Green Tea: Benefits, Dosage, Contraindications

Antioxidant

The antioxidant effects of green tea and its constituent catechins are thought to explain some of its clinical effects, particularly its anti-inflammatory, anticancer, hepatoprotective, and cardiovascular benefits. In human studies, it has been shown that green tea and its catechins protect against oxidative damage caused by exercise and smoking, as well as general exposure to oxidation. The antioxidant effects of green tea and its constituents may be related to their ability to scavenge free radicals, chelate redox-active transition metal ions, inhibit pro-oxidant enzymes, and induce antioxidant enzymes.

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Anti-inflammatory

The catechins in green tea may have anti-inflammatory activity. Green tea-derived catechins inhibit cyclooxygenase-1 (COX-1). Green tea catechins may also inhibit the production of leukotriene-B4 and the activity of 5-lipoxygenase. Epigallocatechin gallate (EGCG) inhibits the activity of COX-2 and interleukin (IL)-1 beta-induced nitric oxide synthase. EGCG and other green tea catechins may also reduce inflammation and protect cartilage by inhibiting the breakdown of proteoglycans and collagen. In vitro, the anti-inflammatory activity of catechins may be due to the inhibition of the activity of inflammatory cells and cytokines, or the inhibition of proteins promoting the chemotaxis of these cells. In human research, the effects of green tea on inflammatory mediators are not clear. Studies show that green tea increases IL-6 levels, decreases TNF-alpha levels but does not affect C-reactive protein levels.

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Anticancer

The polyphenols in tea appear to have antimutagenic effects and may protect DNA. In humans, green tea reduces DNA damage in lymphocytes, probably by increasing the activity of the DNA repair enzyme, oxoguanine glycosylase 1 (hOGG1), and the antioxidant enzyme, heme oxygenase-1. Preliminary animal research suggests that epigallocatechin gallate (EGCG) may prevent the growth of new blood vessels (angiogenesis) in tumors. EGCG may also inhibit tumor cell proliferation by causing cell cycle arrest or apoptosis through the formation of reactive oxygen species. Green tea is also thought to be beneficial in preventing skin damage and cancer due to ultraviolet (UV) rays because of the antioxidant effects of green tea polyphenols.

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Digestive Effect

The tannins in green tea may have antidiarrheal effects. The polyphenols in green tea may increase the levels of lactobacilli and bifidobacteria and reduce the number of enterobacteria. On the other hand, the caffeine in green tea stimulates gastric acid secretion.

Lipid-lowering

In human research, orally taken green tea appears to reduce total lipoproteins and low-density lipoprotein (LDL cholesterol). Some of this research also suggests that green tea could reduce triglycerides and increase levels of high-density lipoprotein (HDL cholesterol). In animals, green tea has also led to decreased levels of total cholesterol, LDL cholesterol, and triglycerides.

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Antibacterial

In vitro, green tea extract has moderate inhibitory effects on the growth of many types of pathogenic bacteria, including strains of Staphylococcus, Streptococcus, and Salmonella, Escherichia coli, and a strain of Corynebacterium suis. Another in vitro study demonstrated that green tea can significantly reduce the release of cytokines induced by bacterial endotoxins and may therefore reduce mortality from sepsis.

Antiviral

A number of in vitro studies have shown that epigallocatechin gallate (EGCG) strongly inhibits HIV replication and inhibits HIV virus attachment to T cells. Antiviral activity has also been identified against Epstein Barr virus, herpes simplex virus 1, influenza A and B, rotavirus, and enterovirus. In vitro research shows that green tea extract and EGCG inhibit human papillomavirus in cervical cancer cells. The antiviral activity is likely due to interference with viral adsorption.

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Cardiovascular

Studies in animals and humans reveal that green tea has multiple effects on the cardiovascular system, including lipid-lowering, cholesterol-lowering, anti-inflammatory, antithrombotic, antioxidant, antihypertensive, and antiatherosclerotic effects and also shows a beneficial effect on endothelial function in healthy volunteers.

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Neurological

In vivo studies have found a neuroprotective effect of epigallocatechin gallate (EGCG) on dopaminergic neurons, which is being studied as a potential preventive treatment for Parkinson's disease. Two animal studies have shown that EGCG can prevent neuron oxidation and apoptosis and reduce beta-amyloid deposits. This may have implications in the development and progression of Alzheimer's disease. On the other hand, the caffeine in green tea stimulates the central nervous system. The possible mechanisms include blocking adenosine receptors and inhibiting phosphodiesterase. By blocking adenosine receptors, caffeine is thought to increase the release of neurotransmitters such as dopamine. It has also been proposed that caffeine may decrease GABA and serotonin signaling.

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Hypotensive

In vitro experiments show that green tea extracts inhibit the angiotensin-converting enzyme (catalyzing the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor). Animal experiments have also shown that a green tea extract protected against angiotensin II-induced high blood pressure. In humans, the results of clinical research remain contradictory. Some clinical research suggests that taking green tea extract reduces blood pressure in hypertensive or normotensive patients.

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Dermatological Effect

Some evidence suggests that oral intake and topical application of products containing green tea may improve skin elasticity and hydration. Evidence suggests that epigallocatechin gallate (EGCG) and epicatechin gallate (ECG) catechins could inhibit 5 alpha-reductase and could potentially be useful in androgen-mediated skin disorders such as androgenetic alopecia, hirsutism, and acne. Green tea is also thought to be beneficial for preventing skin damage and cancer due to ultraviolet (UV) rays because of the antioxidant effects of green tea polyphenols. Green tea polyphenolic extracts, particularly EGCG and epicatechin-3-gallate, appear to produce dose-dependent topical protection against UVA and UVB rays.

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Hepato-protective

Clinical research shows that green tea consumption is associated with decreased concentrations of liver enzymes, including aspartate aminotransferase and alanine transferase, indicating a protective effect against liver disorders.