Green tea has become one of the most popular beverages consumed globally with about 120 ml in volume drunk by virtually everybody, every year (Graham, 1992) and that amount has certainly risen in recent times.
This tea has been consumed for thousands of years in its home of China and helps to reduce stress and even helps with weight management. It is first thought to have been brewed during the reign of Emperor Shennong in 2737 B.C. as a lightly steamed infusion. It is produced from the freshly picked leaves of the Camellia sinensis plant which are steamed. We are probably more familiar with black, oolong and processed tea leaves which are fermented.
Leaving the tea leaves relatively unprocessed allows the important components which are generally antioxidants to remain largely unchanged. Green tea contains B vitamins, folate, manganese, potassium, magnesium, caffeine and the antioxidants, notably catechins.
There are seven reasons why this tea should be drunk:-
Reducing heart disease
Japanese scientists found that only one cup of green tea daily could lower the risk of heart disease and early death. In a four-year study of more than 90,000 people aged between 40 and 69, researchers found that the more green tea people drank , the less likely they were to die from cardiovascular heart disease, stroke and respiratory disease. Women drinking just one cup a day had a 10 per cent lower risk of dying prematurely, but this rose to 17 per cent if they drank five or more cups daily. The study also saw a similar trend in men.
Potentially reduces blood pressure.
A 2014 survey assessed various research studies as to whether drinking green tea could help lower blood pressure. There was some evidence for a modest reduction in people with high blood pressure who consumed green tea. However, whether this would lead to clinically significant results, such as preventing the onset of heart disease or stroke, remains unproven and further research is required to understand the precise mechanisms underlying the observations .
Reduction of cholesterol.
A review of 11 research studies in 2013 involving over 800 people found daily consumption of green and black tea either as a drink or a capsule could help reduce cholesterol and blood pressure. A similar review found drinking green tea enriched with catechins produced a small reduction in cholesterol, a main factor in causing heart disease and stroke. However, it’s still not clear from the evidence how much green tea we’d need to drink daily to see a positive effect on our health. The role of catechins as with other polyphenols is being examined.
Green tea may help those of us with seasonal allergy, because it’s been proven to be anti-allergenic. One compound especially, called epigallocatechin gallate (EGCG), appears to be the most potent. A 2007 study published in the journal Cytotechnology found this particular tea polyphenol could alleviate the impact of pollen allergies. Quercetin, a naturally occurring flavonol in tea, can also alleviate a histamine response.
L-Theanine is an amino acid naturally found in green tea leaves and is thought to generate a relaxing and tranquilizing effect, though you may have to drink six cups to really feel any impact (Juneja et al., 1999). It is known to block the binding of L-glutamic acid to glutamate receptors in the brain, hence the relationship with anti-stress effects by inhibiting cortical neuron excitation. It is worth noting that green tea contains a certain amount of caffeine which works counter to L-theanine. It is not clear then if the two components cancel each other out !
Prevent tooth decay.
Drinking black tea and coffee has a poor reputation because it stains teeth. A study in 2014 looked at how effective a green tea mouthwash was in preventing tooth decay compared with the more commonly used antibacterial mouthwash chlorhexidine. The results suggested they were equally effective and the green tea mouthwash was cheaper.
Drinking green tea appears to improve insulin sensitivity and possibly help against against the onset of type-2 diabetes (Iso et al., 2006). It also helps iron out rapid rises and crashes in blood sugar levels. The same effect was observed with black tea and coffee incidentally.
Antioxidants In Green Tea May Lower Risk Of Some Cancers.
Green tea may reduce some types of cancer based on various studies in human clinical research. Cancer is still the leading cause of death in many countries. Cancer, which occurs through uncontrolled cell growth is attributed to oxidative free-radical damage of DNA (Reuter et al., 2010). Antioxidants are known to protect cells by mopping up oxygen based free radicals.
Green tea contains a great array of antioxidants and a few studies illustrate this potential:-
Prostate Cancer – Prostate cancer is one of the leading causes of death for men but is lower for Asian men than for men in Western countries. Dietary habits may be at the heart of this explanation because the Japanese for example imbibe large quantities of green tea. A very large study known as the JPHC Study (Japan Public Health Center-based Prospective Study) began in 1990 with a second start –up in 1993. Based on questionnaires, 49,920 men aged between 40 and 69 were assessed for their health including cancer development and the amount of green tea they consumed. It’s important to stress one conclusion: when men drank at least 5 cups of green tea everyday they had a 48 per cent lower risk of advanced prostate cancer (Kurahashi et al., 2008) compared to those drinking just one cup.
Colorectal Cancer – One review of 29 different studies of varying quality found that those who drank sufficient green tea reduced their risk of developing this type of cancer (Chen et al., 2017).
Breast Cancer – Women who drink green tea were found to have between 20 and 30 per cent lower risk of developing breast cancers. This finding was based on a meta-analysis of various observational studies (Ogunleye et al., 2010). There are some positive outcomes to be seen even in prevention where early breast cancer has been observed (Lazzeroni et al., 2017).
It’s important to note that green tea is not a standard drink in many of these studies and that it is not a cure. There is a however a gathering body of evidence which increasingly finds for green tea in reducing the risk of developing certain types of cancer. Recent evidence suggests it may even prevent both primary and tertiary cancer formation but that remains to be worked on (Fujiki et al., 2015).
Components In Green Tea
Green tea continues to receive considerable medical interest because it contains such a variety of functional components. It includes flavonoids, caffeine, catechins, fluoride and flavonol glycosides (Graham, 1992; Engelhardt et al., 2000; Yuwono, 2005). Among these constituents, catechins account for about 8–15% of the leaf dry weight (DW). These have a significant and beneficial medical effect on human health, including strong antioxidative, antitumor, antibacterial and antiviral effects (Kim et al., 1994; Rice-Evans, 1999; Sudano Roccaro et al., 2004; Song et al., 2005). In addition, tea catechins have proven to play an important role in preventing cancer and cardiovascular disease (CVD), decreasing low-density lipoprotein (LDL) cholesterol and systolic blood pressure (Jankun et al., 1997; Kuriyama et al., 2006; Nagao et al., 2007). The latest research identifies reduced mortality from a variety of diseases including protection against colorectal cancer (Suzuki et al., 2009).
The catechins of green tea are composed of four major compounds, epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC) and epigallocatechin gallate (EGCG). We also find four important minor compounds, catechin (C), catechin gallate (CG), gallocatechin (GC) and gallocatechin gallate (GCG). All these were identified using HPLC with electrochemical detection (Yoshida et al., 1999; Sano et al., 2001). Of those catechins, EGCG is the most predominant one in green tea, accounting for about 50% of the total content of catechins, and is shown to possess about a five-fold higher antioxidant capacity than other better known antioxidants such as vitamins C and E (Rice-Evans, 1999; Cabrera et al., 2003; Saffari & Sadrzadeh, 2004). All these components need to be absorbed into the blood stream for them to have any benefit. There is about 1 to 5% (dry weight) of caffeine too.
Studies to understand the molecular mechanisms by which componentry, the catechins generally in green tea could exert a positive benefit recently talk a significant turn based on cell receptor studies by Paul Kroon’s team at the Institute Of Food Research In Norwich.
One of the key targets for EGCG in particular is a cell receptor heavily implicated in blood vessel growth with implications for cancer growth too. Epigallocatechin gallate from green tea and procyanidin oligomers from apple could inhibit the signalling process of vascular endothelial growth factor (VEGF). High levels of this growth factor promote angiogenesis which in turn adds to increased growth of cancer cell masses, tumours and promotion of atherosclerosis. The excitement from the research has been the inhibition of VEGF production by the components in green tea.
The EGCG specifically inhibited the signalling by directly interfering with VEGF when activating the VEGF receptor-2 (VEGFR-2) in human umbilical vein endothelial cells (Moyle et al., 2014). When VEGF binds to its receptor it activates a kinase, catalysing phosphorylation which is a common initiating step in receptor systems. Preventing the activating molecule such as VEGF from functioning properly would be of keen interest to those looking for suitable pharmaceuticals to control such activity.
Although studies at the cellular level are essential to understand the mechanisms of any activity, research in animal models will be the next logical step to understanding what is happening between green tea (and apple) components and various cells.
Some useful web-sites to follow include Authority Nutrition which examines 10 evidence-based health benefits for tea and Lifehack. The evidence for green tea’s nutritional benefits continue to be collected and no doubt we will see ever more research being presented.
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