Since few years, green tea has received a lot of attention thanks to its health benefits. Better known for its antioxidant properties, green tea may be used as a treatment against cancer. As cancer is one of the deadliest diseases, it is crucial to increase the research of new therapies. But is green tea an accurate response against cancer ?
Tea is the second most drink in the world, it is more than ever present in our cup. Green ea is associated to health benefit and some national institutes, such as la Fondation Contre le Cancer in France (1), classified green tea as a dietary supplement, in cancer case. So, why are we so interested by green tea against cancer ?
Did I said cancer ?
We can compare the human body as a house made of bricks. The bricks, the furniture and all equipment of a room can be represented as our cells. All of these elements form each living room : our organs. A kitchen allows us to cook, the heart sends blood to our organs. One living room = one function.
But what is going on when we have a cancer ? (2)
A cancer is a deregulation of our cells that can be caused by our environment or by genetics. Our cells have specific programs that protect them from errors that can appear during their multiplication. When they cannot prevent these errors, the behaviour of the cells will be abnormal, they can multiply way too fast : this is the tumour.
A home appliance needs electricity to function, a cell needs molecules from the blood to function. When the tumour achieves a certain size, the cells at the core are suffocating because they are poorly irrigated : they tend to be necrotic.
So, how can tumoral cells survive and develop properly ?
Tumoral cells secrete signals in their environment. These signals attract surrounding blood vessels, which allow the tumour to create its own vascular system. More than that, the signals modify the other surrounding cells (fat cells, immune system, etc) but also the other tumoral cells : this is the cancer. The cancer is also called malignant tumour. Depending on the organ and the cancer development, the current environment of the cancer could be not sustainable to develop properly. Peripheric cancer cells can change in order to migrate, reach the blood or lymphatic flow and invade other organs : these cells are metastasis.
And in this cute story, where is the green tea ?
In scientific research, green tea has been studied for 30 years for its anticancer properties. If you type “tea and cancer” in pubmed, you will find 5143 articles. To understand why green tea is so interesting, we need to bring back some chemistry.
The leaves of green tea contain polyphenols. These polyphenols have two major functions : they can bind to iron and have antioxidant activity. The oxidation is a chemical reaction where one electron is transferred to another molecule. This is the case with rust formation.
During cancer development, oxidation occurs and generates toxic chemicals called ROS (reactive oxygen species). These ROS modify cancer cells to help the progression of the disease. Polyphenol can transfer electrons to reverse the oxidation and so reduce ROS production. However, some studies have shown that some polyphenol parts can auto-oxidize and generate new ROS. This is the oxidant and antioxidant activity of polyphenol that could perturbate the environment of cancer cells and could limit their development (3)
In green tea, the most active polyphenol is called catechin. The most potent catechin is called EGCG, which are massively studied in 1789 scientific papers as “EGCG and cancer”.
EGCG, how it functions ?
Nowadays, we do not know what the specific targets of EGCG are. It seems that EGCG is capable to link with a lot of pro-cancerous molecules, which could stop their action for cancer progression. In 2014 (4), Yang et al. recapitulated the molecules that are blocked by EGCG in cell culture. These molecules are implicated in cell multiplication, cell migration and angiogenesis : the development of blood vessels.
In practice, how functions the EGCG in green tea ?
The in vivo (animal) results are fluctuant. In 2010, (5) Li et al. demonstrated that water supplemented with EGCG decreased the lung cancer growth in mice. These results were compared with lung cancer cells cultured in vitro. The non-toxic dose of EGCG in mice tumour was 133 less times important than in cell culture. The discordance found in these results could be explain with the different dose used but also the exposition duration (daily with animals, less than 48 hours in cells). Also, EGCG is degraded in intestine, specially by intestine bacteria which could limit the accumulation of EGCG in tumour and could limit the efficiency of a therapy based on green tea.
The results are even more contrasted in humans. Most of epidemiological studies of 80’s and 90’s didn’t demonstrate an improvement of the disease with daily green tea diet (6). Green tea isn’t the only thing that impacts our body : tabaco, food, environment, all of these parameters are sources of biases. Moreover, there are any standardized protocols for tea consumption in scientific studies : which dose to use ? what quality of leaf to use ? which type of green tea must be used, as Japanese green teas tend to have more EGCG ?
Other limits exist such as interaction with current therapeutic : chemotherapy. In a recent clinical study, it has been observed that the association of chemotherapy and green tea is far more potent to reduce the size of prostate cancer (7) whereas other studies did not demonstrate an improvement of current therapeutic with EGCG (8)
Where are we in the research of green tea against cancer ?
In this article, I demonstrated that green tea may not have an effect against cancer. However, in 2008, a Japanese scientific paper (9) recapitulated the clinical results of 1990 and 1993 studies for prostate cancer prevention. 49920 men of 40 to 69 years old participated to these studies until 2004. They drinked 5 cups of green tea per day (1 liter or 33 oz per day). The risk of prostate cancer development decreased by half when compared with men that drinked only one cup of green tea per day. This preventive anticancer effect has also been demonstrated in cell and animal models, particularly for the prevention of metastasis (skin, prostate, colorectal, breast, pancreas, lung) (10)
Green tea, as interesting as it could be for cancer, is still an open book …
Comentarios