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תה ירוק א'- מרכיבים

Constituents of green tea
Catechins - The polyphenols are generallyconsidered to be the most important elements of green tea, with the catechinsbeing the most important polyphenols. Catechins found in green tea include EC, EGC, ECG, and EGCG. Of these, EGCG (-epigallocatechin-3-gallate) is by far the most active by itself, butthe combination of catechins can be especially effective for cancer protection 3, and EC appears to increase the incorporation of EGCG and ECG into lipidbilayers 4, emphasizing the importance of synergistic effects between thevarious components of green tea. Although the numbers vary, green tea isgenerally made up of about 10% polyphenols, 50% of which are EGCG. The green teacatechins have been studied extensively and will be the primary focus of thisarticle.
Pheophytins, chlorophylls, and carotenoids - Despite theimportance of catechins, they are not the only constituents of green tea thatoffer health benefits. Some studies find that catechins depend on othercomponents or are not even major players in some of the anti-genotoxic andantioxidant effects of green tea 5, 6. The numerous other active compounds ingreen tea identified include chlorophylls a and b, pheophytins a and b, lutein, and beta-carotene. All of them have antioxidant properties 7, and the healthbenefits of beta-carotene and lutein (such as prevention of maculardegeneration) are well known. Chorophylls and pheophytins from green tea arealso anti-carcinogenic 8, 9, but a discussion of the many benefits thesephytonutrients have to offer is beyond the scope of this article.
Theanine - L-theanine is an amino acid found in high concentrationsin tea, with a typical cup of tea containing 30 mg or more. Theanine decreasesblood pressure 10, 11 and has been found effective in increasing the antitumoractivity of cancer drugs 12, 13, but its most important properties are in thearea of the brain. L-theanine bears structural similarity to glutamic acid andhence competes with it in binding to glutamate receptors, offering protectionagainst glutamate neurotoxicity 11. This glutamate receptor competition alsoprovides a variety of differential effects on the brain. These include anincrease in serotonin and/or dopamine in some areas of the brain, notably thestriatum, hypothalamus and hippocampus 14, an increase in GABA 15, and anincrease in brain alpha wave activity 15. While beta wave activity is associatedwith periods of high stress, alpha waves are associated with a state of beingawake, but relaxed (such as the period right before the onset of sleep). Theanine is said to promote a state of "alert relaxation" because of thesedifferential effects 15, and it also antagonizes the stimulation and anxiogenic (anxiety promoting) effects of caffeine 11, 16 which may be responsible for theparadoxical calming effect of green tea despite the caffeine content.
Caffeine - The amounts of caffeine in green tea are relatively low, with 20 mg in a typical cup (about half as much as a Coke). Surprisingly, caffeine can play a significant role in the anti-mutagenic effects of green teain some cases, especially in the prevention of UV-induced skin cancer 17, 18. The presence of caffeine in green tea will be of most importance in the upcomingdiscussion of thermogenesis

חלק ב-השפעה על הרכב גוף (צמצום יחס שומן-שריר)

Recent research has confirmed that green tea can cause weight loss throughmultiple pathways. In addition to being a potent appetite supressant, green teaincreases thermogenesis, preferentially burning fat over protein in a similarmanner to many other thermogenics. Green tea has been demonstrated to be as ormore effective than some prescription weight loss medications, and may rival theECA stack in terms of fat loss. In addition, green tea is one of the few weightloss medications that has not been associated with negative effects on thecardiovascular system (such as increased heart rate and blood pressure) or CNSside effects (such as overstimulation and irritability). More research is neededbefore the level of effectiveness of green tea can be more conclusivelydetermined, but the present data shows a clear benefit.
Green tea hasbeen known to be a thermogenic agent for quite some time, but the thermogenesiswas usually attributed to the caffeine content. It was then found in an in vitroexperiment with brown adipose tissue that the thermogenic effect of green teawas "much greater than can be attributed to its caffeine content per se" 19. Other in vitro experiments also demonstrate that green tea inhibits lipogenesis (the creation of fat) 20, 21. In rodents, studies with both green tea and greentea powder have shown that it decreases body weight and food intake 22, 23 andinhibits lipogenesis 24.
Two important studies have been done assessingthe thermogenic effect of green tea in humans. The first was a preliminary studythat compared the effects of green tea extract (containing 150 mg caffeine and 270 mg EGCG), caffeine (150 mg), and placebo on 24-hour energy expenditure. While caffeine alone increased energy expenditure by about .6% over placebo (which was not statistically significant in this study), the green tea extractincreased it by 3.5%. Additionally, while the oxidation of fat contributed to 31.6% of energy expenditure in the placebo group, in contributed 41.5% in thegreen tea extract group, indicating that the increase in energy expenditure wasdue to the breakdown of fat, not protein. Measurements of urinary nitrogenexcretion, which were significantly different between treatments, furthersupported this contention. It is also interesting to note that thermogenicresponse was not correlated with body mass index (BMI), implying that green teamay be equally as effective in relatively lean individuals. Finally, theincreased thermogenesis was not accompanied by an increase in heart rate, whichmakes green tea distinct from other thermogenic drugs 25.
The secondstudy was a three month open trial with 70 subjects using the same dosage of thesame extract as in the above study (150 mg caffeine, 375 mg total catechins, 270mg EGCG per day). Treatment with green tea was well tolerated and associatedwith a body weight reduction of 4.6% and a reduction of waist circumference of 4.5% 1. Hopefully these promising results will be followed by largerplacebo-controlled studies..

המשך חלק ב

Green tea has many mechanisms of action in stimulating weight loss. The mostimportant is probably the inhibition of catechol-O-methyl-transferase (COMT) byEGCG 1, 19, 25. COMT is the enzyme that breaks down norepinephrine (NE), one ofthe body's most important lipolytic hormones. Caffeine also plays a synergisticrole by inhibiting phosophdiesterases (enzymes that break down cAMP, which isfurther down the lipolytic pathway) 19, 25. Although EGCG is the mostresponsible, some flavanoids found in small amounts in green tea such asquercetin and myricetin also inhibit COMT and may play a minor role 25.
Secondly, green tea decreases the digestibility of dietary fat 1, 26. The proposed mechanism of action is inhibition of both gastric and pancreaticlipase, which has been demonstrated in vitro 1. These enzymes both play majorroles in the digestion of fat, so when they are inhibited a smaller proportionof fat is absorbed and a greater proportion excreted.
Green tea is alsoa potent appetite suppressant. This can be partly explained by the fact that itincreases both NE and dopamine 14, 25, but further mechanisms of action havebeen hypothesized. Specifically, tea polyphenols have been known to elevatelevels of cholecystokinin (CCK) 2, a hormone which depresses food intake 2, 22. It is not yet known whether this plays a significant role in the action of greentea, and one of the effects of elevated CCK is an increase in pancreatic lipase, which is actually inhibited by green tea. It could be that green teasimultaneously elevates CCK and decreases pancreatic lipase, conferring thebenefits of both appetite suppression and decreased fat digestibility.
Finally, the antioxidant properties of green tea may play a role in thelipolytic effect 20-22. One cell culture study suggested that green teainhibited lipogenesis by increasing superoxide dismutase activity andsubsequently decreasing the formation of free radicals 20, while anothersuggests that vitamin C from green tea plays a role in its lipolytic activity 21. Even if the antioxidant activity turns out to have little to do with thebreakdown of fat, it leads to many health benefits that will be discussed ingreater detail in following sections

חלק ג:תה ירוק וסרטן

Hundreds of studies have been done in recent years on the effect green tea hason both the prevention and treatment of cancer, as well as cardiovasculardisease. Epidemiological studies tend to yield different results, with somefinding no effect 27-28 and others finding that green tea drinkers have lessincidence of certain types of cancer 29-30. However, when a less specificvariable such as total cancer deaths or lifespan is measured in a largerpopulation it can be shown that green tea has a definite effect.
One ofthe more comprehensive studies was conducted on a Japanese population over 13years and the results were published this year in Ageing Research Reviews. Itmeasured 90 lifestyle factors in a population of 8552 individuals over 40 yearsof age and determined the correlation of various factors with death from anycause. Consumption of over ten cups a day of green tea, when compared with lessthan three cups, was correlated with an increase in lifespan of 4.3 and 3.8years in men and women respectively. Greater associations were seen in smokers (implying green tea had a protective effect) and those who died before age 80. Green tea was correlated with a 7.5 year longer lifespan in male smokers. Otherlarge scale studies by this research group found that drinking green tea delayedcancer onset by 4.1 and 7.6 years in males and females respectively, cancerdeath by 3.9 and 5.9 years, and cardiovascular death by 1.9 and 1.4 years. Notsurprisingly, these studies also found that smoking was correlated with earlierdeaths from cancer and cardiovascular disease. The conclusion of this group wasthat green tea increases lifespan and decreases the risk of pre-mature death, particularly that caused by cancer 31.
Many animal and in vitro studieshave also been conducted on the effect green tea has on specific types ofcancer. In mice, both oral and topical administration of green tea significantlydecreases the risk of UV-induced skin cancer 32. However, it is possible thatthis is due to decreased tissue fat 33, and caffeine is a necessary constituentin this case 17, 33, so more research is needed in this area. Green teaselectively destroys breast cancer cells 34, 35 and epidemiologic data impliesthat it aids in the prevention of early stage breast cancer 30. A case controlstudy also found that green tea decreases the risk of developing ovarian cancer 36, while other in vitro data has found that green tea inhibits theproliferation of cervical cancer 37, prostate cancer 38, leukemia 39, head andneck carcinoma cells 35, and pancreatic carcinoma cells 40. In the case of lungcancer, green tea has a definite and significant effect in smokers andnonsmokers alike. In vitro data 41 and epidemiologic data in both smokers 42 andnonsmokers 29 support this. Finally, green tea may have the ability to preventcancer in the gastrointestinal tract. An epidemiologic study in China found thatgreen tea drinkers had less incidence of gastrointestinal cancer 43, although adifferent study in Japan found no statistically significant difference 28. Invitro and in rats, polyphenols also protect against colon cancer 44, 45.

המשך חלק ג

As if this wasn't enough, green tea has also been tested against manycarcinogens and almost always proves successful. Two toxins that humans are verycommonly exposed to, tobacco and alcohol, are probably of most importance. Anepidemiological study in China compared the incidence of various types of canceramong cigarette smokers and alcohol drinkers. In alcohol drinkers, those thatregularly consumed green tea had 81%, 78%, and 39% decreased risk of gastric, liver and esophageal cancer, while smokers that drank green tea had 16%, 43%, and 31% decreased risk of these cancers 42. In both mice and rats, green teasignificantly protects against tumorgenesis induced by nitrosamines, which arefound in tobacco as well as many food products 46, 47. It also protects againstthe toxic effects of the environmental pollutant pentachlorphenol (PCP) and thechemotherapeutic drug cyclophosphamide (CP) in mice 48, 49 and the cancerinduced by the carcinogens DMBA and DMH in other rodents 45, 50.
As withthe thermogenic effect, there are multiple mechanisms for the proctive effectgreen tea has against cancer, and many of them are tissue specific. One of themore important ones is the protection against oxidative stress, which may play aprimary role in certain types of cancer 48, 49-52. The antioxidant properties ofgreen tea and their importance will be discussed in more detail later on. Ofmore importance is that when it comes to cancer cells, green tea is actually apro-oxidant which results in the death of these cells 53. Thiscondition-dependent antioxidant/pro-oxidant duality is a common feature of manyantioxidants.
EGCG plays a fundamental role in cancer prevention as itinhibits many proteins and the activity of many protein kinases involved intumor cell proliferation and survival. These include (but are by no meanslimited to) the large multi-catalytic protease and metaloproteionases involvedin tumor survival and metastasis and the epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGF), platelet-derivedgrowth factor receptor, mitogen-activated protein kinase, and IkB kinase 52. Theinhibition of EGFR and VEGF are of importance because they are known to playroles in angiogenesis 35, 28, which contributes to tumor formation in many cases 54. The inhibition of metalloproteinases MMP-2 and MMP-9 are also known to playimportant roles in the anti-carcinogenic action of green tea 38, 41.

חלק ד: יתרונות נוספים של תה ירוק

Many of the ingredients in green tea are potent antioxidants. In vitro, greentea and/or EGCG prevent the development of or directly quench a variety ofreactive oxygen species (ROS) including superoxide 55, peroxynitrite 56, 57, andhydroxy radicals 57. In preventing lipid peroxidation by hydrogen peroxide, itwas superior to both lipoic acid and melatonin 58. In humans, acuteadministration of green tea significantly improves plasma antioxidant capacity 59-61, with 450 mL (which would contain about 375 mg EGCG) causing an increaseof 12.7% after two hours in one study 61. In turn, this enhanced protectionagainst oxidative stress offers a variety of health benefits.
Asdiscussed earlier, drinking green tea may delay death from cardiovasculardisease by 1-2 years with a greater increase seen in men 31. A study on 14,000Japanese workers adjusted for dietary factors, age, BMI, alcohol consumption, tobacco use, coffee intake, and type of work found that green tea drinkers hadsignificantly lower cholesterol levels 62 while a smaller study on 500 Japenesesubjects found a statistically significant reduced risk of atherosclerosis inmen, but not in women 63. Although the present epidemiologic data does notsupport the idea that green tea increases HDL ("good") cholesterol in humans 62, it has been demonstrated that it does so in rats 64. The data in this regard isstill relatively preliminary so it is hard to draw too many conclusions, but theproposed mechanism of action for the delay in cardiovascular death is preventionof oxidative damage 57.
Due to its ability to increase superoxidedismutase and glutathione levels, green tea also may aid in the prevention ofinsulin resistance and type II diabetes 65, which is often closely interrelatedwith other cardiovascular conditions. In normal rats, green tea significantlyincreases glucose tolerance, while in diabetic rats it significantly reducesserum glucose 65. Another study also found that green tea improved kidneyfunction in diabetic rats 66.
Another area in which the activity ofgreen tea is particularly important is in the brain. It goes without saying thatgreen may protect against the development of brain tumors 67. It also protectsagainst oxidative damage in the brain 57 and improves brain recovery fromischemia/reperfusion injury in rats 68. Green tea may also be useful inpreventing Parkinson's disease through a fairly specific mechanism, and this hasbeen an area of much study as of late 69.
Green tea also exert aprotective effect in the liver, acting in a synergistic fashion with vitamin E 70, as well as the digestive organs. It protects against or lessens liver damagecaused by alcohol and carbon tetrachloride in rats 71, 72 and protects livercells from a variety of toxins in vitro 73. In the gastrointestinal tract, itreverses intestinal damage induced by fasting in rats 74 and inhibits theproduction of a toxin (produced by Helicobacter pyroli) associated with somegastric diseases 75. Other preliminary studies indicate that green tea may beuseful in the treatment of arthritis 76 and cataracts 77.
Finally, recent cell culture studies have found that green tea may have strong antiviralactivity. It has been tested sucessfully against influenza A and B and has beenfound to inhibit their growth 78, and it may also decrease the chance of HIVinfection 79. As of yet, it is unknown whether these observations are relevantin humans, in which the concentrations of the active ingredients wwould be muchsmaller.

תופעות לוואי אפשריות

Green tea, even in large amounts, is associated with very few side effects. Inmice, signs of toxicity were only observed when doses reached 2 g/kg daily of an 80% polyphenol abstract (this is about the equivalent of 8 cups per pound ofbody weight, per day – 1200 cups of tea in a 150 lb. individual) 80.
Itis possible that large amounts of polyphenols could impair mineral absorption, making extra mineral supplements a wise course of action. Of primary importanceis a significant inhibition of nonheme iron absorption, which is relevant ifmost of your iron comes from sources other than meat 81, 82. It is possible thatthere is also mild inhibition of calcium, manganese, and zinc 83, 84, althoughthere is a study that disputes the finding of impaired zinc absorption 84. Theonly place where the phenomenon of significant mineral blockage by green tea haseven been of worry is in Tunisia, where there is high tea intake and non-meatproducts are the primary source of dietay iron 81. However, it may be prudent totake a multivitamin supplement that provides adequate amounts of these mineralsat a separate time or a low-dose slow release iron supplement if one decides toconsume large amounts of polyphenols.
A concern that is commonly broughtup about green tea is the idea that it decreases levels of androgens such astestosterone and DHT. One study reported that after administered to rats, EGCG "significantly reduced food intake; body weight; blood levels of testosterone, estradiol, leptin, insulin, insulin-like growth factor I, LH, glucose, cholesterol, and triglyceride" 22. However, a different study with green teacatechins in rats found decreased body weight but increased thyroid stimulatinghormone (TSH), LH, and testosterone levels 85. A final study in mice with greentea indicated drastic increases in both testosterone and DHT from green teatreatment, but levels of both were synergistically inhibited when it wasadminstered along with soy phytochemicals 86. In vitro, EGCG inhibits type I 5AR 2, 87, which is partially responsible for the conversion of testosterone to DHT (for this reason, it has been proposed in the topical treatment of acne and hairloss 2), while in rats, green tea is an aromatase inhibitor, which (in theory) would be responsible for an increase in tesotsterone levels 85.
So, howdoes one make sense of this mess of contadictory data? It should first be notedthat 5AR inhibition has only been shown in vitro 87, while it has not beenobserved in live animals – in fact, drastic increases in DHT have been found - and it seems that other constituents of green tea counteract the 5AR inhibition 88. The conclusions of the first study, which found drastic decreases in bodyweight, testosterone, and other hormones, are very misleading. A drasticreduction in levels of all of the substances mentioned can unfortunately beexpected from weight loss in any situation (which is one of the reasons losingweight without losing muscle mass is so difficult). Indeed, when the authors ofthe same study restricted the food intake of the rats to cause a similar weightloss to that induced by EGCG, similar effects were seen, implying that thechanges were not due to EGCG but due to drastic body weight loss 22. The effectgreen tea has on levels of androgens and other hormones is complex and dependson various factors, but no detrimental effects have as of yet been seen in humanpopulations.