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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, antimutagenic and anticarcinogenic effects. Other possible benefits include treatment of cardiovascular disease, diabetes, dermatological problems, obesity, and oral health problems [2]. 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 for cancer protection [3], 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 (such as prevention of macular degeneration) are well known. Chorophylls and pheophytins from green tea are also anti-carcinogenic [8, 9], but a discussion of the many benefits these phytonutrients have to offer is beyond the scope of this article.
Theanine - L-theanine is an amino acid found in high concentrations in tea, with a typical cup of tea containing 30 mg or more. Theanine decreases blood pressure [10, 11] and has been found effective in increasing the antitumor activity of cancer drugs [12, 13], but its most important properties are in the area of the brain. 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). Surprisingly, caffeine can play a significant role in the anti-mutagenic effects of green tea in 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 upcoming discussion of thermogenesis.
Green tea and body composition
Recent research has confirmed that green tea can cause weight loss through multiple pathways. In addition to being a potent appetite supressant, green tea increases thermogenesis, preferentially burning fat over protein in a similar manner to many other thermogenics. Green tea has been demonstrated to be as or more effective than some prescription weight loss medications, and may rival the ECA stack in terms of fat loss. In addition, green tea is one of the few weight loss medications that has not been associated with negative effects on the cardiovascular system (such as increased heart rate and blood pressure) or CNS side effects (such as overstimulation 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 thermogenic drugs [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]. Even if the antioxidant activity turns out to have little to do with the breakdown of fat, it leads to many health benefits that will be discussed in greater detail in following sections.
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 for cancer protection [3], 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 (such as prevention of macular degeneration) are well known. Chorophylls and pheophytins from green tea are also anti-carcinogenic [8, 9], but a discussion of the many benefits these phytonutrients have to offer is beyond the scope of this article.
Theanine - L-theanine is an amino acid found in high concentrations in tea, with a typical cup of tea containing 30 mg or more. Theanine decreases blood pressure [10, 11] and has been found effective in increasing the antitumor activity of cancer drugs [12, 13], but its most important properties are in the area of the brain. 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). Surprisingly, caffeine can play a significant role in the anti-mutagenic effects of green tea in 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 upcoming discussion of thermogenesis.
Green tea and body composition
Recent research has confirmed that green tea can cause weight loss through multiple pathways. In addition to being a potent appetite supressant, green tea increases thermogenesis, preferentially burning fat over protein in a similar manner to many other thermogenics. Green tea has been demonstrated to be as or more effective than some prescription weight loss medications, and may rival the ECA stack in terms of fat loss. In addition, green tea is one of the few weight loss medications that has not been associated with negative effects on the cardiovascular system (such as increased heart rate and blood pressure) or CNS side effects (such as overstimulation 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 thermogenic drugs [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]. Even if the antioxidant activity turns out to have little to do with the breakdown of fat, it leads to many health benefits that will be discussed in greater detail in following sections.
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