Tea – health benefit or risk? What is the evidence?

Active Compounds in Tea 

Tea is made from the leaves of the plant Camellia sinensis. Black tea, more common in Europe and America, is made from the older leaves, while white and green picked from the younger leaves, are all widely hailed in the media for their miraculous properties. I will guide you through the scientific evidence.

Tea contains molecules called polyphenols and catechins which have gained lots of media attention for their reported antioxidant, anti-inflammatory, anti-aging, and anti-cancer properties. Approximately 40% of dried tea are polyphenol compounds. Epigallocatechin gallate (EGCG) is one of these compounds, found predominately in green tea.

Cancer

There have been lots of studies looking at the effects of ECGC in the laboratory. These are primarily performed on single cancer cells and some animal models. EGCG has been found to suppress colorectal cancer formation in cellular models in the laboratory, and also increase the sensitivity to the standard chemotherapy1. In these cellular laboratory models, EGCG was associated with upregulation of tumour suppressor genes that help prevent the cell becoming cancerous. These anti-cancer effects were also confirmed in an animal model of colorectal cancer that was already resistant to the standard chemotherapy. ECGC was able to re-sensitise the tumour to the chemotherapy1. While these studies are important, interpretation of their results should be cautioned, since single cell models have a number of well recongised limitations. For example, the activity of the active compounds when directly applied to a single cell is potentially very different to after they have been ingested and under gone what is termed ‘first pass metabolism’, where they are altered by the gut and the liver. Therefore, more research is needed to see if this effect is also seen in patients with colorectal cancer. 

Similar effects have also been seen in the laboratory with other cancers such as lung, prostate, skin, breast, glioma, and head and neck, but again more research is needed to see understand the mechanism of action, and if the same effects are seen in patients with these tumours. 

Small proof of principle trials in patients with pre-cancerous lesions or early cancer have been performed with some promising results. One study that gave either a placebo (no treatment) or green tea supplement to men with pre-cancerous lesions for prostate cancer. This study was ‘double-blinded’, meaning neither the medical team nor the patients knew which treatment they received until after the results had been analysed. The study showed that the green tea supplements significantly decreased progression to prostate cancer compared with the placebo group after two years of follow-up2. A similar effect was seen in patients with pre-cancerous lesions in the mouth. Green tea supplementation significantly reduced the number of patients who progressed to oral cancer compared with placebo3. Again, another study found similar effects with pre-cancerous lesions in the large bowel. Supplementation with green tea significantly reduced the number of lesions which became cancerous, compared with the placebo group4.

At present, there isn’t conclusive evidence in the population that these effects translate into patients, as very large, time consuming studies are needed. An epidemiological study that combined 41 studies following over 3 million people, failed to find an association between drinking tea and cancer risk5. However, this was an aggregation of lots of studies, looking at slightly different endpoints, and a range of cancers. instead research that takes a large group of people and randomly assigns them to tea consumption or not (randomised controlled trial), would provide more robust evidence of any association. However, these types of trials are expensive, and unlikely to be funded by a pharmaceutical company or a tea manufacturer.

We also don’t yet know how drinking tea compares with supplementation, and how many cups of tea would be equivalent or have the same effect.  

Chronic Inflammatory Diseases

Tea is also thought to potentially have a role in treating chronic inflammatory disease such as liver, gut and neurodegenerative conditions because of their antioxidant properties. One study found that green tea supplementation in animal models with chemical induced inflammatory bowel disease was as effective as standard therapy6. How this translates into treating patients with spontaneous chronic inflammatory disease is not yet known.

Toxicity

There are occasional reports of people developing liver failure after taking green tea supplements. It is unclear what dose of supplementation these were and if there were any associated medical conditions. However, there have been no reports of toxicity from drinking tea in moderation. 

Summary

In summary, drinking tea, in particular green tea, in moderation, may provide a safe way of reaping the potential benefits of polyphenols and catechins that have already been shown in the laboratory. While drinking tea is unlikely to be a first line treatment for cancer, it might have a role in cancer prevention, and prevention of recurrence2,7.

References

1.      Toden, S., Tran, H.-M., Tovar-Camargo, O. A., Okugawa, Y. & Goel, A. Epigallocatechin-3-gallate targets cancer stem-like cells and enhances 5-fluorouracil chemosensitivity in colorectal cancer. Oncotarget7, 16158–16171 (2016).

2.     Brausi, M., Rizzi, F. & Bettuzzi, S. Chemoprevention of human prostate cancer by green tea catechins: two years later. A follow-up update. Eur. Urol.54, 472–473 (2008).

3.     Tsao, A. S. et al.Phase II randomized, placebo-controlled trial of green tea extract in patients with high-risk oral premalignant lesions. Cancer Prev Res (Phila)2, 931–941 (2009).

4.     Shimizu, M., Adachi, S., Masuda, M., Kozawa, O. & Moriwaki, H. Cancer chemoprevention with green tea catechins by targeting receptor tyrosine kinases. Mol Nutr Food Res55, 832–843 (2011).

5.     Yu, F. et al.Tea consumption and the risk of five major cancers: a dose-response meta-analysis of prospective studies. BMC Cancer14, 197 (2014).

6.     Oz, H. S., Chen, T. S., McClain, C. J. & de Villiers, W. J. S. Antioxidants as novel therapy in a murine model of colitis. J. Nutr. Biochem.16, 297–304 (2005).

7.     Lambert, J. D. Does tea prevent cancer? Evidence from laboratory and human intervention studies. Am. J. Clin. Nutr.98, 1667S–1675S (2013).

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