Green tea is consumed widely across the world and has is believed to possess several health-promoting qualities. Green tea is derived from Camellia sinensis and is high in phenolic compounds which are composed predominantly of derivatives of catechin, in addition to low proportions of other compounds such as phenolic acids and flavanols.
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The bioactivity exerted by these compounds is associated with a reduced risk of many cancers, cardiovascular and neurodegenerative diseases, among several other effects.
The composition of green tea
Green tea consists of polyphenols (~ 90%), amino acids (~ 7%), thionine, proanthocyanins, and caffeine (~3%). Catechins and flavanols are among the polyphenols; and are the major constituents of green tea. Catechins include epigallocatechin-3-gallate (EGCG), which is the predominant C in green tea among other catechins such as epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epicatechin (EC).
EGC and EC constitute a sizeable proportion of the total, and their levels are found to increase with age. EGCG, representing 50 to 80% of the total catechins, is the major contributor to the health benefits attributed to green tea.
In vitro studies have demonstrated the powerful antioxidant properties of catechins, particularly EGCG, and studies in humans have demonstrated an increase in total antioxidant plasma capacity shortly after the ingestion.
Alongside catechins, green tea also contains chlorogenic acid and coumarylquinic alongside the tea-specific compounds theogallin (3-galloylquinic acid) and theanine (5-N-ethylglutamine). Caffeine is also present in green tea, as in other teas and coffees, with trace amounts of methylxanthines, theobromine, and theophylline.
The optimum consumption of green tea with antioxidants results in several benefits to the body which alongside preventing cancer and cardiovascular/neurodegenerative diseases, can help regulate cholesterol, mediate weight loss, reduce the inflammatory response, and regulate aging.
The polyphenols present in green tea are also attributed to the ability to inhibit tooth decay and reduce blood pressure.
Chemoprotective effects of green tea
Core features of polyphenols are their antioxidant, antibacterial, and anti-tumor properties. Among the polyphenols, EGCG has been observed to modulate several oxidative stress-mediated signaling pathways through their antioxidant effects – playing a role in cancer chemoprevention and treatment.
Several studies have ratified the chemopreventive potential of green tea catechins against a diverse range of cell cultures and preclinical studies of skin, lung, colon, mouth, liver, breast, esophagus, stomach, and prostate cancers.
Green tea polyphenols also block angiogenesis, a key hallmark of cancer, thereby inhibiting the development of new blood vessels. It further impairs the interaction of carcinogens with DNA that leads to carcinogenic mutations.
The effect of EGCG in vivo is, however, limited. The compound shows poor biopharmaceutical and pharmacokinetic properties because of reduced stability in the gastrointestinal tract as well as poor intestinal permeability and a short half-life in the plasma. Therefore, to optimize its in vivo potential, exploration to enhance its bioavailability is underway. These include coadministration with piperine, vitamin C, or sucrose and encapsulation techniques.
Green tea in anti-aging, neuroprotection, and autophagy
Flavonoids comprise <10% of total polyphenols and include known as kaempferol, quercetin, and myricetin. These are potent antioxidants, which exert their effects by preventing or limiting the effect of oxidative damage. Research suggests that green tea can delay collagen aging by an antioxidant mechanism and studies have corroborated its potential anti-photoaging effect, protecting against sun damage.
In the context of neurodegenerative diseases, EGCG, in particular, has been found to suppress the neurotoxicity induced by AB through its activation of glycogen synthase kinase which is involved in the development of the nervous system.
In addition, green tea polyphenols induce autophagy, producing a revitalizing effect. Apoptosis and autophagy are two mechanisms that are essential in neuro- and system-wide protection, as the reactive oxygen species (ROS) and reactive nitrogen species (RNS) produced regularly inside the body can harm different biomolecules and organelles and lead to an inflammatory response in the host.
This response could lead to neurological, age-associated diseases like Parkinson’s disease (PD) and Alzheimer’s disease (AD) as well as cancer due to the accumulation of damage-induced mutations that seed replicative in mortality genome instability and mutation unrestrained proliferative signaling doctor results in cancer.
Green tea and metabolic effects
Many scientific studies have indicated that the consumption of green tea is associated with weight loss. Green tea is a popular ingredient in weight loss supplements as it is tenuously linked to increasing the metabolic resting rate and alter lipid metabolism.
Available evidence shows that green tea can disrupt lipid emulsification, reduce adipocyte differentiation, increase thermogenesis via noradrenaline release and reduce food intake. Green tea has also been shown to decrease HDL cholesterol and limit the absorption of fats and cholesterol. Consequently, green tea can alter systemic metabolism in a manner that decreases fat mass.
On a metabolic level, the intake of green tea is also associated with a reduced risk of type II diabetes. Catechins, particularly EGCG, are implicated in the reduction in the levels of blood glucose; EGCG may mimic the action of insulin and improve insulin resistance. In a high-fat diet (HFD) mouse model, green tea has been shown to regulate lipid and glucose metabolism.
There are some negative health consequences associated with the overconsumption of green tea. Possible side effects of excessive consumption include nausea, dehydration, vomiting, tiredness, stimulation of the central nervous system, and heart rate irregularities. Polyphones present in green tea may also stain the teeth.
Overall, habitual green tea consumption, more notable in green tea extracts, is associated with several health benefits, including anti-obesity, anti-diabetes, anti-cancer, and anti-inflammation alongside neuro- and cardioprotective effects.
It should be noted that the effects of green tea are dose and time-of-treatment dependent and alongside adopting other health-promoting lifestyle changes, such as increased consumption of nutrient-rich foods and exercise, plays a role in preventing diseases and enhancing overall health.
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