Tea is the second most widely consumed beverage in the whole world after water. It is produced from the buds, leaves, or delicate stems of plants belonging to the genus Camellia. The frequently used plant species for tea production is Camellia sinensis (L.) Kuntze (Graham, 1992).
Tea is manufactured in three primary forms, that is, black tea (fully fermented), green tea (unfermented), and oolong tea (semifermented). Green tea is prepared by preventing oxidation of polyphenols present in the green leaf. Black tea is produced through complete oxidation of the said polyphenols, while oolong tea is produced through partial oxidation of polyphenols (Graham, 1992).
History has it that tea was consumed for the first time in China, around 2737 BC. China, India, and Kenya are said to be the major world producers of tea, though the crop is cultivated in six continents. Approximately 3 billion kilograms of tea is said to be produced and consumed each year (Hayat et al., 2015).
Tea possesses significant thermogenic, cholesterol-lowering, neuroprotective, antihypertensive, anticarcinogenic, antimicrobial, anti-inflammatory, and antioxidative properties. Several epidemiological studies, meta-analyses, and research investigations suggest that tea and its biologically active (bioactive) polyphenolic constituents have countless beneficial outcomes on health, including prevention of many diseases, such as arthritis, cancer, cardiovascular disease (CVD), diabetes, genital warts, obesity, and stroke (Hayat et al., 2015).
Tea is mainly comprised of caffeine, minerals, polyphenols, and traces of amino acids, carbohydrates, and vitamins. The type of polyphenols present in a particular form of tea is determined by the level of fermentation that it has undergone. Green tea is comprised of catechins while black tea is comprised of tannins (Prasanth et al., 2019).
The European Food Safety Authority (EFSA) reported that, 126 mg of catechins are present in every 100 ml of green tea. On the other hand, the Food and Drug Administration (FDA) reported that, 71 mg of epigallocatechin gallate is present in 100 ml of green tea. In case of black tea, it was reported that 200 mg of flavonoids are present in 100 ml of the beverage (Rietveld & Wiseman, 2003).
Green tea is widely known for its health promoting properties largely attributed to its constituent compounds called polyphenols (flavanols and flavonols). The dominant polyphenols in green tea are known as catechins. Unfermented green tea is touted by scientists as the best source of catechins (Musial et al., 2020).
Catechins are said to be responsible for the anti-inflammatory, antioxidant, and chemopreventive properties exhibited by green tea. Catechins have several derivatives including epicatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate (Musial et al., 2020).
As antioxidant agents, catechins have shown ability to neutralize excess reactive oxygen species (ROS) and nitrogen species (RNS) produced in body cells. This action helps to prevent oxidative damage to body cells and tissues.
In addition, the antioxidant property of catechins may help to diminish the impact of a process called photoaging, that is, skin-aging induced by exposure to ultra violet radiation. According to research findings, photoaging can lead to edema, erythema, hyperplasia, melanoma/non-melanoma skin cancers, premature aging, and sunburn (Prasanth et al., 2019).
It has also been reported that green tea exhibits stress resistance and neuroprotective properties. Its ability to scavenge for free radicals makes it a potent stress mediator, in regulating stress induced by metal ions (Prasanth et al., 2019).
Green tea has also shown ability to induce expression of various types of antioxidant enzymes. This action helps to prevent oxidative damage of DNA molecules (Prasanth et al., 2019).
Polyphenols present in green tea have shown ability to induce autophagy. Autophagy refers to the process of cell regeneration which occurs in the body in order to replace damaged and worn-out cells (Prasanth et al., 2019).
Catechins present in green tea exhibit anticancer properties that could be useful in preventing various types of cancer including breast, esophageal, liver, lung, pancreatic, prostate, and stomach cancer (Musial et al., 2020).
Research findings also indicate that green tea polyphenols can inhibit tooth decay and reduce blood pressure (Cabrera et al., 2006).
Claims have also been made to the effect that consumption of green tea can help in the clinical management of diabetes. However these claims are still riddled in a lot of controversy necessitating the need for more research (Iso et al., 2006).
After consuming green tea, the catechins undergo phase II metabolism after which they get assimilated into blood plasma in conjugated and unconjugated forms.
Following are the health benefits and uses of green tea as derived from various sources of literature.
Prolonged exposure to ultraviolet (UV) radiation from sunlight may result in premature skin aging, also known as photoaging. This is because prolonged exposure to UV radiation activates an enzyme called tyrosinase. This enzymes is known to enhance production of melanin in skin cells. Excess melanin in skin cells is known to cause conditions such as freckles, melisma, and senile lentigo (Cichorek et al., 2013).
In addition, long-term exposure to UV radiation increases the levels of enzymes called matrix metalloproteinases in skin cells. These enzymes are known to cause degradation of skin collagen and elastin fibers. Furthermore, exposure to UV causes thickening of the dermis and epidermis. This leads to altered skin pigmentation, loss of skin tone, and subsequent wrinkle formation (Fisher et al. 1997).
Also, exposure to UV radiation is known to cause immunosuppression in skin cells. This action interferes with the main function of the skin which is to offer protection against invasion by pathogenic microbes and other destructive agents. In addition, immunosuppression may interfere with a process called autophagy, which entails regeneration of damaged and worn-out skin cells (Fisher et al. 1997).
Scientists have carried out a series of investigations to establish the association between consumption of green tea and photoaging using animal models and human subjects.
In this regard, researchers Lee et al. (2014) examined the anti-wrinkle effects of green, white, and black teas on photoaged hairless mice. The mice were first exposed to UV radiation in order to induce the process of photoaging. Thereafter, the mice were treated with water extracts of green, black, and white teas.
It was reported that the treated mice registered significant improvements in skin conditions. Specifically, these mice exhibited increased collagen and elastic fiber content as well as diminished epidermal thickness, which are the key signs of skin restoration.
Also observed was a significant reduction in the expression of an enzyme called matrix metalloproteinase-3 (MMP-3), which is involved in the degradation of collagen present in the skin. In addition, treatment with green tea extract was shown to significantly inhibit wrinkle formation in the hairless mice.
In another study, Lim et al. (2014) examined the protective effect of standardized green tea seed extract on skin photoaging in hairless mice. Photoaging process was induced in the mice by exposing them to ultraviolet B (UVB) irradiation. The mice were then subdivided into six groups each of which received different dietary supplements.
According to the findings, mice that received a diet rich in green tea seed extract recorded a significant reduction in wrinkle formation. Also observed was a significant reduction in the dermal content of collagen fiber in these mice. It was also noted that the expression of MMP enzymes significantly decreased by the end of the trial period.
These results suggest that dietary supplement of green tea seed extract could be useful in reducing the intensity of skin photoaging and wrinkle formation caused by prolonged exposure to UVB irradiation.
In another recent study involving human volunteers, subjects were made to consume capsules containing green tea catechins. After the trial period was over, it was reported that green tea catechins and metabolites were found to be present in human skin samples, that is, blister fluid and biopsies (Clark et al., 2016).
The implication of these results is that catechins and metabolites can reach the skin in both unconjugated (free) and conjugated forms following continuous consumption of green tea supplements. The presence of these catechins in skin cells can therefore help protect against the effects of UV radiation-induced photoaging.
In another study, researchers Mnich et al. (2009) assessed the efficacy of green tea extract against UV-induced p53 (a tumor protein) expression in keratinocytes. It was observed that green tea extract significantly reduced the number of apoptotic keratinocytes (sunburn cells).
In addition, the green tea extract was found to be highly effective in protecting from DNA damage, sunburn, and erythema.
In yet another study, researchers Li et al. (2009) examined the effect of varying concentrations of green tea extract in protecting the skin from UV induced photoaging and photoimmunosuppression.
It was reported that different concentrations of green tea extracts exerted significant protection against the UV induced processes of photoaging and photoimmunosuppression.
All the preceding studies suggest that consumption of green tea could actually be beneficial in protecting the skin against anatomical modifications caused by exposure to UV radiation.
Scientists have shown that accumulation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in body cells leads to oxidative stress that causes oxidative damage. Oxidative damage is known to cause chronic inflammation in body cells and tissues which leads to the development of chronic diseases such cancer, cardiovascular disease, among others.
Some empirical and non-empirical epidemiological studies have indicated that green tea may possess cancer-preventive capabilities. This action is attributed to polyphenols (specifically, catechins) present in green tea extract. These substances have shown powerful antioxidant properties that could be useful in preventing oxidative damage. This helps to prevent proliferation of cancerous cells in the body (Filippini et al., 2020).
i. Prevention of breast cancer
Green tea extract has been shown to possess compounds that are effective in preventing breast cancer. A study conducted by researchers Ogunleye et al. (2010) examined the link between consumption of green tea and the risk of breast cancer incidence or recurrence. The researchers did a systematic search of several databases and performed a meta-analysis of studies on risk of breast cancer recurrence.
It was reported that increased consumption of green tea (more than 3 cups a day) was inversely associated with recurrence of breast cancer.
ii. Prevention of prostate cancer
Consumption of green tea has also been associated with decreased risk of prostate cancer. In a study carried out by Kurahashi et al. (2008), the findings revealed that green tea consumption was associated with reduced risk of advanced prostate cancer.
The researchers established that green tea and its constituents have ability to inhibit cell growth, induce apoptosis, and interfere with the process of cell cycle. In addition, they reported that the catechins present in green tea inhibited invasion of tumor cells as well as expression of enzymes known as matrix metalloprotease (MMP). These enzymes have been associated with the process of metastasis (that is, spread of cancer cells) in patients with prostate cancer.
iii. Prevention of colorectal cancer
Green tea has also been associated with reduced risk of colorectal cancer incidence in females. Researchers Chen et al. (2017) demonstrated that an inverse relationship exists between consumption of green tea and risk of colorectal cancer. It was reported that green tea had a protective effect among female patients with rectal cancer.
Scientists have demonstrated that oxidative stress and inflammation significantly contribute towards the development of neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases.
Empirical and non-empirical studies have claimed that consumption of green tea in right doses can prevent occurrence of neurodegenerative diseases as one advances in age.
Human epidemiological and new animal data suggests that consumption of tea significantly reduces the incidence of dementia, Parkinson’s, and Alzheimer’s diseases. Specifically, research findings have identified a catechin known as epigallocatechin gallate (EGCG) as the substance responsible in exerting the neuroprotective/neurorestorative activities against neurodegenerative disorders (Mandel et al., 2008).
A study conducted by Kuriyama et al. (2006) explored the link between consumption of green tea and cognitive functions in human subjects. It was reported that a higher consumption of green tea contributed to a lower prevalence of cognitive impairment.
In another study carried out by Checkoway et al. (2002) it was reported that daily consumption of 2 or more cups of green tea reduced the risk for Parkinson’s disease among cigarette smokers.
Pervin et al. (2018) also explored the efficacy of green tea consumption against neurodegenerative diseases. According to the findings, a green tea catechin known as epigallocatechin gallate was shown to inhibit accumulation of fibrous proteins (such as α-synuclein and Aβ) associated with neuronal cell impairment and death in the cerebral cortex.
In vitro studies have shown that catechins present in green tea can inhibit the growth and multiplication of oral pathogenic microbes. For instance, Steinmann et al. (2013) examined the activities of epigallocatechin gallate against various pathogenic microbes. Specifically, the study examined the effect of green tea catechins on growth and proliferation of an oral pathogenic bacteria known as Streptococcus mutans. This bacteria is responsible for causing tooth decay or dental caries.
In a similar study, researchers established that epigallocatechin gallate present in green tea could inhibit growth of oral pathogenic bacteria such as Porphyromonas gingivalis, Prevotella intermedia, and Prevotella nigrescens. In addition, the study established that epigallocatechin gallate could inhibit adherence of Porphyromonas gingivalis onto the epithelial cells of the buccal cavity. The three bacteria are known to cause periodontal infections (gum/bone infections) such as periodontitis and gingivitis (Sakanaka et al., 1996).
In another study, researchers showed that polyphenols present in green tea extract could prevent proliferation of bacteria known as Solobacterium moorei. This bacteria is responsible for causing halitosis or bad smelling breath (Morin et al., 2015).
Still on the same subject, researchers Lodhia et al. (2008) demonstrated that green tea extract have ability to reduce concentration of bacteria that cause bad smell in the oral cavity. In this study, it was observed that green tea powder reduced the amount of volatile sulfur compounds such as hydrogen sulfide and methanethiol produced in the oral cavity. This action was attributed to the antimicrobial and deodorant activities of polyphenols present in green tea.
Therefore, by preventing the growth and proliferation of all the mentioned bacteria green tea is said to be an effective promoter of better oral health.
Green tea contains caffeine, which has ability to stimulate the central nervous system (the brain and spinal cord) by activating noradrenaline neurons and effecting the release of dopamine. Both noradrenaline and dopamine are known to enhance transmission of impulse (messages) from the central nervous system to the rest of the body (Nehlig et al., 1992).
Research findings indicate that caffeine has ability to enhance brain function by improving physical endurance, memory, mood, reaction time and vigilance (Ruxton, 2008).
Green tea also contains another brain-boosting compound called L-theanine. This is a non-protein amino acid with ability to enhance alertness in an individual. It also enhances production of dopamine and alpha waves in the brain (Nobre et al., 2008).
Both caffeine and L-theanine are said to work synergistically, which results in enhanced brain functioning.
People who drink green tea intimate that it makes them more energetic and productive compared to when they take coffee (Simon et al., 2008).
Research findings indicate that consumption of green tea may reduce the risk of cardiovascular diseases such heart disease and stroke. This observation is linked to the antioxidant property of catechins in green tea preparations (Kuriyama, 2008).
Specifically, green tea catechins help to improve risk factors associated with cardiovascular diseases. This occurs because the catechins have shown ability to improve the levels of low density lipoprotein cholesterol and total cholesterol in the blood.
Accumulation of calcium in the coronary artery is one of the conditions that contributes towards occurrence of cardiovascular diseases. Consequently, scientists have demonstrated that regular consumption of green tea may actually slow down coronary artery calcification. This in effect reduces the risk of cardiovascular diseases (Miller et al., 2017).
Diabetes is associated with elevated levels of blood sugar due to inability to synthesize insulin or insulin resistance.
Researchers have established that green tea extracts can improve insulin sensitivity as well as reduce the level of sugar in the blood. For example, Liu et al. (2013) explored the effect of green tea consumption on glucose control and sensitivity to insulin.
It was reported that consumption of green tea extract contributed to a decline in fasting glucose in the blood as well as haemoglobin concentrations. Also reported was a significant decrease in fasting insulin concentrations.
In another study, researchers explored the relationship between consumption of green tea and the risk of type 2 diabetes. It was reported that individuals who consumed green tea continuously had lower chances of contracting type 2 diabetes. In addition, this observation was more pronounced in women compared to overweight men (Iso et al., 2006).
Research findings indicate that consumption of green tea can increase fat burning and enhance metabolic activities in the body (Diepvens et al., 2007).
Consumption of green tea-caffeine mixture can help maintain body weight by promoting thermogenesis and oxidation of fats. This is action is attributed to the catechins and caffeine present in green tea (Western-Plantenga, 2010).
Thermogenesis involves dissipation of heat energy by the brown adipose tissue and skeletal muscles. Oxidation of fats entails breakdown of fats into fatty acids and glycerol. It can thus be seen that the two processes help the body to get rid of excess/undesired fats.
In the same vein, researchers Rains et al. (2011) showed that consumption of green tea catechins (270 mg to 1200 mg per day) may help a person to reduce body weight and fat. Their study established that green tea catechins can influence the activities of the sympathetic nervous system, induce energy expenditure through the process of thermogenesis, and enhance oxidation of fats in the body.
The researchers also intimated that catechins can modify appetite patterns, up-regulate enzymes involved in fat oxidation, and decrease absorption of nutrients into body cells/tissues.
All these activities help the body burn excess fats and reduce the risk of obesity in an individual.
Given that consumption of green tea could enhance metabolism in the short term, it could also be an effective weight loss tool.
A number of studies have intimated that consumption of green tea could actually reduce body fat, more so, in the abdominal area. For instance, a study by Auvichayapat et al. (2007) explored the effectiveness of green tea consumption on weight reduction in obese individuals. The findings revealed that consuming green tea significantly reduced body weight in obese individuals by enhancing energy expenditure and oxidation of fats.
Another study demonstrated that daily consumption of green tea for 6 months resulted in a slight reduction in body weight, improved high density lipids (HDL) and enhanced glucose homeostasis in overweight subjects (Stendell-Hollis et al. (2010 ).
On the same note, Nagao et al. (2007) explored the efficacy of high intake of green tea catechins in reducing body fat and risk of cardiovascular disease. Subjects in the experimental group ingested 583 mg green tea catechins each day for 14 weeks.
According to the results, individuals in the treatment group recorded a significant decrease in hip circumference, waist circumference, body fat mass, body fat ratio, body mass index, and body weight. Also noted was a significant decrease in both systolic blood pressure and low-density lipoprotein cholesterol.
These results suggest that continuous ingestion of a green tea extract rich in catechins could lead to reduction in body fat, systolic blood pressure, and low density lipoprotein cholesterol. This suggests that green tea extract could be an effective tool for reducing obesity and the risk of cardiovascular disease (CVD).
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