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Next to water, tea is presently the most widely consumed beverage in the world [1]. The three forms of tea, which are differentiated by processing method, are green, oolong, and black tea. Of these, green tea undergoes the least amount of processing, and it has been used for medicinal purposes for thousands of years. Green tea has become well known for its antioxidant effects. This article will examine the constituents of green tea and the various benefits they have to offer.
Constituents of green tea
- Catechins - The polyphenols are generally considered to be the
most important elements of green tea, with the catechins being 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, but the combination of catechins can be especially effective, and EC appears to increase the incorporation of EGCG
and ECG into lipid bilayers [4], emphasizing the importance of synergistic
effects between the various components of green tea. Although the numbers
vary, green tea is generally made up of about 10% polyphenols, 50% of which
are EGCG. The green tea catechins have been studied extensively and will
be the primary focus of this article.
- Pheophytins, chlorophylls, and carotenoids - Despite the importance
of catechins, they are not the only constituents of green tea that offer
health benefits. Some studies find that catechins depend on other components
or are not even major players in some of the anti-genotoxic and antioxidant
effects of green tea [5, 6]. The numerous other active compounds in green
tea identified include chlorophylls a and b, pheophytins a and b, lutein,
and beta-carotene. All of them have antioxidant properties [7], and the health
benefits of beta-carotene and lutein are well known.
- Theanine - L-theanine is an amino acid found in high concentrations
in tea, with a typical cup of tea containing 30 mg or more. L-theanine bears structural similarity to glutamic
acid and hence competes with it in binding to glutamate receptors, offering
protection against glutamate neurotoxicity [11]. This glutamate receptor
competition also provides a variety of differential effects on the brain.
These include an increase in serotonin and/or dopamine in some areas of the
brain, notably the striatum, hypothalamus and hippocampus [14], an increase
in GABA [15], and an increase in brain alpha wave activity [15]. While beta
wave activity is associated with periods of high stress, alpha waves are
associated with a state of being awake, but relaxed (such as the period right
before the onset of sleep). Theanine is said to promote a state of "alert
relaxation" because of these differential effects [15], and it also antagonizes
the stimulation and anxiogenic (anxiety promoting) effects of caffeine [11,
16] which may be responsible for the paradoxical 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). The presence of caffeine in green tea will be of most importance in the upcoming discussion of thermogenesis.
Green tea and body composition
Recent research has confirmed that green tea can support weight loss. In addition to being an appetite suppressant, green tea increases thermogenesis, preferentially burning fat over protein in a similar manner to many other thermogenics. In addition, green tea is one of the few weight loss supplements that has not been associated with negative effects like over stimulation and irritability. More research is needed before the level of effectiveness of green tea can be more conclusively determined, but the present data shows a clear benefit.
Green tea has been known to be a thermogenic agent for quite some time, but the thermogenesis was usually attributed to the caffeine content. It was then found in an in vitro experiment with brown adipose tissue that the thermogenic effect of green tea was "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 green tea powder have shown that it decreases body weight and food intake [22, 23] and inhibits lipogenesis [24].
Two important studies have been done assessing the thermogenic effect of green tea in humans. The first was a preliminary study that 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 extract increased 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 the green tea extract group, indicating that the increase in energy expenditure was due to the breakdown of fat, not protein. Measurements of urinary nitrogen excretion, which were significantly different between treatments, further supported this contention. It is also interesting to note that thermogenic response was not correlated with body mass index (BMI), implying that green tea may be equally as effective in relatively lean individuals. Finally, the increased thermogenesis was not accompanied by an increase in heart rate, which makes green tea distinct from other thermogenics [25].
The second study was a three month open trial with 70 subjects using the same dosage of the same extract as in the above study (150 mg caffeine, 375 mg total catechins, 270 mg EGCG per day). Treatment with green tea was well tolerated and associated with 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 larger placebo-controlled studies.
Green tea has many mechanisms of action in stimulating weight loss. The most important is probably the inhibition of catechol-O-methyl-transferase (COMT) by EGCG [1, 19, 25]. COMT is the enzyme that breaks down norepinephrine (NE), one of the body's most important lipolytic hormones. Caffeine also plays a synergistic role by inhibiting phosophdiesterases (enzymes that break down cAMP, which is further down the lipolytic pathway) [19, 25]. Although EGCG is the most responsible, some flavanoids found in small amounts in green tea such as quercetin 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 pancreatic lipase, which has been demonstrated in vitro [1]. These enzymes both play major roles in the digestion of fat, so when they are inhibited a smaller proportion of fat is absorbed and a greater proportion excreted.
Green tea is also a potent appetite suppressant. This can be partly explained by the fact that it increases both NE and dopamine [14, 25], but further mechanisms of action have been hypothesized. Specifically, tea polyphenols have been known to elevate levels 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 green tea, 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 tea simultaneously elevates CCK and decreases pancreatic lipase, conferring the benefits of both appetite suppression and decreased fat digestibility.
Finally, the antioxidant properties of green tea may play a role in the lipolytic effect [20-22]. One cell culture study suggested that green tea inhibited lipogenesis by increasing superoxide dismutase activity and subsequently decreasing the formation of free radicals [20], while another suggests that vitamin C from green tea plays a role in its lipolytic activity [21].
Other benefits of green tea
Many of the ingredients in green tea are potent antioxidants. In vitro, green tea and/or EGCG prevent the development of or directly quench a variety of reactive oxygen species (ROS) including superoxide [55], peroxynitrite [56, 57], and hydroxy radicals [57]. In preventing lipid peroxidation by hydrogen peroxide, it was superior to both lipoic acid and melatonin [58]. In humans, acute administration of green tea significantly improves plasma antioxidant capacity [59-61], with 450 mL (which would contain about 375 mg EGCG) causing an increase of 12.7% after two hours in one study [61]. In turn, this enhanced protection against oxidative stress offers a variety of health benefits.
Green tea also exerts a protective effect in the liver, acting in a synergistic fashion with vitamin E [70], as well as the digestive organs.
Possible side effects and precautions
Green tea, even in large amounts, is associated with very few side effects. In mice, 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 of body weight, per day – 1200 cups of tea in a 150 lb. individual) [80].
It is possible that large amounts of polyphenols could impair mineral absorption, making extra mineral supplements a wise course of action. Of primary importance is a significant inhibition of nonheme iron absorption, which is relevant if most of your iron comes from sources other than meat [81, 82]. It is possible that there is also mild inhibition of calcium, manganese, and zinc [83, 84], although there is a study that disputes the finding of impaired zinc absorption [84]. The only place where the phenomenon of significant mineral blockage by green tea has even been of worry is in Tunisia, where there is high tea intake and non-meat products are the primary source of dietay iron [81]. However, it may be prudent to take a multivitamin supplement that provides adequate amounts of these minerals at a separate time or a low-dose slow release iron supplement if one decides to consume large amounts of polyphenols.
Green tea pharmacokinetics
This last section will examine the optimal dosage and dosing schedule for green tea. 270 mg EGCG is all that is needed for fat loss [1], and this would still have positive health benefits. When we look at the pharmacokinetics of green tea, we can see that there may be ways to maximize the effect of a given amount. The most important thing to note is that green tea polyphenols undergo saturable presystemic elimination. This means that low doses only increase plasma EGCG levels marginally, but once the point of saturation is reached, the same amount of tea will have much greater effects.
The amount required to achieve saturation in humans varies from study to study. Using tea leaves, one study found 3.0 grams to increase plasma EGCG levels by 2.7-3.4 times as much as 1.5 grams, while the difference between 3.0 grams and 4.5 grams was not statistically significant [90]. However, a study measuring antioxidant potential of green tea leaves found that 2.5, 5.0, and 7.5 increased plasma antioxidant potential by 2.1%, 6.2%, and 12.7% respectively at the 120 minute mark, indicating that 7.5 grams was over twice as effective as 5.0 grams [61]. 7.5 grams equates to about 375 mg of EGCG (note that this is only an approximation), and a pharmacokinetic study with green tea extract yielded a similar result. Amounts containing 225, 375, and 525 mg EGCG raised plamsa concentrations by 657, 4300, 4410 pmol/mL, respectively [91]. However, in a last study, despite the fact that 400 mg EGCG (from an extract) overcame saturation as opposed to 200 mg (the AUC, in this case a measure of total bioavailability, was 23.0 and 64.9 respectively), 600 mg and 800 mg amounts kept getting significantly more effective, with AUCs of 111.1 and 258.2 respectively [92]. The maximum plasma concentrations similarly increased. It is also noteworthy that this study found that it took 600, not 400 mg of pure EGCG (without other catechins) to overcome saturation, once again emphasizing that the other catechins operate synergistically with EGCG.
With this data in mind, we could safely say, for example, that taking an extract containing 400 mg EGCG once daily would be considerably more effective than 200 mg twice daily. Increasing this amount to 600 mg may or may not have a significant effect, depending on the study we look at. Either way, a minimum of 400 mg EGCG should be taken at a time to overcome saturation. An ideal dosing schedule would be 400 mg 2-3 times daily, while a more economical (but still very effective) one would be 400 mg once daily (preferably in the pre-workout period).
Here are some of the best supplements containing green tea:
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