The green tea (camellia sinensis)
The Composition of Green Tea (Camellia sinensis, family Theaceae)
Green tea is a frequently used beverage worldwide and is a hot-water extract of the leaves of the plant Camellia sinensis (Bruneton, 1999; Duthie & Crozier, 2003; Wang, Kim, & Lee, 2000; Weisburger, 2004). Green tea is also a medicinal plant and its leaves contain interesting phytochemicals, the polyphenols epigallocatechins gallate and an enzyme, polyphenol oxidase. In particular, green tea leaves contain, in quantities equivalent to 20-30% of the dry weight, a range of polyphenols, the flavan-3-ols or flavonols, the so called (+)-catechin and (-)-epicatechin and their gallate derivates, caffeine (2.5%-4.5%), dimeric theaflavins, oligomeric procyanidins, flavonoid glycosides, small quantities of theobromine (0.05%), theophylline (0.02%-0.05%) and volatile oil (Braun & Cohen, 2005; Bruneton, 1999; Duthie & Crozier, 2003; Evans, 2002; Jochmann, Gert; & Stangl, 2008). In addition, according to Duthie and Crozier (2003), green tea provides a small intake of macronutrients; less that 2% of the hot-water-soluble solids of tea are proteins, 4-5% are carbohydrates and 2-3% are lipids, mainly linoleic and linolenic acids. Also, green tea contains a range of micronutrients including manganese, potassium, niacin, riboflavin, folate and zinc.
The epigallocatechin gallate (see appendix 1), is one of the most abundant polyphenols in tea and is regarded as the most significant pharmacologically active component (Braun & Cohen, 2005; Inoue et al., 2008). It is important to mention that polyphenols are known as major components of natural products, such as fruits and vegetables, in terms of their functional properties, for example; inhibition of tooth decay, allergy and oxidation, reduction of blood pressure, and prevention of gout. The high presence of catechins indicates that green tea extracts have considerable antioxidant activity that compares favourably with the antioxidant nutrient vitamin E. Furthermore, research studies revealed that catechins are chiefly responsible for the chemoprotective, antiviral, antibacterial, antioxidative, anticarcinogenic and thermogenic activity of green tea (Braun & Cohen, 2005; Bruneton, 1999; Inoue et al., 2008).
Affects of Processing
Green tea, particularly catechins, must be stable in finished products in order to deliver health benefits. However, catechins may be adversely affected (oxidation) by tea processing, manufacturing and prolonged storage as well as by the presence of other components in green tea (Ortiz, Ferruzzi, Taylor, & Mauer, 2008; Manzocco, Anese, & Nicoli, 1998).
In terms of processing, although black, oolong and green tea are produced from the same plant, they all differ in their polyphenol content regarding the way their leaves are treated. For instance, comparing to black and oolong tea which are made from oxidised or partially oxidised leaves respectively, green tea leaves are not oxidised (Cabrera, Artacho, & Giménez, 2006). This is because the enzymes which catalyze the catechins oxidative polymerization, are deactivated by heat treatment (pan-roasting or steaming) soon after plunking. The deactivation of the polyphenol oxidase (PPO) by the pan-firing of the fresh leaf in the green tea process halts this fermentation process and a much lower loss of catechins is observed (Astill, Birch, Dacombe, et al., 2001). Since the oxidising process converts many polyphenolic compounds into others with less activity, thus green tea (unfermented tea) is considered to have the strongest therapeutic effects and the highest polyphenol content (Astill, et al., 2001; Braun & Cohen, 2005; Evans, 2002).
According to Ortiz and colleagues (2008), storage conditions (moisture above 69% RH) and the presence of other components played a significant role in catechin degradation. The authors also revealed that additional ingredients to powdered green tea such as sucrose (at 75% RH), citric acid (at 58% RH) and/or ascorbic acid (at 75% and 85% RH) promoted catechin degradation when exposed to environmental relative humidity. It was pointed out that catechins degraded more at a pH close to 2, whereas they were more stable at a pH close to 5. Furthermore, the length of storage (12 weeks), the interactions of time and RH, time and formulation, and all three factors, had a significant effect on catechin degradation. A research study evaluating the effects of heat and storage on catechins of green tea extract revealed that catechins appeared to play a key role in the changes of both colour and taste of green tea as result of the flavanol oxidation (Wang, et al., 2000). Additionally, the authors concluded that steamed tea due to its stability to colour and taste changes and its high content of catechins appeared to be better than roasted tea for the preparation of canned or bottled beverages.
It is also noteworthy to mention that preparation variables such as infusion temperature, water quantity, initial concentration, brew time and amount of agitation are factors that will reduce the amount of polyphenols and caffeine extracted into a tea infusion (Astill, et al., 2001). However, according to Duthie and Crozier (2003), the tea drunk today may be less rich in macronutrients than in previous decades as infusion times have decreased from 5-6min to 40-60s. With respect to the caffeine, variation in caffeine content within green tea is further influenced by growing conditions, manufacturing processes and particle size of the tea leaves (Astill, et al., 2001; Braun & Cohen, 2005).
Evaluating effects in the body
The possible beneficial effects of drinking green tea have received considerable coverage in the medical and national press (Cabrera, et al., 2006; Braun & Cohen, 2005; Evans, 2002; Wang, et al., 2000).
There have been extensive in vitro and animal studies of possible mechanisms by which green tea polyphenols may potentially influence both carcinogenic initiation and promotion and protect against cancers (Bruneton, 1999; Duthie & Crozier, 2003; Inoue et al., 2008; Manzocco, et al., 1998). These mechanisms include direct and indirect antioxidant protection of DNA, reduction of oxidative stress, the modulation of enzyme systems as cytochrome P450 complexes that metabolise carcinogens or procarcinogens to genotoxins and the modulation of malignant transformation, apoptosis and gene expression. EGCG blocks urokinase, an enzyme which appears to be implicated in cancer cell proliferation and circulation (Bruneton, 1999). According to Weiburger (2004) polyphenols decreased the mutagenicity of different types of carcinogens. Selenium potentiated the effect of green tea on the mutagenicity of 2-amino-3-methylimidazo-[4,5-f] quinoline. Furthermore, according to Evans (2002) possible beneficial effects are inhibition of angiogenesis, a process involving the growth of blood vessels necessary for tumour growth and metastasis. It can be argued that green tea
With respect to antioxidative activity, green tea polyphenols act as powerful antioxidants that inhibit the oxidation of LDL cholesterol caused by reactive oxygen species, and lead to atherogenesis (Manzocco, et al., 1998; Yang & Landau, 2000; Weiburger, 2004). Similarly, Braun and Cohen (2005) and Bruneton (1999) stated that green tea inhibits lipid peroxidation and scavenges hydroxyl and superoxide radicals. Weiburger (2004) pointed out that green tea had a more powerful effect, expressed as vitamin C equivalents, than the vegetables and fruits tested. He also revealed that the tea polyphenols suppress the growth of the enterobacteriaceae in the intestinal tract. Then the consequence is a lower Ph of the intestinal contents since there is formation of organic acids. Therefore, regular green tea drinkers have a healthier intestinal bacterial flora.
Furthermore, green tea catechins have been reported to have antibacterial and antiviral activity (Cabrera, et al., 2006; Mitscher, Jung, Shankel, et al., 1997). Then, immune system is improved since green tea protects it against oxidants and radicals. Drinking green tea it leads to an increase of gamma delta lymphocytes and in turn of gamma-interferon, reflecting a raised immune response.
Furthermore, green tea has been found to increase thermogenesis, to inhibit gastric and pancreatic lipases and to control body weight and lower body fat (Yang & Landau, 2000). Jochmann, Baumann and Stangl (2008), revealed that green tea has antiobese properties by suppressing body fat accumulation and reducing body weight. According to the authors, in humans, consumption of green tea is associated with significant BMI and body fat reduction. Mechanisms behind could be the green tea thermogenic properties attributable to the joint-effect of caffeine and catechins, an increased energy expenditure and promoted fat oxidation (Dulloo, Duret, Rohrer, et al., 1999).
Epidemiological and experimental studies have established a positive correlation between the consumption of green tea and protection against cardiovascular diseases and risk factors (Evans, 2002; Jochmann, et al., 2008). A possible mechanism by which green tea may reduce heart disease risk is via the ability of catechins to prevent the oxidation of low-density lipoprotein cholesterol to an atherogenic form as an oxidised LDL is important in the pathogenesis of the atherosclerotic lesion (Hertog, et al., 1997; Jackson, 2003). According to Duthie and Crozier (2003) green tea may prevent atherosclerosis by preventing platelet adhesion and aggregation by inhibiting the cyclooxygenase pathway and reducing the cyclic 3’,5’ –adenosine monophosphate (AMP) response of platelets to prostagladin I2. In addition, vasodilatory effects of tea extracts and polyphenols may be due to their affecting enhanced nitric oxide generation, cyclic 3’, 5’ –guanosine monophosphate (GMP) accumulation and other endothelium-dependent relaxation factors. Also caffeine in tea may reduce blood coagulation by inhibiting thrombin-stimulated thromboxane formation. Although caffeine stimulates the central nervous system and has a week diuretic action, theobromine acts in the reverse way. Theophylline also has a powerful diuretic action and relaxes involuntary muscles more effectively than either caffeine or theobromine (Cabrera, et al., 2006). Additionally, studies investigating long-term effects revealed a link between green tea consumption and hypertension claiming the antihypertensive properties of green tea (Jochmann, et al., 2008). The vasodilator and antioxidant effects of polyphenols appeared to contribute to lower blood pressure and improve endothelial function (Yang, Lu, Wu, J., Wu, C., & Chang, 2004).
Conclusion
As shown, a large and growing body of evidence suggested that green tea is one of the most promising dietary agents for the prevention and treatment of many diseases and other health related issues. Green tea catechins, presented in high quantities because of the difference in the processing of tea leaves, possess powerful antioxidant, anti-inflammatory, anti-bacterial and anticarcinogenic properties. Recent human studies suggest tea may contribute to a reduction in the risk of cardiovascular disease and some forms of cancer as well as to the promotion of oral health and other physiological functions. As tea is already one of the most popular beverages worldwide, future studies, designed to accurately assess tea consumption and tea polyphenol status, should be directed to quantifying its role in the primary and secondary prevention of chronic diseases.
Recommendations
Since green tea beneficial effects are being increasingly proved, it would be advisable to consider its regular consumption in Western diets (Cabrera, et al., 2006). Then the long consumption of green tea by humans recommends it as apparently safe to use in reasonable doses (Mitscher, et al., 1997). Several investigations confirmed a lower risk of “cardio-vascular” diseases (11%) by intake of 3 cups (about 700mL) of green tea per day. Differently, habitual moderate green tea consumption (120 ml/day or more) for at least 1 year has been suggested to exert preventive effects with regard to development of hypertension in a Chinese population (Jochmann, et al., 2008; Yang, et al., 2004).
A number of human intervention studies with green tea demonstrated a significant increase in plasma antioxidant capacity by drinking 6 or more cups of tea per day which in turns facilitates healthy aging (Buttriss, 2003; Cabrera, et al., 2006; Weiburger, 2004). According to Bruneton (1999) the tea leaf can be used orally for the symptomatic treatment of mild diarrhea, functional asthenia, as adjunctive treatment in weight loss programmes and to enhance the renal excretion of water (diuretic).
With regard to green tea consumption and cigarette smoking, it has been suggested to drink more than 10 cups of green tea per day resulting in prevention of tobacco-associated increase in cancer incidence (Litt, Suga, & Nakachi, 1997). It is noteworthy to mention that because green tea infusions are thought to be bitter when infused in a high temperature, then it is recommended a brewing temperature between 70 and 80 0C. In the UK most tea is consumed by adding milk to it. However, because flavonoids in tea bind to milk proteins and are therefore not absorbed from the gastrointestinal tract. Also, adding milk to the tea abolish the antioxidant-raising capacity of tea (Hertog et al., 1997).
In sum, given the above it can be argued that drinking several cups of green tea a day keep the doctor away. However, herbs contain active substances that can trigger side effects and interact with other herbs, supplements, or medications. For these reasons, people should take herbs with care, under the supervision of a practitioner knowledgeable in the field of botanical medicine.
By Konstantina Fyta, Ph.D
Educator and Cellular Nutrition Advisor
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