Green tea is not only a worldwide popular stimulant, but also has therapeutic properties. In addition to theobromine, theophylline and caffeine, the active ingredients are catecholine compounds, which have strong antioxidant properties.
Tea is drunk all over the world. Next to coffee, tea is one of the most consumed beverages, with black and green tea enjoying particular popularity. Both types of tea are made from the tea plant (Camellia sinensis L.) – the processing makes the difference. Camellia sinensis, in addition to caffeine, theophylline and theobromine, is rich in catecholins, which belong to the group of polyphenols and have very strong antioxidant properties. These catechol dyes include epicatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate. During harvesting, the catechin tannins are enzymatically degraded. Black tea contains none or very few of these tannins. If the leaves of the tea plant are gently dried immediately after harvesting and steamed at 60 -70 °C, the catechin tannins remain. This is green tea. Since only green tea has the protective properties described below, the catechin tannins are obviously significantly involved in the effects.
Green tea as medicine
Studies continue to be published that provide evidence of possible medicinal uses for green tea. A 2004 study shows that in Taiwan, the greater the daily green tea consumption, the more the risk of developing hypertension decreases [1]. The beneficial effect of green tea in a patient with systemic light chain amyloidosis was reported in a 2008 article.
In recent years, scientific interest has focused on the possible preventive effect of green tea against cancer. For example, articles have appeared reporting the association of green tea intake and risk reduction of cancers, such as breast cancer [3] or GIT cancer [4]. In 2005, a review paper on cancer prevention by green tea was published, which found that from seven cups of green tea per day, a reduction or a time delay of chronic degenerative diseases can be expected [5].
Does green tea help against prostate cancer?
A review study published in 2017 looked at green tea intake and a possible reduction in the risk of developing prostate cancer [6]. The review evaluated ten studies that examined the association between green tea intake and the risk of developing prostate cancer: Four cohort studies, three case-control studies, and three randomized controlled trials.
The results were pooled and showed a dose-dependent reduction in the risk of developing prostate cancer. However, this reduction becomes significant only at a level of >7 cups/day – this in agreement with [5]. The study authors attributed negative results of previous studies to various study biases such as insufficient amount of green tea intake (number of cups/day, duration of green tea consumption) and failure to distinguish between green tea and black tea, which contains only a fraction of the catechins normally found in green tea due to enzymatic degradation after harvest.
Higher doses of green tea, e.g., >10 cups/day, further reduce the risk of disease. However, a pharmacological study in dogs showed an increasing risk of hepatotoxicity at higher doses [7]. Therefore, the FDA has limited the daily intake of green tea extract to 400 mg. In contrast, pharmacokinetic studies show no risk at daily doses of 9 -16 cups [8].
Review should provide clarity
Much has been written about the effectiveness of green tea against cancer in general. However, a systematic review in which all available studies would be evaluated has not yet been published. Recently, such a review has now been announced [9]. In this, the available databases will be reviewed and all case-control studies and RCTs will be evaluated for a possible protective effect of green tea against various cancers. The primary outcome variable is the overall responder rate. Secondary outcome variables include overall survival, progression-free survival, disease control rate, and adverse events.
At least seven cups!
The available studies provide evidence for a possible preventive effect of green tea against hypertension as well as on symptom relief in light chain amyloidosis. Various studies show a protective effect against prostate cancer, documenting non-significant efficacy at a few cups per day and indicating significance at a consumption of >7 cups/day.
Literature:
- Yang YC, et al: The protective effect of habitual tea consumption on hypertension. Arch Inter Med 2004; 164(14): 1534-1539.
- Mereles D, Wanker EE, Katus HA: Therapy effects of green tea in a patient with systemic light-chain amyloidosis. Clin Res Cardiol 2008; 97(5): 341-344.
- Yuan JM, et al: Green tea intake, ACE gene polymorphism and breast cancer risk among Chinese woman in Singapore. Carcinogenesis 2005; 26(8): 1389-1394.
- Borrelli F, et al: Systematic review: green tea and gastrointestinal cancer risk. Aliment Pharmacol Ther 2004; 19(5): 497-510.
- Bertram B: Cancer prevention by green tea – what is proven? Swiss Journal of Nutritional Medicine 2005; 3(2): 4-9.
- Guo Y, et al: Green tea and the risk of prostate cancer: A systematic review and meta-analysis. Medicine 2017; 96(13): e6426.
- Kapetanovic IM, et al: Exposure and toxicity of green tea polyphenols in fasted and non-fasted dogs. Toxicology 2009; 260(1-3): 28-36.
- Johnson R, Bryant S, Huntley AL: Green tea and green tea catechin extracts: an overview of the clinical evidence. Maturitas 2012; 73(4): 280-287.
- Cheng K, Chi NN, Liu JD: Green tea extract for treatment of cancers. Medicine 2019; 98(15): e15117.
HAUSARZT PRAXIS 2019; 14(6): 3-4
