (2015) “The effect of green tea intake on risk of liver disease: a meta analysis” <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4538013/>
- Holbrook et al (2005) “Systematic overview of warfarin and its drug and food interactions” <https://www.ncbi.nlm.nih.gov/pubmed/15911722>
- Kamao et al. (2007) “Vitamin K content of foods and dietary vitamin K intake in Japanese young women” <https://www.ncbi.nlm.nih.gov/pubmed/18202532>
- Samman et al. (2001) “Green tea or rosemary extract added to foods reduces nonheme-iron absorption” <https://www.ncbi.nlm.nih.gov/pubmed/11237939>
- BBC Website “’The food supplement that ruined my liver (25/10/18)’” <https://www.bbc.com/news/stories-45971416>
- Mazzanti et al. (2015) “Hepatotoxicity of green tea: an update” <https://www.ncbi.nlm.nih.gov/pubmed/25975988>
- EFSA (2018) “EFSA assesses safety of green tea catechins” <https://www.efsa.europa.eu/en/press/news/180418>