Health Benefits Derived from Green Tea
Catechins are the most active compounds
Catechins - a very interesting class of chemical constituents in green tea - are believed to have a range of beneficial health effects such as neuroprotective activity, and anti-inflammatory, antiulcer, antiviral, antibacterial, and antiparasitic effects. The best known catechin is epigallocatechin 3-gallate (EGCG), which constitutes 50-75% of the total flavonoid content in green tea, and is the single most studied catechin in relation to health contributing potential. EGCG is a potent antioxidant, and acts to inhibit a number of physiologically important enzymes including histone acetyltransferase, DNA methyltransferase, fatty acid synthase, glutamate dehydrogenase, and topoisomerase I and II, and it has been shown to interfere with the epidermal growth factor receptors (ErbB-1 and ErbB-2). There is some evidence that it might have therapeutic applications in the treatment of some types of cancer, chronic fatigue syndrome, endometriosis, neurodegeneration, periapical lesions, regulating the HIV viral load, Sjögren's syndrome, and spinal muscular atrophy, to name a few.
The closely related compound, gallocatechin gallate (GCG), can be produced by the epimerization of epigallocatechin 3-gallate, a process that can occur at high temperatures. GCG also has beneficial health effects, and in some cases exhibits a stronger effect than EGCG.
Oral hygiene and antibacterial effects
When bacteria such as Streptococcus mutans cause tooth decay, they first build a carbohydrate-based polymeric adhesive plaque on the tooth surface, and then produce lactic acid that can dissolve the tooth. Preventing the formation of this plaque is a way to avoid tooth decay. If a dilute sugar solution is inoculated with the S. mutans organism, introduced onto a glass slide, and left for a number of days, a large amount of plaque will accumulate on the slide. Mixing an aqueous solution of tea into the sugar solution will result in a remarkable control of the plaque formation. A normal cup of tea has a catechin concentration of 500-1,000 ppm, but tests with serial dilutions of tea while holding the concentration of the sugar solution constant found that only a slight amount of plaque formation remains, even with 8-fold dilution. This is because the enzymes in the tooth decay bacteria that produce the plaque are inhibited by the tea polyphenols.2
Green tea polyphenolic catechins, in particular EGCG and epicatechin gallate (ECG), can inhibit the growth of a wide range of Gram-positive and Gram-negative bacterial species with moderate potency. Evidence is emerging that these molecules may be useful in the control of common oral infections, such as dental caries and periodontal disease. Sub-inhibitory concentrations of EGCG and ECG can suppress the expression of bacterial virulence factors and can reverse the resistance of the opportunistic pathogen Staphylococcus aureus to β-lactam antibiotics. For example, relatively low concentrations of ECG can sensitize clinical isolates of methicillin-resistant S. aureus (MRSA) to levels of oxacillin and other beta-lactams (but not of other antibacterial agents tested) that can be readily achieved in clinical practice. Catechin gallates such as ECG can intercalate into phospholipid bilayers and it is likely that they affect both virulence and antibiotic resistance by perturbing the function of key processes associated with the bacterial cytoplasmic membrane.2 The antibacterial action of epicatechin gallate plus oxacillin was a bactericidal one.3
In other matters related to oral hygiene, it has been observed that tea catechins exert a substantial deodorizing action toward breath odor (methyl mercaptan) and fishy odor (trimethylamine).4,5
Stomach cancer and ulcers
A study reported that when animals were fed carcinogenic substances that are known to give rise to esophageal, pregastric, and gastric tumors, tea catechins given orally exhibited an antitumor effect, although this has yet to be demonstrated in humans.6 These researchers had observed that the population in a region of Shizuoka prefecture had a gastric cancer mortality rate (corrected for age) that was much lower (~20%) than the national average. When they investigated, they found that this area was the leading tea-producing region in the entire country (Kawane tea, from Kaminagare, Ooigawa), and the inhabitants drank it on a daily basis it was ample and abundant. Recently, a bacterium that inhabits the gastric mucosa of humans, Helicobacter pylori, has been suspected of inducing acute gastritis and eventually stomach cancer. Tea polyphenols in vitro at or below levels of 50-100 ppm inhibit the growth of H. pylori.7 Thus, in a study of 34 people who were carriers of H. pylori, consumption of 700 mg of catechins every day (equivalent to 10 cups of tea) for a period of one month led to significantly reduced bacterial counts in nearly all of the participants, while the bacteria had been completely eliminated in six individuals.
Blood sugar, obesity, and diabetes
The tea polyphenols that clear the stomach undergo partial absorption into the body through the small intestine, while the majority remains in the gut to perform its beneficial function. The α-amylase secreted into the small intestine by the pancreas and the sucrase that is present in the wall of the small intestine play a role in the digestion of sugars. It has been recognized that tea catechins exhibit an inhibitory action on these digestive enzymes. It has further been demonstrated that in rats fed starch or sucrose, the increase in blood sugar levels can be controlled when tea catechins are given beforehand.8
Daily consumption of tea containing 690 mg catechins for 12 weeks reduced body fat, which suggests that the ingestion of catechins might be useful in the prevention and improvement of lifestyle-related diseases, mainly obesity.9 This demonstrates the potential efficacy of tea catechins in conjunction with controlling increases in blood sugar and in dieting.
The effects of tea on obesity and diabetes have received increasing attention. Tea catechins, especially EGCG, appear to have antiobesity and antidiabetic effects. While few epidemiological and clinical studies show the health benefits of EGCG on obesity and diabetes, the mechanisms of its actions are emerging based on the various laboratory data. These mechanisms may be related to certain pathways, such as through the modulations of energy balance, endocrine systems, food intake, lipid and carbohydrate metabolism, the redox status, and activities of different types of cells (i. e., fat, liver, muscle, and beta-pancreatic cells). Because the EGCG receptor, the so-called 67-kDa laminin receptor (LR), has been discovered with colocalization of other types of LR and cytoskeleton in both cancer cells and normal cells, this may explain how EGCG possesses such a broad range of activity.10
Improving cardiovascular health
Studies have established a positive correlation between green tea consumption and cardiovascular health.11,12 The catechins in tea have been studied for their effects in the prevention and treatment of cardiovascular diseases. They exert vascular protective effects through several mechanisms, including antioxidative, anti-hypertensive, anti-inflammatory, anti-proliferative, anti-thrombogenic, and lipid lowering effects.
Tea catechins can:
• Exert antioxidant activity by scavenging free radicals, chelating redox active transition-metal ions, inhibiting redox active transcription factors, inhibiting pro-oxidant enzymes and inducing antioxidant enzymes;
• Inhibit the key enzymes involved in lipid biosynthesis and reduce intestinal lipid absorption, thereby improving blood lipid profile;
• Regulate vascular tone by activating endothelial nitric oxide;
• Prevent vascular inflammation that plays a critical role in the progression of atherosclerotic lesions. The anti-inflammatory activities of catechins may be due to their suppression of leukocyte adhesion to endothelium and subsequent transmigration through inhibition of transcriptional factor NF-kB-mediated production of cytokines and adhesion molecules both in endothelial cells and inflammatory cells.
• Inhibit proliferation of vascular smooth muscle cells by interfering with vascular cell growth factors involved in atherogenesis.
• Suppress platelet adhesion, thereby inhibiting thrombogenesis.
Controlling high blood cholesterol
The GCG concentration in some tea beverages can account for as much as 50% of the total catechins. In a study aimed at examining the effects of GCG-rich tea catechins on hyperlipidemic rats and the mechanisms associated with regulating cholesterol metabolism in the liver, it was found that heat epimerization of EGCG gave a mixture of catechins that had a GCG content of approximately 50%.13,14 In sucrose-rich diet-induced hyperlipidemic rats, the GCG-rich tea catechins exhibited strong activity in reducing plasma cholesterol and triglyceride concentrations. Furthermore, the hepatic cholesterol and triglyceride concentrations that had increased as a result of the sucrose-rich diet were reduced due to consumption of GCG-rich tea catechins. When the hepatic expressions of LDL receptor and HMG-CoA reductase were in hyperlipidemic rats, it was found that purified GCG increased LDL receptor protein level and activity to a greater extent than EGCG. This study indicated that GCG-rich tea catechins in tea beverages may be effective in preventing hyperlipidemia by lowering plasma and hepatic cholesterol concentrations.13