Green Tea Extract
Green Tea extract contains phytochemicals, especially catechins like epigallocatechin gallate(EGCG) and caffeine. EGCG and caffeine have been shown to synergistically influence health indices, including body composition, but the contents within green tea extracts vary among brands. Notably, adverse effects have also been reported upon consuming green tea extract.
Sources and Composition
Origin and Composition
About 30% of the leaves by weight are either favonols or flavanols, the latter of which is comprised mostly of compounds called catechins. These catechins are then broken down into four main classifications of: epigallocatechin-3-gallate (EGCG), epi-gallocatechin (EGC), epicatechin gallate (ECG) and epicatechin (EC). Although all catechins share similar properties, EGCG appears to be most potent in regards to many of them. These four catechins are the main catechins, although some other isomers or conjugates may be present (with either catechin or epicatechin as a backbone, and varying levels of gallic acids).
Common sources of green tea catechins (primarily the four stated) include:
- Tea from the leaves of Camellia Sinensis, used to prepare White, Green, Oolong, and Black teas.
- Chungtaejeon Tea (Korean fermented tea) which has lesser amounts of catechins but a higher anti-oxidant potential
There are five 'catechins' in Green Tea. The parent compound is found in two isomers (catechin is the trans isomer, and epicatechin the cis isomer), and epicatechin also exists with a hydroxylation as epigallocatechin. Both epicatechin and epigallocatechin can become acylated with Gallic Acid to form gallates. For the purpose of this article, all five will be referred to as 'Green Tea Catechins'.
The two molecules that have gallic acid moeties hanging off the bottom right hand side of the above picture (EGCG and ECG) share properties that are not seen with the other two (EGC, EC, and C); this is because the gallic acid moiety itself may contribute to structure-function.
Formulations and Variants
A combination of green tea catechins and L-theanine called LGNC-07 has been investigated for its ability to improve memory. In one study on patients suffering from non-clinical memory decline (average age 58) it shows some efficacy, mostly in increasing recognition speed, word reading, and selective attention. LGNC-07 is a 6:1 ratio of green tea catechins to theanine, with 360mg catechins and 60mg theanine per 430mg capsule.
The mechanism of action of this combination, as seen in animal models, may be through acetylcholinesterase inhibition. The combination was more potent at alleviating toxin-induced memory loss than either compound in isolation.
LGNC-07 is a 6:1 standardized ratio of green tea catechins with L-theanine investigated for cognitive effects
COMT is an enzyme that is able to methylate and (usually) inactivate a few compounds, including green tea catechins and adrenaline/dopamine (monoamines) and L-DOPA. It exists in a cytosolic soluble form as well as a membrane-bound form, with the former being fairly distributed in the body; as assessed by erythrocyte COMT levels, rats may have more active COMT than humans, as assessed by species differences in COMT inhibiting drugs.
COMT is an enzyme that aims to inactivate several molecules in the body, in order to prevent their excessive elevation
All four green tea catechins appear to be a substrate for this enzyme, being methylated in the presence of incubated COMT. At 1uM EGCG, it rapidly undergoes methylation and may be methylated once again to 4′,4"-dimethylEGCG, which is a slower methylation and its production can be greatly reduced with higher concentrations (3uM or more) of EGCG, the preferred substrate.
Interestingly, catechins also appear to inhibit COMT as well as act as substrate for it. Flavonoids with a catechol B ring appear to be effective (such as rutin, isorhamnetin, and quercetin) and EGCG has an IC50 value of 0.15-0.20uM in rat and mouse liver cells (in inhibiting EGC and L-DOPA, respectively); this has been measured at 0.07uM in human liver cells. In assessing the metabolites of EGCG, the first methylation product (4"-mEGCG) appears to be slightly more potent (IC50 0.1-0.16uM) than EGCG while the second metabolite (4′,4"-dmEGCG) is slightly less potent (0.2-0.3uM), the glucuronides were less potent on EGC and failed to inhibit methylation of L-DOPA; this inhibition applies to both human liver cells and rodent liver cells. Production has been noted in human endothelial cells as well.
The inhibition is mixed-type in regards to EGCG and noncompetitive in regards to methylated derivatives. In particular, EGCG that is twice methylated appears to compete with S-adenosyl methionine (SAMe) for binding to COMT, which is a product required for methylation of COMT subtrates (as SAMe donates the methyl group).
Green tea catechins, particularly EGCG, appear to be both subject to being inactivated by COMT, but the catechins and their supposed inactivated forms can inhibit the enzyme from acting further
It has been hypothesized that inhibition of COMT by EGCG, which theoretically would lead to an increase of adrenaline, may underly fat burning effects of green tea. This is due to persons with a low-active genotype of COMT being highly associated with higher serum adrenaline levels at rest and exercise.
COMT does appear to be of significance in vivo, as at least one study conducted on the vascular effects of green tea that noted nonsignificant results overall found highly significant results in the low-active COMT genotype subset; this study noted that blood pressure decreased only in the low-active genotype and another noted that this may be due to favorable pharmacokinetics (less excretion in the urine, resulting in more time to affect the body).
At least one study, however, has noted that week-long consumption of high dose catechins (1200mg) was unable to significantly influence adrenaline levels any differently than placebo at rest or during low-intensity exercise.
One study that noted suppression of oxygen radical release from HUVEC cells in vitro in response to oxidation (overall, an ant-oxidative effect) noted that this may have been due to NADPH Oxidase inhibition similar to Spirulina. Essentially non-existent inhibition was seen with the base catechin and epicatechin molecules, but methylated metabolites (via COMT) had inhibitory values (IC50) of 15.1+/-4.1uM. Other molecules tested that had inhibitory capacities on NAPDH oxidase were EGCG at 3.5+/-1.1uM, Procyanidin B2 (from Grape Seed Extract) at 3.8+/-0.8uM, various Quercetin derivatives (ranging from 4.6-12uM) and Resveratrol at 16.0+/-4.7uM. Vitamin C, Trolox, and Aspirin were inactive.
NADPH oxidase inhibition can result in both antiinflammatory and antioxidative effects in a manner not related to the catechins directly scavenging free radicals
Intestinal uptake (bioavailability) of green tea catechins is low, ranging from 1.68% in humans (similar in rats) to up to 13-26.5% in mice. This low intake is partially due to the physical structure of catechins (due to being a hydroxylated polyphenols) which form a large hydration shell; they are absorbed via passive diffusion (between intestinal cells) rather than via a transporter. Catechin absorption is improved on an empty stomach for both pure EGCG and a decaffeinated green tea extract containing multiple catechins.
There is also a relatively large degree of immediate efflux due to rapid metabolism by glucuronidation enzymes (UGTs) contributing to this low bioavailability (mice), and inhibiting these enzymes with piperine (from black pepper) appears to increase absorption of EGCG; while an increase in bioavailability was not calculated in this study, both AUC (20-30%) and Cmax (10%)are increased while fecal EGCG indicative of malabsorption decreased to half or less.
Green tea catechins, notably EGCG, seem to be poorly absorbed in the intestines mostly due to there not being a transporter that takes them up from the gut into the body. They are better absorbed on an empty stomach. Absorption might be able to be increased with piperine, but studies need to be conducted in rats or humans (since the animals used, mice, show interspecies differences when compared to humans)
Green Tea Catechins, primarily EGCG, are inhibitors of intestinal sulfurotransferase enzymes which may metabolize other supplements or drugs. This property is shared with flavonoids found in green tea like Quercetin. The inhibitory potential on this enzyme can also extend to the colon, and exert anti-carcinogenic effects on colonic tumor cells via inhibiting the metabolism of some procarcinogens.
Transportation in Serum
One study using 400mg and 800mg EGCG with a light breakfast noted that >92% of circulating EGCG was in the free form which has been replicated in other pharmacokinetic studies. Interestingly, the higher dose of 800mg EGCG is associated with an increased half-life, suggesting either saturation of EGCG excretion or sufficient inhibition of COMT to preserve itself from methylation. Taking EGCG fasted results in a higher Cmax (maximal amount to hit blood) of about 3.5-fold, and the overall AUC was greater fasted. More significant differences are seen at lower dosages.
Peak serum levels of green tea catechins (Tmax) tend to be around 2 hours after oral consumption, although they can be shifted sooner by consuming them in a fasted state to around 60 minutes.
In the fed state, 400mg, 800mg, and 1200mg EGCG result in an AUC (Area under Curve) of 36.7, 90.89 and 299.4min/ug/mL respectively, and in the fasted state result in 126.96, 254.48 and 685.53min/ug/mL respectively. A higher inter-individual variation exists. The Cmax at the same dosages were 798.7+/-573.1, 1,522.4+/-1,357.8 and 3,371.6+/-1,651.2ng/mL fasted compared against 141.8±89.1, 294.0±113.5, and 923.6±755.3ng/mL; respectively. Other studies note that a once daily oral dose of 400mg hits a Cmax of approximately 234.9+/-140.9ng/mL with a 24-hour AUC of 95.6+/-46.8min·μg/ml, and an 800mg dose hits a Cmax of 390.3+/-231.4ng/mL with the AUC as 145.6+/-85.1min·μg/ml. Increasing the dosage to 1.5g has a Cmax (EGCG) of 326ug/mL whereas increasing the dose further to 3.0g increased the Cmax significnatly (2.5 to 3.4-fold) and shifted the Cmax to the left (sooner).
Free EGC and EC are not detectable, as they exist as conjugated metabolites, of either the sulfate or glucuronide variety. Many conjugates may also be methylated, and up to 39 different combinations of catechin isomer, methylation status, and location of (or absent) conjugation to either sulfate or glucuronide have been ntoed. The gallic acid moiety on ECG and EGCG seem to keep them in the free form, and prevent conjugation.
These amounts are the same whether green tea is administered as a pill or via decaffeinated tea.  The EC, EGC, and ECG do not affect (beneficially or adversely) the serum pharmacokinetics of EGCG, as assessed by EGCG versus mixed green tea catechins.
Results show high variability, but suggest that there is a dose-dependent increase in circulating levels of EGCG. There seems to be a tipping point at around 800mg, where the amount in the blood then increases more dramatically. Very high doses of EGCG in the blood are associated with nausea.
EGCG can be formed into unique Cysteine derivatives (EGCG-2′-cysteine and EGCG-2′'-cysteine) when dosed near the toxicity threshold (200-400mg/kg injections); these are thought to possible play a role in toxicity.
Another urinary metabolite known polyhydroxyphenyl-γ-valerolactones, which are produced from colonic microflora, is also excreted in the urine after green tea consumption. The two main valerolactins (M6 and M6') can be created from any catechin, and from EGC/EGCG respectively. Thus, it is possible that unabsorbed catechins (which exist, due to low bioavailability) may still be bioactive not just in the colon but also as valerolactones.
In regards to excretion rate, at least one study that used repeated dosing of EGCG (92% purity) noted that after 10 days of 200mg supplementation that the AUC and Cmax was approximately 10% lower, suggesting enzymes in the elimination pathway are induced with chronic EGCG treatment; this trend was reversed at 400mg and significantly at 800mg.
This may be a key to individual differences in green tea responsiveness, as COMT activity may be 40% reduced in some individuals via a genetic polymorphism. However, these polymorphisms are not associated with the individual pharmacokinetics of Green Tea Catechins in vivo.
It has been noted in one meta-analysis that green tea was more effective in Asian populations for the purpose of weight management than it was in Caucasian populations, although this effect did not reach statistical significance.
Learning and Memory
Due to EGCG being able to easily cross the blood brain barrier and (at 300mg) increasing brain activity up to 2 hours after ingestion, biological relevance for cognition has been investigated. However, when subjects are measured a similar dose of 270mg was found to not influence mood or cognition. A lower dose of 135mg EGCG reduced cerebral blood flow to the frontal cortex, but was not associated with lesser cognitive abilities.
EGCG, when administed I.V to rats 60 minutes prior to a learning event (passive avoidance test); however, high doses were tested (and statistical significance seen at 15mg/kg bodyweight). Lower dosages fed orally (0.5% of feed) for 8 weeks seems to improve memory in aged rats. These results were replicated with injections of 10-20mg/kg, where an improvement in spatial memory was observed.
Mechanistically, 5-40uM of EGCG has been shown to augment proliferation of adult neural progenitor cells (NPCs) in vitro, although these concentrations were unable to induce differentiation with higher (80uM) concentrations still augmenting proliferation but actively inhibiting differentiation. This was seen in vivo with 10-20mg/kg injections of EGCG in old rats. An additional mechanism may be acetylcholinesterase inhibition, as less activity was noted in the brains of aged rats fed green tea catechins.
Green tea might be beneficial for cognition
Anxiety and Mood
In healthy persons, a single dose of 270mg EGCG was not found to influence mood in a positive or negative manner.
In regards to anxiety, the primary catechin (EGCG) has been shown in vitro to negate negative modulation of GABA(A) receptors, and in vivo has reduced anxiety in rats in a dose dependent manner, although significance was seen at 30mg/kg bodyweight.
However, L-Theanine seems to be able to potentiate anxiolytic effects and despite not reducing anxiety itself can confer a reduction in anxiety when paired with Midozolam. As EGCG has anxiolytic properties itself, the pair may be synergistic in reducing anxiety.
In a mouse study on chronic fatigue syndrome, it was found that green tea at 25-100mg/kg bodyweight taken before forced swim tests was able to preserve physical performance in the fatigued state and acted to normalize some brain biomarkers that are changed by chronic stress; TNF-a and glutathione (increase and decrease; respectively). Some other parameters of chronic fatigue, such as weight loss and spleen and thymus hypertrophy, were alleviated at 50 and 100mg/kg bodyweight.
Interactions with Cardiovascular Health
Supplementation of Green Tea Catechins has been found to reliably increase the presence of catechins in the blood and overall antioxidative potential of the blood which then leads to a reduction in LDL oxidation. This has been noted with 5 g green tea powder as well as 1,000 mg of gallated catechins (53.6% EGCG) where a 22% increase in the time for LDL to oxidize when tested ex vivo was found to be due to gallated catechins associating with LDL directly.
Ingestion of high levels of green tea catechins seem to be able to reduce LDL oxidation, a factor in atherosclerosis, by nestling on the LDL particles and directly sequestering the radicals nearby
Blood Flow and Vasorelaxation
Green tea consumption (whole tea) has been reported in a meta-analysis to increase endothelium-mediated vasodilation. At a median dose of 500mL daily, arterial diameter was increased 40% relative to control groups' or baselines' 6.3%.
A possible mechanism of action may be the green tea catechins' ability to increase Nitric Oxide bioavailability in vivo. Possibly through increasing production of Nitric Oxide through Akt-mediated NOS activation. One study noted that the addition of milk to tea negated the cardioprotective effects mentioned here but it has been criticized for taking only one measurement 2 hours after green tea ingestion;
Green tea catechins are potent inhibitors of the enzyme Squalene epoxidase, a rate limiting enzyme that turns squalene into cholesterol. The effects on squalene epoxidase appear to be from binding to the enzyme through a C3 galloyl group, as galloyl esters appear to have this ability per se. Additionally, its anti-oxidant effects can sequester oxygen that is required for the reaction mediated by squalene epoxidase to occur.
Interactions with Glucose Metabolism
When taken alongside sucrose, black tea (containing Green Tea Catechins) at 110-220 mg does not appear to alter the insulin response in healthy and diabetic subjects whereas another study pairing brewed green tea (300mL) with a light breakfast found that green tea did not alter the overall exposure to either insulin or glucose when compared to water. One study has been conducted using an Oral Glucose Tolerance Test (OGTT) which found a benefit overall but to a mild degree with numerous individual differences (some patients seeing no change and others an elevation of blood glucose with green tea).
When assessing how the catechins affect insulin after a meal in regards to COMT, whether the subject had a higher active or lower active COMT did not appear to matter when it came to elevations in insulin after a test meal.
When investigating how green tea catechins affect insulin and glucose acutely (one dose taken alongside carbohydrates) it does not look like they have much benefit after a single dose.
When assessing studies using Green Tea Catechins in overweight and obese individuals who do not have type II diabetes, it was found that there was overall no effect of 843mg EGCG on fasting insulin levels after one year although this may be due to differences in efficacy based on insulin level going into the study as, it was found, benefits may exist in subjects with a fasting insulin level exceeding 10 μIU/mL with no efficacy in subjects with lower insulin concentrations. When assessing type II diabetics, green tea extract (500mg; 57% EGCG) appeared to show some benefit to insulin concentrations (although the decrease was not significantly different than placebo) with benefit to insulin resistance.
While it is possible that some subjects with high fasting insulin concentrations may see benefit to green tea supplementation, overall it seems like there is not a major appreciable benefit in reducing insulin levels.
Studies assessing Green Tea Catechins and insulin sensitivity have found in non-diabetic individuals no significant differences from placebo with both 300mg EGCG, a nonsignificant trend towards improved insulin sensitivity in overweight women (960mL brewed; estimated 32.21mg EGCG), and no effect with 500mg green tea extract (57% EGCG). This seems to carry on with type II diabetics where no noticeable difference is seen across the entire group despite some responders and in borderline diabetics/diabetics where 456mg catechins for two months showed a trend towards improving insulin sensitivity.
When assessing human studies where green tea (in various forms) is given to either non-diabetic or diabetic subjects, supplementation does not appear to be associated with significant improvements in insulin sensitivity when compared to placebo.
COMT is an enzyme that, despite being considered the 'main' mechanism of how green tea catechins exert their effects, is present in the pancreas and implicated in regulation of blood glucose. It has been noted to be suppressed in instances of diabetes in rodents due to high blood glucose and high active COMT (G/G homozygous) seems to be more associated with insulin resistance and type II diabetes when compared to the lower active COMT enzymes (G/A heterozygous and A/A homozygous).
When looking at how the genotype interacts with Green Tea Catechins, it has been noted that postmenopausal women with COMT (G/G) see an increase in fasting insulin concentrations and reductions in adiponectin (a hormone from body fat that sensitizes the body to insulin) although these changes did not result in a statistically significant increase in insulin resistance despite trending.
Inhibition of COMT in people with the high active variation of this enzyme shows trends to actually worsen insulin sensitivity, and may be a reason why there are differing levels of response to green tea and insulin/insulin sensitivity
When ingesting a black tea mixture (containing Green Tea Catechins) alongside sucrose, both 110 mg and 220 mg appear effective at suppressing the amount of glucose that enters the bloodstream over the next 60-120 minutes to a mild degree with no significant difference between doses; this effect occurred despite no apparent changes in insulin release.
Green tea shows a favorable nutrient partitioning effect during times of carbohydrate metabolism, suppressing adipocyte GLUT4 translocation and stimulating myocyte GLUT4 translocation. This benefit was noted from brewed green tea and not isolated catechins or EGCG.
Obesity and Fat Mass
Green tea affects thermogenesis via synergistically acting in concert with its caffeine content, and the addition of 300mg EGCG to 200mg caffeine can increase the thermic response to food more than 200mg caffeine. Caffeine is able to increase noradrenaline levels in the body, which is synergistic with EGCG's ability to inhibit the enzyme catechol-o-methyl transferase(COMT) which degrades catecholamines like noradrenaline and methylates polyphenols. The end result of the pairing is higher levels of catecholamines induced by caffeine, and this synergism seems to be equipotent at various dosages of EGCG. Caffeine also inhibits the phosphodiesterase enzyme, which degrades cAMP. It appears that this pathway (catechol-o-methyltransferase inhibition) may be active in vivo.
In studies with beta-adrenergic antagonists (beta-blockers) fat burning effects of green tea are reduced partially, suggesting that green tea exerts fat-burning effects via beta-adrenergic agonism and other means as well.
Green tea can augment other fat burning compounds, usually through COMT inhibition. This does apply to endogenously produced adrenaline, so green tea taken in isolation could augment adrenaline's ability to increase the metabolic rate
Interactions with receptors/enzymes
All four catechins of green tea are effectively able to prevent adipocyte and preadipocyte differentiation when in the presence of a pro-differentiation (fat-gaining) stimuli. The mechanisms by which Green tea acts is via suppressing multiple transcription factors involved in adipocyte differentiation such as PPAR-gamma2, SREBP1c and C/EBP-α, as well as reducing levels of the cell cycle regulator Cdk2 and Fox01. EGCG is the most potent of the catechins in all the above regards. These effects carry over to the preadipocyte and can inhibit its differentiation.
Many of the above enzyme changes have been noted in animal models, but seem to be associated with the complete tea extract rather than isolated EGCG.
Green tea, via its EGCG component (and possibly other present flavonoids) is able to partially inhibit the Fatty Acid Synthase enzyme. This is the lone enzyme responsible for de novo lipogenesis and also a mechanism of anti-cancer effects from Green Tea Catechins.
In vivo studies
When administered to humans, green tea has mixed results. Some studies show no differences between between control and experimental group in regards to fat oxidation whereas others show significant differences. In studies where effects fail to show statistical significance, there are individuals that do have clincially relevant results (such as 2% increased energy expenditure at 600mg EGCG). The inconsistencies in fat oxidation may be due to individual differences, such as caffeine tolerance; the lower one's tolerance to caffeine is, the more effective Green tea catechins appear to be on fat loss.
When looking at longer term studies and fat loss overall (rather than fat oxidation, which is rate of loss) green tea catechins are associated with 1.2kg weight loss in 90 days at 886mg daily. When paired with exercise, green tea catechins are associated with a loss of 2.2kg bodyweight in the obese on an exercise program over 12 weeks where control group lost 1kg. A recent meta-analysis noted that, on average, green tea extract was causative for about 1.27kg of body weight loss (at least 12 weeks in length) with more weight loss occurring in those that did not habitually consume caffeine.
When taken with a meal, there seems to be no changes in overall metabolic rate from green tea (although trending towards significance) but a greater proportion of energy comes from dietary fat rather than carbohydrate after 300mg EGCG. This greater fat oxidation rate does not appear to occur during exercise at low doses (270mg) and may be vicariously through green tea being able to inhibit carbohydrate digestion (as fat oxidation is a rate, rather than a gross amount; more fat is burnt for energy when there is no presence of dietary carbohydrate). This theory is somewhat backed up by green tea losing its effectiveness as a fat burning agent when on a high protein diet, as there are less carbohydrate (relative to adequate protein diets) to block.
Higher dosages of green tea (945mg) are able to increase fat oxidation and oxygen consumption during exercise however, and may be through forcing systemic reactions.
Overall, green tea seems to be a very good fat loss agent, either in low doses through consumption of green tea (as a tea) or in high supplemental dosages. Low doses via tea have a large safety threshold, and superloading in supplemental form seems to be safe in isolation, although nausea is reported with combining high doses of green tea with other stimulants
Skeletal Muscle and Physical Performance
In several animal studies, rats experience either an increase in body weight or (if body fat is measured) lean mass. That being said, the majority of studies note a decrease in overall body weight. In one study looking at both Black and Green tea, this was only seen with Green Tea and isolated EGCG, and was 4-5% higher than control after 27 weeks (with no differences in food intake, a bit less water consumption). No changes in fat burning genes were seen in muscle cells in this study.
When assessing studies that use high doses of green tea catechins with light exercise in sedentary or light active individuals changes in muscle mass are not readily observed.
As the primary green tea catechin, EGCG, is known to inhibit COMT and may lead to increased catecholamines it is thought that supplementation of EGCG prior to exercise can change how many calories are burned overall and the percentage of these calories coming from fat (rather than carbohydrate).
When assessing Green Tea Catechins and their interactions with exercise over the short term, two days supplementation of a high dose extract (1,000 mg in three divided doses; 45% EGCG) failed to influence overall energy expenditure or substrate utilization at rest or during exercise when taken an hour before exercise with another study that the increase in fat oxidation observed (during rest) with four weeks supplementation of 1,136 mg catechins (45% EGCG) disappeared during moderate exercise. This is not to say the catechins are without any effect as one study, despite not finding any increase in fat oxidation, did note an increase in plasma glycerol and fatty acids during both exercise and rest suggesting an increase in the rate of lipolysis.
Generally speaking, studies using green tea in order to increase the rate of fat oxidation (percentage of energy derived from fatty acids rather than other sources such as carbohydrate) fail to find a significant effect of supplementation
When assessing studies using green tea paired with fasted training (no food prior to exercise), in both men supplementing green tea for 12 weeks prior to testing (250mg thrice daily; 125mg EGCG and 20mg caffeine) and women using this dose for one day before taking a final dose in the morning before training have found increases in fat oxidation relative to placebo both during exercise and up to 75 minutes after exercise.
Some studies have found an increase in fat oxidation rates associated with green tea and exercise, both studies utilizing the fasted (unfed) state with exercise
Green tea has been shown in mice to be able to enhance time to exhaustion during endurance events. This is suspected to be via increased mobilization of intramuscular fatty acids, of which may be due to increased expression of fatty acid translocase in skeletal muscle with green tea supplementation.
When tested in non-athletic but otherwise healthy adult men, 570mg of green tea catechins (stated 6-7 cups of tea equivalence, providing 218.4mg EGCG and 15mg caffeine; caffeine at a similar level in placebo) was able to increase the subjects aerobic endurance. Specifically increasing their ventrical threshold (VT) when paired with 8 weeks of cycling when placebo saw no change; neither group saw a change in VO2 max nor body composition.
Green tea may have some mild benefits to aerobic capacity when given to sedentary or light active individuals. It is not yet tested whether this effect persists in heavily trained individuals.
Bone and Joint Health
When tested in vitro, a combination of green tea catechins with another herb known as salvadora persica L. was found to possess synergistic antibacterial effects with relevance to the build-up of dental plaque (adhesion of microbes to the teeth plays a central role in plaque buildup) later confirmed to have efficacy when formulated into a mouthwash containing 0.25mg/mL brewed green tea. This effect appeared to outperform the active control of chlorhexidine (0.12%) which was similar to placebo although it should be noted that the other plant used, salvadora persica L., was at a higher concentration of 7.82mg/mL and has evidence to suggest it has anti-plaque properties independently.
Studies assessing the effects of green tea alone in mouthwash and in dentrifices have found beneficial effects in instances of periodontitis attributed to the antioxidative effects, with the dentrifices outperforming fluoride-triclosan and the mouthwash reporting an 8-fold increase of antioxidative capacity of the gingival crevicular fluid (to assess local antioxidant effects) when compared to control.
Green tea, when incorporated into mouthwash or anything else providing exposure to the gums, seems to have anti-microbial effects that may be beneficial to dental diseases. While not too much information is available to assess overall potency this does appear to be a relevant benefit
Bone mineral density in postmenopausal women seems to be related to overall body weight with women who weigh more having more instances of osteoporosis and fractures from falls (although mostly in the ankle and lower leg); as such, green tea has been thought to play a protective role as the consumption of tea seems to be associated with both greater bone mineral density in Japanese women.
In a 12 month study involving postmenopausal women taking either Green Tea Extract (843mg EGCG) or placebo, supplementation failed to significantly influence bone mineral density.
Green tea consumption, when taken by postmenopausal women for a year, may not have a rehabilitative effect on bone mass. It is uncertain if lifelong green tea consumption is a protective factor.
Inflammation and Immunology
Natural Killer Cells
Natural Killer (NK) cells are an immune cell which, beyond preventing sickness, have a role in killing cancer cells. When investigated in mice it seems that administration of EGCG appears to enhance phagocytosis of NK cells at 40mg/kg oral intake; an estimated human intake of 3.2mg/kg.
A metabolite of EGCG, 5-(3',5'-dihydroxyphenyl)-γ-valerolactone (EGC-M5, most predominant metabolite in urine) appears to enhance the activity of CD4+ T cells, increased the cytotoxicity of Natural Killer (NK) cells, and increased IFN-γ production from splenocytes (10-50 μM; stimulated by PHA) without affecting IL-2 secretion; these effects were seen in rats given EGC-M5 where NK activity, but not overall amount of NK cells, seemed to be enhanced.
EGCG, potentially through a metabolite, seems to be able to enhance the activity of NK cells when given to rodents.
EGCG has been found to inhibit the enzyme known as NADPH oxidase in mast cells in vitro, and hindered mast cell degranulation (induced by compound 48/80) between 10-100 μg/mL. This function seems to occur with most catechins but is stronger with the gallated forms (even more-so when methylated) and, while it reduces granulation of mast cells when intracellular oxidation is low it seems to exacerbate it in vitro when oxidation is already high prior to the introduction of EGCG.
Due to its interactions with NADPH oxidase, EGCG seems to interact with mast cells. It is uncertain if there is a practical interaction when it comes to allergic reactions with supplementation.
Interactions with Hormones
Green and white tea can inhibit the UGT2B17 enzyme, which conjugates testosterone into testosterone glucuronide; a less potent form of testosterone designed for urinary excretion. Via this mechanism, green tea may increase the active AUC of testosterone. The IC50 of this reaction was 64uM. This possible mechanism does not result in increased testosterone levels in post-menopausal women at a dose of 400-800mg EGCG daily over 2 months, most likely due to low initial circulating testosterone levels.
One study in rats suggest dosages of 1.25-5% green tea catechins in the diet (equivalent to 5-20 cups a day) is associated with decreased sperm motility in rats over a period of 26 days. Inhibition of two testicular enzymes also led to decreased serum testosterone levels from the control's 3.5ng/mL to 1ng/mL at the highest dose. This mechanism (inhibition of steroidogenesis enzymes in the testes) has been seen in vitro as well, and the gallic acid moiety of EGCG and ECG seems relatively important as pure epicatechin (EC) does not exert these effects. However, the same study established that lower concentrations of EGCG in the testes (20ug/mL EGCG, or 13.8ug/mL Green tea catechins) may actually increase testosterone by being at such a concentration to inhibit P450scc (side chain cleavage) enzymes, but not at a level to inhibit steroidogenesis enzymes. Interestingly, these dosages (1.25% and 5% of the diet) were used in another study that noted aromatase inhibition and greatly increased (7.2ng/mL at 5% vs. 1.7ng/mL control) testosterone levels at 8 weeks with 5% catechins; 1.25% never reached statistical significance and 5% at 4 weeks did not either. Luteinizing hormone was also significantly higher in this group at this time. The IC50 for the green tea catechin mixture used in this study was 28mcg/mL on aromatase.
Green tea's EGCG can also inhibit 5-alpha reductase in cell free cultures, but fails to do so in whole-cell cultures; its biological significance is questionable. The 5-AR enzyme is responsible for the conversion of testosterone to DHT, a more potent androgen.
There is no consensus on what green tea does when paired with testosterone. It seems to have the ability to increase and decrease testosterone depending on mechanism and concentration, but only one rat study exists for in vivo evidence.
Interactions with Oxidation
In a model of vegetable depletion (where all flavonoids are removed from the diet), a small intake of 18.6mg green tea catechins is able to increase systemic anti-oxidant capacity after a meal; fasted anti-oxidant potential was not changed, and thus green tea catechins appear to exert transient effects lasting up to 6 hours.
Peripheral Organ Systems
Green tea consumption (via tea, over a lifespan) is associated with a decreased risk of Hepatocellular Carcinoma (OR=0.44) and, retrospectively, has strong interactions with alcohol consumption and other carcinogens. In effect, green tea is associated with reduced risk of Hepatocellular Carcinoma and this reduce risk may be through abrogating the adverse effects of select toxins or habits that lead to Hepatocellular Carcinoma, such as oxidative stress and inflammation and smoking and hepatitis.
Other large-scale epidemiological research proves somewhat inconsistent, with some results showing benefit and others null. In regards to association, green tea appears to either exert a relatively small effect or the results are confounded by lifestyle.
It is possible green tea has some benefits to liver functioning but, overall, there isn't convincing evidence that this effect is strong enough to be called a 'liver health' supplement
800mg EGCG daily, for four weeks, does not result in modifications to CYP2D6, CYP2C9, CYP1A2 but may result in a decrease in CYP3A4 activity. A lower dose of EGCG (504mg, divided into two doses) is not associated with this decrease in CYP3A4 activity and still does not influence CYP2D6, although this study did not have sufficient statistical power to assess small changes.
CYP1A induction is sometimes seen in animal models with green tea, but may be due to the confounder of caffeine which may induce CYP1A activity in the liver, as mentioned in discussion this is not seen when studies use decaffeinated PolyPhenon-E (95% EGCG).
It is possible that green tea can influence the activity of some CYP enzymes (predominately expressed in the liver)
Interactions with Cancer Metabolism
Effective Dose and Mechanisms
It has been postulated that for green tea catechins to be effective as an anti-cancer agent (via one mechanism), a dose of over 100uM would be needed in the interstital fluid at any given time. This is based off of two in vitro studies on tNOX showing said concentration being where effects on cancer growth are first noted.
Green tea catechins can influence cancer metabolism in a multitude of ways. Through telomerase inhibition, inhibiting topoisomerases, tNOX inhibition, selective inhibition of COX2 enzymes without influencing COX1, induction of cellular apoptosis and regulation, and possibly inhibition of BCL-2 proteins.
One secondary analysis that measuring urinary metabolites of women who ingested Polyphenon E (EGCG) at doses of 400-800mg found reduced VEGF levels at the samples taken at 2, 4, and 6 months with reduced Hepatocyte Growth Factor when measured at 2 months only.
In one intervention with 800mg PolyPhenon-E (Green Tea Catechins), supplementation was not associated with an increase in prostate levels of EGCG and the trend towards improved biochemical markers (indicative of risk reduction) came back nonsignificant. Benefit appears to be there, but not overly potent.
The reason may be due to pharmacodynamics. Consumption of 3-5 cups of green tea daily does increase serum levels of EGCG, but approximately half of what makes it to the prostate is methylated (4'-methyl-EGCG) and not as bioactive. Methylation reduces the cancer fighting activity of EGCG in vitro in prostate cancer cells.
Conversely, EGCG intake can work antagonistically (in opposition) with radiotherapy in prostate cells. Radiotherapy works via oxidation and death of cancer cells, whereas EGCG prevents this with its anti-oxidant actions.
Longevity and Life Extension
In vivo results
Green tea catechins have been associated with up to 6% increases in lifespan in the C57BL/6 strain of mice although maximal life did not change; the dose was 80mg/L catechins in the drinking water.
One animal intervention, which administered green tea beginning at 4 months of age and continuing until death at a dose of 2g/kg food, did not significantly influence time of death; although it seemed to suppress midlife death rates in females.
Longevity is always an iffy topic to approach from a research angle. Green tea looks to be one of the more promising agents, but further study is needed
Interactions with Aesthetics
Green tea is thought to help with acne due to not only its anti-inflammatory effects but also by being implicated in reducing sebum production and overgrowth of a bacteria known as Propionibacterium acnes; both of which are heavily involved in the production of acne.
When placed in SEB-1 sebum cells, EGCG is able to reduce lipid accumulation by 55% at 40μM thought to be related to suppressing SREBP-1 proteins and was secondary to the AMPK pathway and sebum reduction has been confirmed with topical administration of green tea extract in humans.
The inhibition of p. acnes by EGCG appears to increase in a concentration-dependent and appears to reduce p. acnes-induced inflammation as assessed by IL-8 production and TLR2 expression in monocytes.
EGCG seems to have strong beneficial effects on the major factors that promote acne in vitro through multiple mechanisms
An 8-week split face study investigating adult men and women using either a 1% or 5% EGCG against control (3% ethanol) showed rather powerful effects, reducing the Leeds score from a baseline value of 5.1+/-0.4 down to 1.2+/-0.4 (1% EGCG) or 1.7+/-0.6; the lesion counts also decreased by 79% and 89% (noninflammatory and inflammatory lesions respectively) with both groups of EGCG performing equally to each other and greater than control. These benefits have been replicated, although to less potent degrees with a 2% green tea solution reducing lesions by 58.33% over six weeks, 2% reducing open-comedos (blackheads) 61% and pustules 28% (with no effect on whiteheads) over 8 weeks, and a 3% green tea solution reducing sebum-production in men in a time dependent manner over eight weeks (eventually exceeding more than a 50% reduction). One single blinded study performed a comparative analysis and suggested that 2% green tea extract, when applied topically, outperformed 5% zinc sulphate over eight weeks.
When 1,500mg of Green Tea Catechins (856mg EGCG providing no caffeine) is taken orally by adult women with acne for four weeks supplementation appears to be associated with reduced acne overall when the subjects were compared to themselves four weeks prior. However, when they were compared against placebo there were only benefits along the nose and below (chin and perioral region) without any significant differences in total facial acne.
Application of green tea solutions or EGCG alone appear to have repeated benefits shown to acne, with concentrations between 1-5% EGCG not behaving too differently (suggesting a 'cap' to its benefits) and 2-3% green tea solutions working well and in a time-dependent manner. When looking at oral ingestion of green tea products and supplements the benefits become more mild and, at times, not statistically significant.
Some studies assess the effects of green tea products and supplements on skin in subjects who do not have instances of acne. One such study assessed the effects of green tea infused with milk (exact catechin dose not disclosed) found minor improvements in facial erythema with no significant changes in skin elasticity, moisture, wrinkles, or melanin index.
Ascorbic Acid (Vitamin C)
Xylitol (11-55µM) and Vitamin C (4-20µM) were coincubated with the four main green tea catechins, and it was noted that vitamin C enhanced the absorption of the catechins without extra galloyl groups (i.e., epicatechin and epigallocatechin) without effect on the gallated catechins; xylitol was inactive.
Butylated Hydroxyanisole, or BHA, is a food additive in some packaged foods that also serves as an anti-oxidant. It seems to greatly increase the ability of green tea catechins to act as an anti-microbial agent against Streptococcus mutans, Candida albicans, and Escherichia coli.
Caffeine and Ephedrine
Green tea catechins, via inhibition of the COMT enzyme, can increase the half-life of adrenaline and noradrenaline which are induced by both caffeine and ephedrine. This increased AUC (Area under curve) of adrenaline confers a greater ability to burn fat, and can provide a large amount of neural stimulation at a lower overall dose of either caffeine/ephedrine or green tea. At least one study assessing caffeine's fat burning ability noted that the efficacy of 50mg rose from an additional 15kcal expended (assessed by metabolic chamber) to 79kcal, which is close to the 110kcal estimate that is seen with 600mg caffeine.
However, caffeine may also be indirectly inhibitory of green tea catechins. Habitual caffeine users (defined as over 300mg daily) tend to show less overall fat loss over time relative to non-habitual (>300mg daily) users. This difference, in some studies, has been the line between weight management and weight regain or prevented weight loss otherwise.
Green tea catechins show synergism with capsicum vanilloids in cancer prevention. One study noted that a 25:1 ratio of catechin:vanilloid increased the cell killing ability (via the protein tNOX) 100-fold relative to green tea in isolation. The protein seems to be a common target for anti-cancer research as it pertains to catechins and vanilloids.
Although capsaicin, or red pepper extract, is a vanilloid compound there are others. An HPLC analysis of the plant Capsicum Annuum notes the existence of vanillylamine, vanillin, vanillic acid, and homovanillic acid.
Carb and Fat
In regards to transports, Green tea catechins show abilities in preventing intestinal uptake of dietary sugar mostly via its ECG component, although the other three catechins also contribute and do so via competitive inhibition of the SGLT-1 transporter that uptakes glucose.
Enzymatically, catechins show weak to moderate effects in inhibiting the sucrase enzyme that breaks down sucrose into its constituent glucose and fructose. Theaflavins (found in high levels in black tea) were much more potent in this regard, and suggest that the tea itself may be more effective than isolated green tea catechins.
Lactase, amylase, alpha-glucosidase, and protein-digestive enzymes are also inhibited by green tea catechins; although these inhibitions (with exception to glucosidase) are reduced 2.6-fold with coingestion of proline-rich foods.
Whether inhibition of these enzymes translates to clinical effects in humans has been investigated, but the data are short-term and sparse. One intervention tested how coingestion of a combined extract from green, black, and mulberry tea leaves (100 mg each) affected carbohydrate absorption from a test meal consisting of mostly rice with a little bit a fat. The researchers found a 25% inhibition of carbohydrate absorption. However, this study was confounded by the mixture of tea extracts used. A later study mitigated this confounder by coingesting pure green tea extract (4 g extract containing 257.6 mg EGCG) with 50 g corn flakes and 100 mL low-fat milk. This study found approximately a 30% reduction in carbohydrate absorption.
Green tea has also been implicated in reducing the activities of gastric and pancreatic lipase, suggesting it may play a role as a fat blocker as well. These mechanisms are statistically significant, but not overly potent in vivo as it seems to hit a certain max inhibition, whereas its effects on reducing cholesterol uptake are dose dependent. Green tea at a dose of 0.5-1% of the diet increased the total percent of dietary fat lost in the feces (rats) from 3.5 (control) to 4.6-5.8%.
Green tea has mechanisms to cause malabsorption of all macronutrients, but this only seems to be a relevant concern for dietary carbohydrates. Protein inhibition is hindered by saliva enzymes, and fat inhibition just doesn't seem to be relevant in humans.
Curcumin also shows synergism with green tea catechins in colonic tumor cells and human larynx carcinoma cells. The inhibition is effective in reducing colonic tumor progression induced by 1,2-dimethylhydrazine. This may be due to similar mechanisms as Quercetin, as Curcumin can inhibit MRPs associated with ejecting EGCG from cells. One in vitro study noted increased levels of EGCG in cells when co-incubated with curcumin.
Interestingly, this synergism works both ways. Epicatechin (EC), one of the four green tea catechins, can increase the actions of curcumin in cancer cells by increasing the time it spends in a cell.
Green tea catechins, primarily EGCG, are effective in inhibiting iron uptake into the body. This is seen with nonheme and heme iron. These effects on non-heme iron are negated when Vitamin C is consumed. At an oral dose of 150mg and 300mg EGCG, the rate of inhibition appears to be around 14% and 27% respectively.
Dietary Phenolic Acids
Green tea catechins, primarily those with a gallic acid moiety (epicatechin gallate, epigallocatechin gallate) have the ability to inhibit the monocarboxylic acid transporter and thus prevent substrate of the transporter from working. This has been demonstrated with salicyclic acid (aspirin) and ferulic acid in vitro. Ferulic acid's relative permeability in the intestinal model was reduced from around 69.9% to 47.6% with the addition of EGCG, and salicyclic acid from 84.5% to 67.9% (mean values).
One study using a short-chain peptide derived from Sardine protein hydrolysate (Valine-Tyrosine dipeptide) found that this product was synergistic with Green Tea Catechins in inhibiting the ACE enzyme, and possibly by reducing blood pressure.
Fish oil, at a dose of 8mg/kg bodyweight in rats, increases the bioavailability of green tea. Additionally, fish oil at this dose was synergistic with green tea catechins, at both 12.5mg/kg bodyweight and 62.5mg/kg bodyweight, in decreasing build-up of beta-amyloid pigmentation in an animal model.
On other parameters where there is crossover, such as markers of fat metabolism (lipids, cholesterol) and glucose metabolism (insulin, glucose) and adiponectin; fish oil and green tea catechins may be additive rather than synergistic.
Green Tea catechins appear to have synergism with Phytic Acid (inositol hexakisphosphate) and Inositol in regards to tumor suppression in the colon as a response to toxin injection. The combination of the three (all compounds at 1-2% of the diet) was able to reduce tumor occurrence from 94% (control) to 46% at 1% of feed and 23% at 2% of feed. Additionally, tumors tended to have a reduced average size when the three were combined (1.30±0.06mm) relative to any combination of two ingredients (2.4-2.8±0.19-0.46). Other studies note a significant group interaction between green tea and phytic acid.
These synergistic effects may extend to other tumor cells, such as pancreatic. More research needs to be done though.
Two studies have been conducted with a combination supplement of EGCG (at doses ranging between 50-105mg) and N-oleyl-phosphatidylethanolamine (NOPE) between 120-170mg, and both suggest that this supplement combination can increase adherence to a low calorie regiment to further weight loss attempts in overweight adults.
Other green tea components
The main green tea catechin, Epigallocatechin-3-Gallate (EGCG), shows synergies in regards to anti-cancer mechanisms when in the presence of the other green tea catechins: epigallocatechin (EGC), epicatechin-3-gallate, and epicatechin (EC). The other three catechins reduced the needed dose to inhibit cancer cell growth 10-fold when present. In other studies, green tea mixtures show more potency in cancer prevention than EGCG in isolation. This may be due to epicatechin (EC) and its ability to increase the time drugs, such as EGCG or curcumin, spend in a cell and thus increase their potency.
The catechins, primarily epicatechin (EC), shows synergy with Theaflavin in regards to anti-bacterial effects.
In vitro studies suggest that quercetin can enhance the anti-proliferative effect of green tea catechins in prostate cells in a synergistic manner.
Quercetin also increases the bioavailability of green tea polyphenols (catechins) in vivo. Via inhibiting the COMT enzyme Quercetin decreased methylation of EGCG 2-fold and 4-fold in lung and kidney cells (with no significant effect on liver cells), adding 0.4% Quercetin to a rat diet led to 2-3-fold increases in circulating green tea in the two aforementioned organs, with little effect on liver tissue.
Quercetin can also inhibit Multidrug Resistance Proteins, which can efflux Green Tea Polyphenols from cells. The efflux actions of MRP1 and MRP2 is protective in a way (prevents foreign compounds from staying in a cell for too long), but also reduces the time EGCG can act in a cell. The above study showing synergism noted highest levels in lung and kidney tissue and less in liver; although liver has highest levels of COMT relative to the other two organs, it has lowest MRP1; thus, via inhibiting MRP1, Quercetin can increase the amount of EGCG in a cell and, via inhibiting COMT, can increase the amount of bioactive EGCG relative to methylated EGCG.
Supplemental Quercetin, or foods that are very high in quercetin content (onions and leeks), are synergistic if consumed at meals with green tea.
Green tea catechins also show synergism with soy isoflavones, in particular Genistein.
Green tea can increase PGE2 production from stimulated macrophages (immune cells) at concentrations of 10 and 0.4uM by 25 and 20%, respectively, This is normally seen as a pro-inflammatory reaction, and green tea's induction of COX2 protein adds to this. The combination of EGCG and Genistein switched the increased production to a 35% suppression and, at 10uM Genistein, led to a 51% suppression of COX2 protein.
A study using Whey Protein concentrate at 3ug/mL alongside green tea catechins (brand name Healthya used) lowered green tea's ace inhibitory potential from 56.9+/-3.2% to 34.9+/-9.8%. These effects were replicated in a rat model of spontaneous hypertension, where systolic blood pressure was decreased to approximately 80% of baseline with the introduction of green tea polyphenols while not being significantly lowered when both Whey and polyphenols are coingested. The peptide in question that appears to adversely interact with green tea polyphenols is the Val-Pro-Pro tripeptide.
The amounts used in this study are beyond food level, but higher concentrations of green tea catechins were demonstrated to interact poorly with whey proteins
Green Tea Catechins (Green tea extract, GTE) appears to enhance the cytotoxicity of Ziziphus Jujuba (Jujube). This one study noted that in HepG2 cells (liver carcinoma cells) that cytotoxicity at 100ug/mL Jujube (chloroform extract) reduced viability of cells to 80%, and under the influence of 30ug/mL GTE this was enhanced to about 60%. Green tea at 30ug/mL itself has no affect on viability, and this enhanced cell death was not seen in noncancerous liver cells under any condition.
Mechanistically, an increased level of ROS (oxidation) occurred in the Jujube condition which correlated with cellular death; this was not enhanced or hindered by GTE but instead the combination appeared to further change the cell cycle relative to control HepG2, where Jujube in isolation and the combination to a greater degree increased the amount of cells in the G1 phase while reducing the amount in G2/M and S phases. The authors concluded that the synergism occured via enhancing G1 cellular arrest, which was confirmed by less DNA synthesis and improved Rb protein (mediator of G1) actions.
Synergistic protection has also been noted when measuring the actions of APRIL, a protein that induces differentiation of HepG2 cells.
Safety and Toxicology
In supplemental form, Green Tea Catechins have been found repeatedly safe in consumption at 800mg One study has found that 1,200mg EGCG in one dose was well tolerated, but was associated with significantly more nausea than the other dosage groups of 800mg and 400mg. Single dosages of EGCG can be tolerated up to 1,600mg.
It has been noted that, through beverage consumption, that there have been "no reports of clinical toxicity".
In humans, the maximum tolerated dose is around 4.2g/m2 once daily, or 1.0g/m2 thrice daily. This is a Body Surface Area (BSA) reading that correlates with blood volume, and using the DuBois formula an adult male at 5'10 weighing 150lbs would suffer from acute toxicity at 7.9g supplemental green tea extract once daily, or 1.9g thrice daily, although this study was influenced by the caffeine content of the supplements (at 7%).
In investigating toxicity of green tea in animals, administration of either Teavigo or Polyphenon E (two brands of green tea catechins) results in dose-dependent toxicity associated with vomiting and diarrhea, resulting in death, in beagle dogs fed excessive dosages (greater than 500mg/kg); beagle dogs were used due to having better absorption rates of green tea catechins from the intestines. Vomiting may be associated with gastric damage, and 2000mg/kg oral administration to rats leads to 90% lethality associated with hemorrhaggic lesions in the stomach and intestines. Interestingly, animals with lower absorption rates of EGCG from the intestines suffer greater intestinal damange and less systemic (liver and kidneys).
In the beagle dogs, toxicity to the liver was confirmed with elevated ALT and with female dogs suffering liver necrosis; other studies injecting green tea extract (150mg/kg) also note increases in ALT and liver toxicity with excessive dosages. Proximal tubule necrosis (kidneys) was noted at the oral doses in dogs.
Excessive levels of green tea catechins (mostly EGCG) have been confirmed for toxicity, mostly in the intestines, stomach, and liver with excessive levels in the blood possibly damaging the kidneys as well. Nausea from green tea supplements is not inherently linked to stomach damage
A few case studies exist in the dosage range of 10-29mg/kg bodyweight (681-1997mg for a 150lb person) which tend to (8/9 cases) been associated with elevated ALT and bilirubin; indicative of liver damage. Causation was placed on the dietary supplements in these cases (due to symptoms appearing upon reintroduction) but the authors could not rule out possible tampering of the supplements.