Tea (all varieties) is one of the most widely consumed beverages all over the world. It is second to water. All of the teas (excluding the herbal and fruit teas) are derived from the leaves of the same plant (Camellia sinensis) and are classified based on different types of processing; Black tea (fermented); Green tea (non-fermented) and Oolong tea (semi-fermented).

The type of tea and method of processing the raw leaf influences the quantity of the beneficial compounds (phytochemicals).

The most significant compounds are polyphenols known as catechins, and green tea is one of its richest sources. This is one of the reasons green tea is one of the most studied types. There are several beneficial health effects, such as lowering hypertension, reducing cholesterol, improving cognition, and reducing cardiovascular disease risk factors.

Several large epidemiological and well-controlled clinical trials have provided evidence that supports this. The same studies have shown that drinking 5-10 cups per day over several months give these effects. Although these amounts are relatively hard to achieve in Western society the studies conducted in Asian populations have shown that these amounts are easily achieved.

Therefore, on a market today it is relatively easy to find several green tea extracts and other purified compounds derived from green tea. However, caution needs to be taken with these products as some of the extracts can contain significant amount of phytochemicals. Some are equivalent to drinking more than 100 cups.

In addition, depending on the dietary intake and health status of the individual, these compounds can pose significant adverse effects. This would be an example of where "too much of a good thing" can actually be a bad thing!

One of the most adverse examples would be a case of an Indigenous individual who required a liver transplant in Western Australia (case reported in The Medical Journal Australia in 2016).

Nevertheless, consuming green tea as a beverage is unlikely to cause adverse effects as the levels required to reach these effects are hard to achieve in common use.

Response by: Dr Nenad Naumovski, University of Canberra

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