Tea is a popular drink all over the world. In 2019 alone, a total of 5,267,240 tons of tea were produced . The top tea consumer in 2020 was Turkey, with sales of 2.7 kg per person . But what makes tea different from other beverages? It’s just hot water, isn’t it? Definitely not! Drinking tea can have a large impact on our health. For example, researchers from China conducted a meta-analysis of observational studies and found that in groups of people with high tea consumption, fewer people suffered from depression compared to groups of people with low tea consumption . Findings indicated a negative linear association between the amount of tea and risk of depression: An increase of three cups per day was related to a 37 % lower risk.
The most studied tea in the context of health benefits of tea consumption is green tea. There are many reviews, meta-analyses and randomized controlled trials that have investigated the effect of green tea on body and mind.
In their review, Mancini et al.  looked at 21 studies that investigated effects of green tea on cognition, mood and brain function. They concluded that consuming a daily dose of 100 ml leads to greater relaxation and a reduced risk of dementia. In addition, beneficial short-term effects on attention and memory were found. Catechins, such as epigallocatechin gallate (EGCG), and the combination of the amino acid L-theanine and caffeine seem to be responsible for those effects on the brain. The calming and stress-relieving effect is likely to be solely ascribed to L-theanine . Compared to other teas, the dose of catechins, L-theanines and caffeine is quite high in green tea. This is mainly due to the processing. Green tea is obtained from the leaves and buds of the Camellia sinensis plant by steaming and drying them without fermentation . This means that the active ingredients of the original leaf are preserved and hardly change in concentration .
However, green tea may not only have positive effects on our brain, but also on our body itself. The meta-analysis by Lin et al.  examined the effect of green tea on obesity in 25 randomized controlled trials and found that with the help of green tea supplementation, both body weight and body mass index could be reduced significantly in patients with obesity. On average, the weight reduction was 1.78 kg. People with a body mass index above 30 even lost 2.53 kg whilst using green tea supplementation. Again, the mechanisms behind these effects are not fully understood yet. It is assumed that the combination of catechins and caffeine increases activity of the sympathetic nervous system, which leads to an enhanced energy expenditure and splitting of fats in adipose tissue . Furthermore, reduced nutrient uptake, e.g. of glucose, caused by the catechins could also be a mechanism leading to weight loss . In addition to reducing body weight, green tea can also lead to a reduction in systolic blood pressure, total cholesterol levels and LDL cholesterol . Thus, green tea supplementation may also help reduce the risk of cardiovascular disease.
In summary, green tea has many positive health effects, although more studies are needed to assess the exact magnitude and mechanisms behind these effects. Of course, green tea is not a magic bullet that can replace a healthy lifestyle and the studies are not without limitations. For example, some samples which were included in the meta-analyses or reviews were quite small   and some of the effects were greater in studies funded by tea companies . Nevertheless, the state of research suggests that it is possible to positively influence mental and physical health through drinking green tea.
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- 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, 34, 26-37. https://doi.org/10.1016/j.phymed.2017.07.008
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- Lin, Y., Shi, D., Su, B., Wei, J., Găman, M. A., Sedanur Macit, M., … & 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
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