Mental Health, nutrition

Is Green Tea Good For You?

Why You Should Be Drinking Green Tea

Green tea is a popular drink of choice for many people including myself. I first started drinking the herbal tea at University as I wasn’t into coffee at the time and often fancied something fresher than a creamy English Breakfast tea. Initially, I found green tea to be bitter and difficult to enjoy, however over time it has become my favourite hot beverage. I write this post both out of love (maybe obsession) for the drink and out of an interest to know more about what it is actually good for.

Green tea has been extensively studied for years due to its medicinal qualities. It is often proposed on social media as a cure for weight loss and as an immunity booster. People have suggested that it improves their wellbeing as well as reduces stress. But, how do these claims stand up to the science, and is there evidence for the health benefits of green tea?

Green tea originated in China, and has been drunk and used as traditional medicine for centuries. Up until the 14th century, it was a sign of affluence in China and only the highest tiers of society found themselves able to relish in its benefits. From the 14th century onwards, green tea became more readily available to all people, regardless of societal hierarchies, and slowly made its journey across the globe.

Many of the superfood claims come from the fact that green tea contains many important properties for our health, including polyphenols and caffeine. Polyphenols are compounds found in plant-foods and are understood to have antioxidant and anti-inflammatory properties.

Fun Fact

A lot of the teas we drink today all come from the same plant, the Camellia Sinensis. It is native to Asia and is used to make green tea, oolong tea and black tea (what we commonly know as English breakfast tea). What makes each tea different to the other is the process that it undergoes after it is harvested. Once harvested, green tea is processed in a way to prevent oxidisation, whereas black tea is allowed to oxidise making the leaves darker and stronger in flavour.

Green Tea and Weight Loss

The first time I heard someone talk about green tea was in a conversation about weight loss. A friend of mine couldn’t recommend it enough as an aid to his fitness and weight loss journey. I have often seen green tea capsules in the ‘Weight Loss’ sections of health stores and pharmacies, and it is often included as an ingredient in popular diet pills. But does it really do anything or is it just marketing?

A systematic review and meta-analysis published in 2020 concluded that green tea intake is associated with a reduction in body weight and body mass. The greatest reductions were seen:

  • For people with a BMI of ≥30 kg/m2.
  • In trials with less than 800mg of green tea a day
  • In trials that lasted for 12 weeks and more.

Currently, it is not yet fully understood why green tea has this ‘anti-obesity’ effect. Some believe that it may help to regulate the hormones that give us our hunger cues. Leptin and ghrelin are two important hormones for appetite regulation. While leptin inhibits the feeling of hunger, ghrelin is produced to increase it. The link between green tea and these hormones is not well established in the literature currently, and any associations made tend to be weak.

A study from 2014 aimed to summarise the other possible mechanisms underlying the ‘anti-obesity’ effect. They suggested that the caffeine in the tea may contribute to weight loss in its role as a metabolic stimulant. Caffeine may increase fat oxidation, thermogenesis and energy expenditure however there also isn’t a lot of research behind this. The study also suggests that the effect of caffeine may only be effective when in combination with a specific type of phenol that is commonly found in teas (epigallocatechin gallate/ EGCG). In other words, caffeine alone will not be as effective. Unfortunately, the study suggests that most other cited mechanisms still remain speculative and as theory.

Green Tea and Cancer

When I began looking for research into green tea and health, perhaps the most common types of study I came across were cancer intervention studies. A Cochrane Review published in 2020 said that, despite overall inconsistencies across results, there may be some benefit of green tea on some specific cancer sites. Specifically, there seemed to be a decreased risk for prostate cancer. Higher consumption of green tea is linearly associated with reduced risk of prostate cancer. This association, however, is seen for 7 cups of tea or more a day.

There have been two recent systematic reviews looking into green tea and breast cancer. Both also suggest a protective effect of green tea and decreased incidence of breast cancer. Both also call for more research to confirm these effects.

While the Cochrane review does note that green tea can pose a decreased risk of certain cancers, they write that these effects need to be studied further. The studies used in the review vary in quality and they consider the association to still be ‘unproven’. Drinking vast quantities of green tea can also have some unpleasant side effects. At the moment, green tea should not be used as a preventative or protective strategy for cancer unless advised by a medical professional.

Green Tea, Mental Health and Cognition

On top of cancer prevention and its ‘anti-obesity’ effects, green tea seems to also have impacts on mood and cognition. It has been found that it can reduce anxiety and stress, improve memory and attention. Interestingly, the anxiety and stress reducing effects of green tea seem to be associated with low caffeine contents. Theanine is an amino-acid found in tea, and is believed to exhibit the stress-reducing effect. Caffeine is understood to antagonise the effect of theanine, or in other words, inhibit its effect. It may be, that for the anti-anxiety and anti-stress effects, a low-caffeine green tea is required.

A higher caffeine content may be more important for the cognitive effects such as attention and memory as often believed to be the case with coffee. However, a systematic review published in 2017 found that the beneficial effects of green tea are due to the combined influence and balance of caffeine and theanine. Individual supplementation with either one were found to have lesser benefits.

Green Tea Warnings

While the herbal tea may have many positive impacts on human health and is generally safe to consume, there are a couple of side effects that should be discussed.

When consumed in high quantities, green tea may cause toxic-like effects in some people. These include headaches, nausea, vomiting, gastrointestinal dysfunction and tachycardia. Some adverse affects have been directly attributed to the caffeine and stimulant effect such as anxiety, nervousness and insomnia. Luckily, as discussed above, a low-caffeine content should in fact have an opposite effect. It is not entirely understood what causes these adverse effects. Some believe it may be the potency of the phenols in the tea, with EGCG (mentioned above) being a key determinant of toxicity.

There may also be some nutrient-nutrient interactions. Tea has high levels of tannins which can inhibit iron absorption. If you’ve ever taken iron tablets, you may have been told not to take them with tea. For people who have low iron levels, or have a limited iron intake, this might be something to consider when planning your diet.

Concluding Remarks

In writing this article, I came across a lot of research about the potential benefits of green tea. Unfortunately to keep the article from being too long I chose to focus on three main aspects of the research base. The most well established benefit of the herbal tea is its weight loss effect. While it is unclear how it works, it is clear that it does work.

The other two areas of focus for this article were the effects on cancer prevention, and on mental health and cognition. Both of these areas require further research however there is a promising benefit of green tea for both.

I love the tea and I do acknowledge some bias! I seem to collect it in different forms and blends (e.g. as matcha). Its status as a superfood has yet to be established, and it does come with side effects but if its never been your thing, maybe you should revisit it.

If you are unsure or wish for more information, I would recommend talking to your GP or a dietitian.


References

Chen, I.-J., Liu, C.-Y., Chiu, J.-P., & Hsu, C.-H. (2016). Therapeutic effect of high-dose green tea extract on weight reduction: A randomized, double-blind, placebo-controlled clinical trial. Clinical Nutrition (Edinburgh, Scotland), 35(3), 592–599. https://doi.org/10.1016/j.clnu.2015.05.003

Cheng, K., Chi, N.-N., & Liu, J.-D. (2019). Green tea extract for treatment of cancers: A systematic review protocol. Medicine, 98(15), e15117. https://doi.org/10.1097/MD.0000000000015117

Filippini, T., Malavolti, M., Borrelli, F., Izzo, A. A., Fairweather-Tait, S. J., Horneber, M., & Vinceti, M. (2020). Green tea (Camellia sinensis) for the prevention of cancer. The Cochrane Database of Systematic Reviews, 3, CD005004. https://doi.org/10.1002/14651858.CD005004.pub3

Gianfredi, V., Nucci, D., Abalsamo, A., Acito, M., Villarini, M., Moretti, M., & Realdon, S. (2018). Green Tea Consumption and Risk of Breast Cancer and Recurrence-A Systematic Review and Meta-Analysis of Observational Studies. Nutrients, 10(12). https://doi.org/10.3390/nu10121886

Grosso, G., Micek, A., Castellano, S., Pajak, A., & Galvano, F. (2016). Coffee, tea, caffeine and risk of depression: A systematic review and dose-response meta-analysis of observational studies. Molecular Nutrition & Food Research, 60(1), 223–234. https://doi.org/10.1002/mnfr.201500620

Guo, Y., Zhi, F., Chen, P., Zhao, K., Xiang, H., Mao, Q., Wang, X., & Zhang, X. (2017). Green tea and the risk of prostate cancer: A systematic review and meta-analysis. Medicine, 96(13), e6426. https://doi.org/10.1097/MD.0000000000006426

Haghighatdoost, F., & Hariri, M. (2019). The effect of green tea on inflammatory mediators: A systematic review and meta-analysis of randomized clinical trials. Phytotherapy Research, 33(9), 2274–2287. https://doi.org/10.1002/ptr.6432

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. Regulatory Toxicology and Pharmacology: RTP, 95, 412–433. https://doi.org/10.1016/j.yrtph.2018.03.019

Huang, J., Wang, Y., Xie, Z., Zhou, Y., Zhang, Y., & Wan, X. (2014). The anti-obesity effects of green tea in human intervention and basic molecular studies. European Journal of Clinical Nutrition, 68(10), 1075–1087. https://doi.org/10.1038/ejcn.2014.143

Jurgens, T. M., Whelan, A. M., Killian, L., Doucette, S., Kirk, S., & Foy, E. (2012). Green tea for weight loss and weight maintenance in overweight or obese adults. The Cochrane Database of Systematic Reviews, 12, CD008650. https://doi.org/10.1002/14651858.CD008650.pub2

Levy, Y., Narotzki, B., & Reznick, A. Z. (2017). Green tea, weight loss and physical activity. Clinical Nutrition (Edinburgh, Scotland), 36(1), 315. https://doi.org/10.1016/j.clnu.2016.11.001

Lin, Y., Shi, D., Su, B., Wei, J., Găman, M.-A., Macit, M. S., Nascimento, I. J. B. do, & Guimaraes, N. S. (2020). The effect of green tea supplementation on obesity: A systematic review and dose–response meta-analysis of randomized controlled trials. Phytotherapy Research, 34(10), 2459–2470. https://doi.org/10.1002/ptr.6697

Mancini, E., Beglinger, C., Drewe, J., Zanchi, D., Lang, U. E., & Borgwardt, S. (2017). Green tea effects on cognition, mood and human brain function: A systematic review. Phytomedicine: International Journal of Phytotherapy and Phytopharmacology, 34, 26–37. https://doi.org/10.1016/j.phymed.2017.07.008

Najaf Najafi, M., Salehi, M., Ghazanfarpour, M., Hoseini, Z. S., & Khadem-Rezaiyan, M. (2018). The association between green tea consumption and breast cancer risk: A systematic review and meta-analysis. Phytotherapy Research: PTR, 32(10), 1855–1864. https://doi.org/10.1002/ptr.6124

Unno, K., Furushima, D., Hamamoto, S., Iguchi, K., Yamada, H., Morita, A., Horie, H., & Nakamura, Y. (2018). Stress-Reducing Function of Matcha Green Tea in Animal Experiments and Clinical Trials. Nutrients, 10(10). https://doi.org/10.3390/nu10101468

Unno, K., Noda, S., Kawasaki, Y., Yamada, H., Morita, A., Iguchi, K., & Nakamura, Y. (2017). Reduced Stress and Improved Sleep Quality Caused by Green Tea Are Associated with a Reduced Caffeine Content. Nutrients, 9(7). https://doi.org/10.3390/nu9070777

Williams, J. L., Everett, J. M., D’Cunha, N. M., Sergi, D., Georgousopoulou, E. N., Keegan, R. J., McKune, A. J., Mellor, D. D., Anstice, N., & Naumovski, N. (2020). The Effects of Green Tea Amino Acid L-Theanine Consumption on the Ability to Manage Stress and Anxiety Levels: A Systematic Review. Plant Foods for Human Nutrition (Dordrecht, Netherlands), 75(1), 12–23. https://doi.org/10.1007/s11130-019-00771-5

Yu, S., Zhu, L., Wang, K., Yan, Y., He, J., & Ren, Y. (2019). Green tea consumption and risk of breast cancer: A systematic review and updated meta-analysis of case-control studies. Medicine, 98(27), e16147. https://doi.org/10.1097/MD.0000000000016147

Leave a Reply