Green tea, made from the leaves of Camellia sinensis, has come a long way from its humble origins in China to its current status as the second most popular beverage worldwide. According to Chinese mythology, Shennong, the legendary ruler of China in approximately 2370 BC, drank the first cup of green tea that was brewed when a tea leaf fell into his boiled water.1 Despite his title as the divine healer, Shennong could not have possibly realized the numerous health benefits contained in the little cup. Green tea benefits health in various ways including cognitive enhancement, improvement of mental ability and alertness,2 and increased reward learning through modulation of dopamine transmission.3 Tea also helps with dieting through increased fat oxidation and prevents cardiovascular disease and diabetes.4 Recently, several studies have also credited green tea for its ability to prevent cancer development.1,4-6
When harvested from the tree, leaves of Camellia sinensis contain a high concentration of flavonoids. Flavonoids are members of the polyphenol group and have demonstrated anti-inflammatory, anti-allergic, and anti-mutagenic effects. In green tea, a group called catechin constitutes a large percentage of the flavonoids. This specific type of flavonoid, especially epigallocatechin gallate (EGCG), prevents the formation and growth of tumors.4 Normal cells take both complex and varying pathways to develop into malignant cells, but there are three crucial stages in the path to malignancy. In the initiation stage, undesirable mutations in the chromosome form due to exposure to carcinogenic substances or radiation. In the second stage of promotion, the mutation is translated and transcribed to the cytoplasm and cell membrane. The last stage is progression, during which cancer cells proliferate. By this point, accumulated mutations in chromosomes produce many genetic alterations that promote uncontrollable growth. While the numerous stages of cancer progression may complicate the search of one specific cure, they provide equal number of opportunities for regulation of carcinogenesis.5
The polyphenol substituents found in tea can suppress cancer at various stages in its progression. First, tea can prevent initiation by inactivating or eliminating the mutagens that can potentially damage the cell DNA. Potential mutagens are surprisingly common in our environment.5 Every day, we are exposed to processes that introduce dangerous reactive oxygen species (ROS) such as hydrogen peroxide and oxygen radicals that can react with DNA and induce detrimental mutations.1 Common ionizing radiation (UV and X-rays) as well as tobacco are well-documented mutagens as well. The flavonoids contained in tea are natural scavengers that destroy these free oxygen radicals.1 Catechin, a type of flavonoid, is especially effective at reducing free radicals by binding to ROS as well as to ferric ions, which are required to create ROS.6 Polyphenols of green tea can also competitively inhibit intermediates of heterocyclic aromatic amines, a new class of carcinogens, thus reducing the danger of accumulating DNA-damaging material.1 Finally, the chemical structure of the polyphenols in tea has strong affinity toward carcinogens, enabling them to bind to and neutralize the harmful substances.6 By blocking common cancer-initiating factors, tea lowers the chance of genetic mutations that may result in a tumor.
Substances in green tea can also prevent cancer by blocking angiogenesis, essentially starving the tumor cells.1 Angiogenesis is the formation of network of blood vessels through cancerous growths. In smaller tumors, cancer cells can use simple diffusion to transport necessary oxygen and nutrients. However, as the number of accumulated cells increase, tumor cells send signals to surrounding host tissues to produce the proteins necessary for blood vessel generation. These blood vessels supply large amounts of oxygen and nutrients that are unavailable through passive diffusion. Catechins in green tea stop angiogenesis by interfering with the tumor cell signals. EGCG has been shown to inhibit epidermal growth factor receptor, and thus production of vascular endothelial growth factor (VEGF), which is in charge of initiating angiogenic blood vessel formation.1 Further studies have shown direct inhibition of VEGF transcription and VEGF promoter activity in breast cancer cells by green tea extract (GTE) and EGCG.4-6 GTE also suppresses production of protein kinase C, which regulates VEGF as well. By inhibiting the signal pathway to blood vessel formation, green tea is able to reduce the progression of angiogenesis.
Another role of tea includes preventing metastasis, which is the most common cause of cancer-related mortality.1 Metastasis represents the full development of a tumor, in which the boundary that enclosed the cancer is broken and the tumor freely migrates to other parts of the body. Green tea’s flavonoids prevent degradation of membranes and proteins on the cell surface that promotes anchorage.1 Once base membranes and proteins that anchor cells to specific locations disappear, tumor cells are unfettered. EGCG in green tea has been shown to block metastasis by inhibition of membrane type 1 matrix metalloproteinase (MMP), which in turn restrains MMP-2, an enzyme crucial to degradation of the extracellular matrix. In experiments, a mixture of EGCG and ascorbic acid showed a significant suppression of metastasis by 65.9%.1
Finally, tea can prevent the unregulated proliferation of cancer cells that drives tumor formation and metastasis. Apoptosis, or the self-destruction of a cell, is actually a common and natural biological process. When a cell loses the ability to undergo apoptosis, it becomes potentially cancerous. Increasing apoptosis in cancer cells should restore balance and eliminate unrequired and harmful cells in the body. The problem lies in specifically inducing apoptosis of cancer cells without harming the normal cells, but research has shown tea’s potential in the selective promotion of apoptosis. In an experiment involving human papillomavirus 16-associated cervical cancer cells, EGCG inhibited cell growth by promoting apoptosis and cell cycle arrest.1 In head and neck carcinoma cells, EGCG also increased the percentage of cells at phase G1, the initial growth cycle of the cell, and induced apoptosis.1 Similar results were found by adding the extracted water-soluble fraction from green tea to mouse epidermal cells JB6, which both inhibited carcinogenesis and induced apoptosis.5
The extensive evidence presented here illustrates the cancer-preventive and inhibitory effects of green tea. However, we must consider that most of the data were collected through in vitro and in vivo experiments. Clinical trials with human beings have yet to confirm the preventive effects of tea polyphenol against cancer.5 Current research does not present significant evidence to determine the true effects of tea. On the other hand, a negative correlation has been observed between green tea consumption and cancer mortality along with general mortality rate in Japanese populations.5 In general, increasing the amount of green tea consumed per day indicated a reduced chance of cancer. These results suggest that tea, even with its vast number of health benefits, is not a cureall. In conjunction with regular exercise and vegetables with each meal, however, many diseases can be prevented. By drinking tea, one can partake in a tradition passed down for centuries while keeping the body healthy.
References
- Jain, N. K. et al. Protective Effects of Tea on Human Health; CAB International: Cambridge, 2006.
- Borgwardt, S. et al. Eur. J. Clin. Nutr. 2012, 66, 1187-1192.
- Zhang, Q. et al. Nutr. J. 2013, 12, 84.
- Dulloo, A. G. et al. Am. J. Clin. Nutr. 1999, 70, 1040-1045.
- Kuroda, Y. et al. Health Effects of Tea and Its Catechins; Kluwer Academic/Plenum Publishers: New York, 2004.
- Yammamoto, T. et al. Chemistry and Applications of Green Tea; CRC Press LLC: New York, 1997.