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Chapter 41

Psychological and Physiological Consequences of Drinking Tea E.L. Gibson and J.A. Rycroft

Abbreviations CVD EGCG LDL DNA EEG GABA CFF

Cardiovascular disease Epigallocatechin gallate Low-density lipoprotein (cholesterol) Deoxyribonucleic acid Electroencephalography Gamma-aminobutyric acid Critical flicker fusion test

41.1 Introduction The beverage tea is second only to water in terms of global consumption of a drink, outstripping all other drinks put together: in the UK, 77% of adults drink tea, averaging nearly three mugs (540 mL) per day, with volume increasing with age (Gardner et al. 2007). Moreover, tea is known to most of the world’s ethnic and cultural groups: therefore, putative effects of tea on health or behavior may assume considerable importance for public health. This chapter considers the evidence that tea may affect both these outcomes, through psychological and physiological consequences. There is now a very substantial literature relating tea to health, but there is only space here to summarize the evidence, concentrating in particular on studies in human beings, and the recent consensus. The impact of tea on psychological and behavioral outcomes is less thoroughly researched, but nevertheless several intriguing findings that have emerged in recent years are considered here. It is important from the outset to define the term “tea” as used in this chapter, since tea can take many forms across the world. Here, tea refers to the most universally recognized form of beverage, a hot drink formed from infusing leaves and leaf buds from the shrub Camellia sinensis (familiarly

E.L. Gibson (*) Clinical and Health Psychology Research Centre, Department of Psychology, Whitelands College, Roehampton University, Holybourne Avenue, London SW15 4JD, UK e-mail: [email protected] V.R. Preedy et al. (eds.), Handbook of Behavior, Food and Nutrition, DOI 10.1007/978-0-387-92271-3_41, © Springer Science+Business Media, LLC 2011

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E.L. Gibson and J.A. Rycroft

Fig. 41.1 The production journey for green, oolong, and black tea. The tea journey: a diagram summarizing the various stages of processing tea leaves, the type of tea produced, and the impact on its key components, the flavonoids (unpublished figure)

known as the tea plant) in hot or boiling water (though once cooled, this beverage can be drunk cold, as “iced tea”; extracts of tea also form the basis of bottled or canned forms). These tea leaves can be cultivated, picked, cured, and processed in a variety of ways, resulting in differing fermentation and oxidation, giving several classes of tea beverage, principally (in order of oxidation): white, green, oolong, black, and pu-erh teas (Fig. 41.1). In oriental countries, “red tea” is another (arguably more accurate) term for black tea, although, red tea is also a name used for an infusion of leaves from the South African “rooibos” (red bush) plant, which thus contains no C. sinensis, or caffeine. Indeed, infusions of a wide variety of other plants, fruits, and flowers are often referred to as “tea” (or its equivalent in the local language) in many cultures across the globe, and many of these may have both psychological and physiological effects relevant to health and well-being (Pardo de Santayana et al. 2005). However, such effects have not been scientifically or extensively researched for most of these other beverages: indeed, the vast majority of research on tea deals with forms of green or black tea, which are also the most commonly consumed varieties globally (Table 41.1); therefore, this is the nature of the evidence summarized in this chapter. Nevertheless, restricting the scope of the chapter to infusions of the leaf of one plant does not result in a simple categorization or interpretation of evidence, since the forms of tea from C. sinensis vary in biochemical content as a result of cultivation of different varieties of the plant, as well as both preparation of the leaves (Fig. 41.1) and preparation of the infusion (e.g., temperature of the water and length of brewing time; Astill et al. 2001). Moreover, studies of the potential impact of tea on health and behavior have often used processed extracts of tea; this has the advantage of controlling dosage and preparation, but somewhat limits the possibility of generalizing results to any effects of drinking tea as a beverage.

41 Psychological and Physiological Consequences of Drinking Tea

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Table 41.1 Key facts about tea Key points Facts Origins of tea drinking Tea originated in southeast Asia, where it has been drunk for at least 3000 years. However, tea did not arrive in Europe until the early seventeenth century, finally becoming fashionable in Britain in the late seventeenth century, whence it spread to the colonies – the British East India Company established tea plantations in India in the early nineteenth century. Types of tea True tea is a drink made by infusing in hot water the leaves from the tea plant, Camellia sinensis. These tea leaves can be cultivated, picked, cured, and processed in a variety of ways. However, the main process that differentiates tea is fermentation (oxidation) of the leaves. Thus, white tea (the youngest leaves), a Chinese yellow tea, and green tea are not fermented, but undergo steaming, roasting, and drying, resulting in delicate, light tea. Oolong tea is semi-fermented, giving a tea part way between green and black. Black tea and pu-erh (“shu” type) tea are fully fermented, although there is a variety of pu-erh tea, “sheng,” which is unfermented like green tea. Within these broad tea types, there are numerous varieties. Most tea is produced in East and South Asian countries, including China, Japan, Global tea production and consumption India, Sri Lanka, Korea, Vietnam, and Indonesia. Other continents produce substantial amounts of tea including East Africa (especially Kenya), Central and South America. Turkey is also a major producer. About 70% of production is black tea, and 22% green tea. These top producers also tend to be big consumers, accounting for at least half of all production: however, the major importers of tea are (from the highest) Russia, UK, Pakistan, USA, Egypt, Japan, and Iran. These countries consume very different forms of tea, in terms of processing, variety, and preparation. Global tea consumption continues to grow, and has more than doubled since 1970. Tea preparation Water temperature is important for correct tea preparation, with the more delicate tea needing lower temperatures than the fermented one. Thus, for white, yellow, and green tea, water temperature should range from 66°C to 82°C (coolest for white tea). Oolong tea is best brewed using water at 82–88°C, whereas black and pu-erh tea require water near boiling point (99°C). Tea should always be steeped (brewed) for at least 30 s (which allows all the theanine to be released). If steeping is limited to 2 min or so, then several separate infusions can be obtained from the same leaves; each will have different flavor characteristics, as well as chemical components. Longer brewing, for 3–4 min, maximizes the release of antioxidant polyphenol compounds, although brewing beyond 5 min will tend to produce a bitter tea. With or without milk? Adding milk to tea was established early on in its arrival in Europe, although it is practiced elsewhere, such as Manchuria. It is the most common way to take tea in Britain, where the popular tea varieties are quite strongly flavored and astringent. There are mixed findings concerning the impact of milk on tea’s health benefits, but it is likely that, when added as less than 10% of the volume, milk will have little impact on tea’s effects. This table gives key facts about the origins and global distribution of tea drinking, its varieties, production, and preparation of the tea drink

The approaches to studying effects of tea, or its components, on health, behavior, and well-being (a sense of positive health that is more than the absence of illness), vary from in vitro studies of chemical activity in “test tube” models, through experimental studies of tea dosing in animals and man, to epidemiological studies of relationships between habitual tea consumption in large populations and psychological and physiological outcomes. These outcomes are varied, but are grouped here as: cardiovascular (heart and circulatory) health; cancer risk; body weight and obesity; mental wellbeing and mood; and cognitive function or mental performance.

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41.2 Cardiovascular Health Drinking a daily cup of tea will surely starve the apothecary (Ancient Chinese proverb)

Earlier epidemiological studies, including prospective studies looking at development of cardiovascular disease (CVD) over several years in large population samples, did not find conclusive evidence of either a beneficial or harmful effect of drinking tea, but instead, inconsistent results, despite experimental evidence that tea contained potentially beneficial chemicals; however, it was acknowledged that this may be due to close associations in some populations between tea drinking and other lifestyle factors that themselves may be detrimental to cardiovascular health (Hollman et al. 1999). In other words, in some populations, for example in the UK, frequent black tea drinking is popular in lower socioeconomic groups in which unhealthy behaviors are also common, and their confounded influence on heart health may not easily be separated, so that in such populations greater tea intake may even be associated with poorer cardiovascular health, at least in unadjusted analyses. However, a recent overall review of many such epidemiological studies, including populations where this behavioral confounding is not apparent, has concluded that (black) tea clearly has a positive association with coronary heart disease, with three mugs per day reducing risk by up to 71%, depending on the study and population (Gardner et al. 2007). Another meta-analysis of the epidemiological studies linking tea consumption to incidence of stroke, using data from nine studies involving 4,378 strokes among 194,965 individuals, also showed that consuming three or more cups of either green or black tea per day may reduce the risk of ischemic stroke by as much as 21% (Arab et al. 2009). What properties of tea might benefit the cardiovascular system? The most likely candidates are the various plant chemicals found in tea, collectively known as polyphenol flavonoid compounds, as these are known to have antioxidant activity in vitro, which could suppress inflammatory processes that otherwise contribute to CVD. These components of tea include the catechin flavanols, particularly epigallocatechin gallate (EGCG), their oxidation (fermentation) products, the theaflavins, thearubigins, as well as the flavonols, quercetin, keampherol, and rutin (Fig. 41.2). Tea also contains a unique amino acid, theanine, which may have important effects on the brain (see next; refer Fig. 41.3 for structures of catechins and theanine). Fig. 41.2 A comparison of the flavonoid contents of typical green and black tea. Pie charts showing the proportions of different classes of flavonoid compounds in average cups of green and black tea (% = % dry weight) (Based on data from Lakenbrink et al. 2000 and Astill et al. 2001. With permission)


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Fig. 41.3 The chemical structures of catechin and theanine. The chemical structures for two key components of tea: (a) the basic catechin structure and (b) the structure of the amino acid, theanine (an analog of glutamate and glutamine). Both structures were downloaded from Wikimedia Commons (http://commons. wikimedia.org/ – public domain images)

In many European populations, tea is the dominant source of flavonoids such as catechins – in the UK, accounting for as much as 80%, though rather less in the USA – although these are also found in red wine, chocolate, and apples, for example. It has been estimated that average intake of flavonoids in Western countries is about 65–250 mg/day (Erdman et al. 2007). Tea’s importance in contributing catechins to the diet was illustrated by findings from a prospective study of elderly Dutch men, in whom high habitual dietary catechin intake reduced risk of dying from coronary heart disease by about 50% compared with low catechin intake; in contrast, once the contribution of tea had been statistically removed, the risk reduction was down to 20% and was not statistically significant (Arts et al. 2001). In determining likely mechanisms for the impact of tea on cardiovascular health, experimental studies of tea, or its components, have revealed beneficial effects on vascular physiology that support probable health benefits of drinking tea on the cardiovascular system, in in vitro laboratory and animal models, and in clinical trials in human participants (Vita 2005). Several clinical studies have investigated two aspects in particular: (1) activation of blood platelets (assessed as aggregation of platelets with white blood cells or activated by factors such as adenosine diphosphate), which indicates risk of clot formation and inflammation of arterial walls, and is a key event leading to coronary heart disease; (2) responsiveness of the vascular endothelium (cellular lining of blood vessel walls) to changes in blood flow (i.e., dilatation vs constriction), which is thought to be an important indicator of the health of the cardiovascular system. In patients with established CVD, 4 weeks of drinking 900 mL of black tea per day did not reduce platelet aggregation compared to water, despite increased plasma flavonoid content, although the design cannot rule out an interaction with change in caffeine intake. In contrast, this same group did find that this tea “treatment” improved endothelial function in these patients (reviewed by Gardner et al. 2007). By comparison, in a recent double-blind, placebo-controlled study in a larger sample of healthy men, where caffeine intake was equated between treatment groups, 6 weeks of drinking four mugs of black tea per day was shown to inhibit platelet activation (aggregation with white blood cells), as well as lowering plasma levels of C-reactive protein, usually regarded as a general indicator of chronic inflammation (Fig. 41.4; Steptoe et al. 2007a). Similar evidence is available from studies of effects of green tea, which is higher in levels of catechins, especially EGCG, than black tea (although the amount consumed will depend on how the tea is brewed). In epidemiological studies, where potentially confounding lifestyle and other factors are controlled for, an inverse association has been described between green tea consumption and CVD, including stroke and hypertension, in oriental populations (Tanabe et al. 2008). Clinical studies have been short term, but beneficial effects on vascular inflammation and blood lipids, including

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Fig. 41.4 Change from pretreatment baseline in measures of platelet activation and C-reactive peptide after drinking four mugs of black tea per day for 6 weeks, or a tea placebo (means adjusted for baseline values). Measures: “Monoplatelet aggr” = monocyte-platelet aggregation; “Neutro-platelet aggr” = neutrocyte-platelet aggregation; “Leukoplatelet aggr” = leukocyte-platelet aggregation. *p < 0.05 for significant differences between tea and placebo treatments (Based on data from Steptoe et al. 2007a. With the authors’ permission)

reduced oxidation of low density lipoprotein (LDL) cholesterol, have been reported. Indeed, particularly impressive results were found in a recent randomized, double-blind placebo-controlled study administering decaffeinated green tea extract capsules for 3 weeks to healthy volunteers aged from 21 to 70: the green tea treatment reduced blood pressure, inflammation and oxidative stress (a cellular process that can damage DNA), and total and LDL cholesterol (Nantz et al. 2009). However, there is still a need for longer-term placebo-controlled clinical studies. Moreover, it should be noted that recent ex vivo experimental assessment of tea flavonoid effects on vascular endothelium vasodilatation found that highly fermented black tea was equally as potent as green tea, suggesting that the theaflavins and thearubigins in black tea, to which green tea catechins are converted by fermentation, also have beneficial effects on endothelial function (Lorenz et al. 2009). Similarly, a very recent study showed that black tea dose-dependently improved flow-mediated dilation (a noninvasive measure of endothelial function) in healthy male volunteers (Grassi et al. 2009). These beneficial cardiovascular effects of tea are reminiscent of similar effects, including lowering of blood pressure, seen for diets high in fruit and vegetables: similar mechanisms may be involved and continue to be intensively researched.

41.3 Prevention of Cancer The ability of components of tea to have physiological activity that benefits cardiovascular health, either by reducing inflammation or improving arterial vasodilatation, makes it possible that drinking tea will have other health advantages; key among these could be a reduction in the risk of cancer. Several mechanisms might account for tea’s anticancer properties, but the principal ones are likely


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to be reduction of DNA damage by oxidative stress, e.g. by scavenging (deactivation) of reactive oxygen species (free radicals) and binding of metals, metabolism and detoxification of carcinogens, modulation of carcinogenic gene expression, and lowering the rate of cell replication (Lambert et al. 2005). In addition, recent evidence suggests that flavonoids in tea may be able to induce apoptosis, i.e. a process of cell death important in regulating cell proliferation and thus cancer, as well as altering biochemical intracellular signaling pathways (de Mejia et al. 2009). Furthermore, even theanine may have anticancer activity (Liu et al. 2009). Nevertheless, most laboratory studies use higher flavonoid concentrations than those likely to occur from normal tea drinking. Thus, it is important that this mechanistic evidence should be supported by evidence of inverse associations between tea consumption and cancer risk at a population level, i.e. epidemiological studies, where other potentially confounding influences are statistically adjusted for (Lambert et al. 2005). Although there are promising results from some studies showing such inverse associations, others have not supported those findings. For example, in prospective studies in older populations used to assess relations between dietary flavonoid intake and death from cancer, an inverse association was found in a Finnish cohort but not in two Dutch cohorts (Hollman et al. 1999). Gardner et al. (2007) recently reviewed epidemiological studies of associations between specifically black tea and cancer, and concluded that there was little evidence of a consistent protective effect. For example, in a large sample of Canadian men, no association was found between (mainly black) tea drinking and prostate cancer (Gardner et al. 2007). Moreover, a recent report from a very large sample of North American women aged over 45 found no association between total or sitespecific cancer incidence and dietary intake of flavonols and flavones (Wang et al. 2009b). However, it should be noted that, in this population, tea is likely to be only a minor contributor to the intake of these flavonoids. In another sample from the USA, no association was found between tea intake and colorectal cancers (Gardner et al. 2007). In a Japanese sample, frequency of green tea intake was also not associated with gastric cancer (one of the most common cancers in Japan; Tsubono et al. 2001). Conversely, green tea was associated with almost a 50% reduction in risk of gastric cancer in a Chinese population (Setiawan et al. 2001), and black tea was strongly protective against gastric cancer in an Indian population (Rao et al. 2002). Moreover, in a Japanese population, drinking more than ten cups per day of green tea reduced the risk of developing any cancer in both men and women by at least 40%, and onset of cancer was delayed, compared with low intake of green tea (Nakachi et al. 2000). These studies are dependent on accuracy of information concerning both tea intake and confounding factors: importantly, in a prospective study of Chinese men, urinary tea polyphenols were measured to estimate tea intake; high tea intake protected against gastric and esophageal cancers, but only in men who had a low intake of carotene, suggesting low consumption of vegetables (Sun et al. 2002). This could suggest that any protective effect of tea against cancer may be obscured if the diet is generally healthy. Such complex interactions are also indicated by a study of Dutch men and women, where protective effects of dietary flavonoids against colorectal cancers depended on the body size of participants, i.e. protection was only evident in overweight men and normal weight women (Simons et al. 2009), which suggests subtle interactions with other lifestyle, and perhaps genetic, factors. In summary, despite very promising evidence from mechanistic laboratory studies suggesting that tea flavonoids could reduce the risk of cancer, epidemiological studies of relations between tea (or dietary flavonoid) intake and cancer incidence have produced inconsistent findings. The US Food and Drug Administration previously assessed all the epidemiological data available on green tea and cancer prevention and concluded that it is highly unlikely that green tea reduces the risk of prostate cancer and that there is no credible evidence to support a relationship between green tea consumption and a reduced risk of gastric, lung, colon/rectal, esophageal, pancreatic, ovarian, and combined cancers (FDA 2005). One reason could be that, to protect against cancer, tea intake may need to be both high and in populations that eat relatively low amounts of fruit and vegetables.

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41.4 Body Weight, Appetite, and Obesity Tea’s proper use is to amuse the idle, and relax the studious, and dilute the full meals of those who cannot use exercise, and will not use abstinence. Samuel Johnson (1757) “Essay on tea.”

Anecdotally, tea has long been believed to alter appetite; however, scientific evidence has been scarce until recently. Laboratory studies investigating potential cardiovascular benefits of black and green tea flavonoids revealed physiological effects that could be of benefit to obese humans at risk of insulin resistance and unhealthy blood lipid profiles (Ramadan et al. 2009). Consistent with this, the green tea flavanol EGCG, was found to promote postprandial insulin secretion in human beings (Weber 2004): this latter result is particularly interesting, as insulin is known to promote satiety and so constrain food intake (see Sect. 1.3 and 1.7 of this publication). In addition, short-term administration of green tea extract plus caffeine to ten healthy men increased fat oxidation and energy expenditure, through stimulation of the sympathetic nervous system (probably by inhibiting enzymatic degradation of the neurotransmitter noradrenaline), whereas caffeine alone was ineffective (Dulloo et al. 1999). Likewise, in 12 healthy men performing a 30-min cycling exercise, green tea extract (without caffeine) increased fat oxidation rate compared with placebo (Venables et al. 2008). Eleven long-term clinical studies have recently been reviewed and meta-analyzed by Hursel et al. (2009): some of those are summarized here (but not cited, if included in that review). In a study of 104 obese Dutch men and women undergoing severe energy restriction for weight loss, the effect of green tea on weight regain after the restricted period was compared to placebo: there was no difference between groups; however, there was evidence that caffeine may reduce weight regain in habitually low consumers of caffeine. This shows that it is important to design studies that distinguish between effects of caffeine and the flavonoid components of tea. In another study by these same investigators, caffeine was standardized to 300 mg/day for both a green tea extract treated group and a placebo group, during a weight loss diet in women. There was no benefit from green tea on weight or fat loss; in fact, the women given green tea actually became hungrier than those on placebo. However, in normal and overweight Japanese men and women, taking a drink containing green tea catechins twice or thrice a day for 12 weeks resulted in greater weight and fat loss than placebo (Kajimoto et al. 2005). During this period, participants were asked to maintain their usual diet; even so, the catechin drink also reduced total and LDL cholesterol. Similar results were found in a study of Japanese men comparing 12 weeks of drinking either oolong tea once per day or the same tea supplemented with green tea extract; body weight and fat loss, and reduction in LDL cholesterol, were greatest for the green tea extract group. Furthermore, in obese Thai men and women on a calorie-controlled diet (8.4 MJ/day) for 12 weeks, green tea treatment reduced body weight and increased resting energy expenditure compared to placebo. By comparison, in Taiwanese obese women taking green tea extract or placebo capsules for 12 weeks, there was no difference in weight loss, but blood cholesterol profiles were markedly improved by the tea extract. Despite these somewhat mixed findings, a meta-analysis of such studies concluded that evidence supports a small effect of green tea or catechins (or combined with caffeine) in enhancing weight loss or weight maintenance (Hursel et al. 2009). Since then, another study monitored the effects of green tea consumption on body weight, body fat mass, as well as the distribution of fat (Wang et al. 2009a). A total of 182 moderately overweight Chinese subjects, aged between 18 and 55 years, were divided into four groups, with each group allocated a regular dose of green tea containing a different quantity of catechins. Amounts consumed ranged from 30 mg to almost 900 mg; an average cup of green tea contains between 50 and 100 mg of catechins. Participants in the study drank their designated tea divided in two daily doses. On days 0, 30, 60, and 90, measurements of body composition were taken to assess the effects that the prescribed tea had on body mass and fat.


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The results showed that, relative to the control group consuming no green tea catechins, body weight, waist circumference, intra-abdominal fat, and the total lean mass all decreased after 90 days in the group that drank the tea with the highest concentration of catechins. The authors concluded that regular consumption of green tea with very high catechin content can, over a 90-day period, reduce body weight, body fat mass, and waist size in moderately overweight Chinese individuals.

41.5 Mental Well-being and Mood If you are cold, tea will warm you; if you are too heated it will cool you. If you are depressed, it will cheer you; if you are excited, it will calm you. W. E. Gladstone (British Prime Minister 1865)

In many cultures, it is an accepted folklore that drinking tea can acutely improve one’s state of well-being, especially the ability to calm oneself, to relax, and escape for a moment from life’s many pressures. However, there has been very little scientific investigation to support this notion. Of course tea normally contains caffeine, and, as described in the next section, this explains some, but not all, of the arousing potential of a regular cup of tea (Hindmarch et al. 2000). Yet, other aspects of tea may have important effects on mood: for example, drinking tea, but not coffee, was associated with feeling more relaxed, for women with high social support at work (see Steptoe et al. 2007b). Furthermore, the amino acid, l-theanine, unique to tea, has been shown to increase a psychophysiological measure of relaxation in human beings, i.e. increased electrical alpha-wave activity on the brain surface, as detected by electro-encephalographic (EEG) recording of brain electrical potential changes, or “brain waves” (Nobre et al. 2008), and to improve relaxation during restful conditions (Lu et al. 2004). However, another study measuring EEG after theanine-enriched green tea intake found evidence of increased attention but not relaxation (Dimpfel et al. 2007). Furthermore, several studies have found that theanine and caffeine can interact in affecting mental function (see below), and one study reported that theanine can ameliorate the increase in blood pressure seen after acute caffeine intake (Rogers et al. 2008). If theanine aids relaxation, one might expect theanine to be of benefit during stress, as seems anecdotally to be the case for tea. There is evidence that this is indeed the case: thus, in participants who were acutely stressed by having to complete a difficult mental arithmetic task, theanine reduced the heart rate response to stress, and also reduced a well-known stress-sensitive response, a rise in salivary immunoglobulin A antibody levels, compared to placebo (Kimura et al. 2007). One study has looked at the impact of drinking black tea (without milk) four times a day for 6 weeks on responses to stress in healthy men (Steptoe et al. 2007b). This was a randomized double-blind placebo-controlled study, where effects of caffeine were controlled by equating caffeine levels between tea and placebo drink groups. Participants underwent stressful laboratory tasks (role-play speech and mirror tracing tasks) at baseline, after a 4-week wash-out phase on placebo tea, and finally after 6 weeks on either active or placebo tea: 75 men completed the study. Blood samples were taken before and after the stress; heart rate and blood pressure were measured continuously during each session, and the hormone cortisol, known to increase under stress, was measured at several time points in saliva samples. The main findings were that, compared to placebo (a) tea treatment did not alter the stress-induced increases in heart rate and blood pressure; (b) tea drinking resulted in a faster poststress recovery of the cortisol response (Fig. 41.5); (c) participants given the active tea were more relaxed after stress than those given placebo (Fig. 41.6). Thus, drinking tea for 6 weeks did not alter the acute physiological responses during stress, but improved the hormonal and psychological recovery from stress. If tea benefits mood and coping with stress, it might be expected to show some ability to protect against depression. There is indeed some support for this: a cross-sectional study of over 2000

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Fig. 41.5 Changes in salivary cortisol before, after, and during recovery from, stressful tasks, after either 6 weeks of drinking black tea or a placebo drink. Levels of the stress hormone, cortisol, in saliva samples taken before and after performing psychologically stressful tasks, and during subsequent poststress recovery. Cortisol levels fell more rapidly during recovery from stress (50 min later) for the group drinking black tea for 6 weeks (solid line), compared to the group drinking placebo tea (dashed line) (The figure is reproduced from Steptoe et al. 2007b. With the permission of the authors.)

Fig. 41.6 Effect of 6-week tea drinking vs. placebo on change in relaxation from before performing stressful tasks to after post-task recovery. Change in rated relaxation from before performing stressful tasks to after posttask recovery, 50 min later. Participants were less relaxed after stress following placebo treatment (dashed line), but more relaxed after drinking active tea for 6 weeks (solid line) (The figure is reproduced from Steptoe et al. 2007b. With the permission of the authors)

Finnish people found that respondents reporting daily tea drinking were significantly less depressed than those drinking tea less frequently, and there was no depression among those drinking five or more cups per day (Hintikka et al. 2005). This finding may also be relevant to the evidence that tea has neuroprotective effects (see next). A similar finding has been reported for a Japanese population, in relation to green tea and psychological well being (Hozawa et al. 2009): in over 42,000 Japanese over 40 years old, those drinking green tea at least five times per day were 20% less likely to report being psychologically distressed than those drinking tea less than once a day, after controlling for other lifestyle and demographic factors. What could be the mechanisms by which tea benefits mood and psychological well-being? Animal studies have shown that the catechins in tea can act in the brain via type-A receptors for


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gamma-aminobutyric acid (GABA-A), a major inhibitory neurotransmitter known to be involved in the calming, sedative, and anti-anxiety actions of benzodiazepine drugs like Valium (Vignes et al. 2006). Moreover, these catechins have recently been found to inhibit activity of neurones in the brain stem nucleus, the locus coeruleus (Chang et al. 2009). As the locus coeruleus is involved in brain arousal systems, this might indicate a mechanism for the calming effects of tea, although it is not clear that catechins absorbed from drinking tea could affect the brain in this way. Theanine also appears to act in the brain via another inhibitory amino acid transmitter, glycine, and via modulation of dopamine release (involved in attention and motivation, as well as motor control). Theanine is a structural analog of glutamate and glutamine, two neurotransmitters involved in excitatory brain transmission. As such, theanine is able to compete with glutamate and glutamine for their transporters, receptors, and metabolizing enzymes. By attenuating the action of glutamate and glutamine, theanine might affect cognitive function, or mood (Bryan 2008). There remains much to be learnt about the potentially complex mechanisms by which tea may modulate brain activity and so mental well-being.

41.6 Cognitive Function My dear, if you could give me a cup of tea to clear my muddle of a head, I should better understand your affairs. Charles Dickens (1894)

This section will consider two sorts of evidence in relation to tea and brain function: (a) that tea can acutely modulate cognitive function, i.e. as assessed by mental performance after short-term dosing with tea or its components; (b) that drinking tea, or ingesting its components, is associated with neuroprotective effects, i.e. effects on neuronal structure and function that prevent or ameliorate neurodegeneration and associated cognitive decline or dementia.

41.6.1 Acute Effects on Cognitive Function More than a decade ago, it was shown that drinking black tea improved alertness acutely (within 10 min; Critical Flicker Fusion test, CFF) – an effect that was not matched by 100 mg caffeine and was more reliable than the effect of coffee on repeated testing over the day (Hindmarch et al. 1998). Nevertheless, that study also found no acute benefit of tea, coffee, or caffeine on tests of short-term memory. Subsequently, in a comparison of tea and coffee over a day, tea improved alertness (CFF) more than coffee (which had twice as much caffeine), whereas coffee showed some additional benefit for reaction times in a choice reaction time task (Hindmarch et al. 2000). Additionally, both these caffeinated drinks delayed and disrupted sleep, although tea less so than coffee. Finally, there are preliminary reports that black tea may improve focused attention, i.e. the ability to select and process only relevant sensory information from among multiple stimuli, although it is not clear to what extent that effect is independent of caffeine (Lipton Institute of Tea Factsheet, “Black tea and mental performance,” 2009, Unilever; de Bruin et al. unpublished data). Thus, there is some evidence that beneficial effects of tea on alertness may differ from those of caffeine per se. It is also worth noting here that there is increasing evidence that apparently beneficial effects of caffeine may largely be due to removal of cognitive impairment following overnight withdrawal from caffeine. Thus, unlike the positive effects seen in overnight withdrawn participants, no beneficial effects of caffeine on performance were found in participants who had abstained from

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c affeine for 3 weeks prior to testing, and among those receiving placebo, these long-term withdrawn participants performed better than overnight withdrawn participants (Rogers et al. 2005). However, it is not known whether beneficial effects of tea, as distinct from caffeine, depend on acute withdrawal. How might tea improve alertness and attention, other than via caffeine? One possibility is via the activity of theanine: as already mentioned, EEG recordings of brain activity after theanine administration suggest that it is able to produce a relaxed state without drowsiness that might improve sustained attention (Nobre et al. 2008; Gomez-Ramirez et al. 2009). One group examined the impact on performance of caffeine (150 mg) or theanine (250 mg) alone or in combination (Haskell et al. 2008). As expected, caffeine improved performance on several measures; however, theanine alone did not, and even impaired performance on a demanding mental arithmetic task, as well as increasing headaches 90 min later. Yet, the combination of caffeine and theanine improved performance above caffeine alone on more complex verbal tasks, and also caused the greatest increase in alertness. However, these effects on mood were not replicated by another group who used 250 mg of caffeine and 200 mg of theanine, alone or in combination (Rogers et al. 2008). In that study, theanine seemed to prevent the increase in alertness caused by caffeine, as well as the increase in blood pressure. The authors noted that this might help to explain why tea is often perceived to be more relaxing than coffee, and it is in line with changes in EEG activity described above. However, it is important to note that these doses of theanine and caffeine are considerably greater than would normally be found in a cup of tea. Nevertheless, when lower doses of caffeine (50 mg) and theanine (100 mg) were tested in another study, the combined treatment showed some improvement in attention and memory over caffeine alone and placebo (theanine was not tested alone), whereas the caffeine-related increase in alertness was again weaker in the presence of theanine (Owen et al. 2008). The effects of these same doses of caffeine and theanine have subsequently been shown to improve attention on a switch task but not to improve intersensory attention or subjective alertness (Einöther et al. 2010). Could the flavonoid components of tea, especially the catechins, also contribute to any acute effects of tea on cognitive function? It may be plausible, given the animal evidence discussed in the previous section that catechins do alter brain neurotransmitter systems – indeed, the inhibition of locus ceoruleus neuronal activity by catechins (Chang et al. 2009) would be compatible with a more relaxed frame of mind after tea, though not obviously with improved attention – though they may not reach the brain in sufficient amounts. Furthermore, catechins and other flavonoids could improve blood flow in active areas of the brain via their vascular epithelial effects (see above). There are also several studies in rodents demonstrating that chronic consumption of catechins improves memory and other aspects of neuronal function (de Mejia et al. 2009), as well as promising results from human interventions administering some types of dietary flavonoids (mainly isoflavones) for weeks or months (Macready et al. 2009). Nevertheless, there do not appear to be any studies demonstrating short-term effects of tea flavonoids on cognitive performance, so the question of their contribution to any such effects from tea remains open.

41.6.2 Chronic Effects on Cognition and Brain Function There is growing evidence from animal studies that various flavonoids, including tea catechins, can benefit neuronal growth and function, and furthermore act as neuroprotective agents, counteracting neurodegenerative processes, such as oxidative stress, that otherwise lead to dementia, Parkinson’s disease, etc. (de Mejia et al. 2009; Macready et al. 2009). Moreover, theanine also appears to have neuroprotective effects in animals (Il Kim et al. 2009). To date, there do not appear to be any controlled interventions examining the impact of chronic tea intake (or tea components) on cognitive function in human beings. Nevertheless, there are several epidemiological studies that have examined relationships between long-term tea consumption and brain or cognitive function. In a cross-sectional study of elderly


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Japanese, higher green tea consumption, but not coffee, was associated with lower cognitive impairment (Kuriyama et al. 2006). In an elderly French population, risk of dementia after 5 years was reduced by 51% in those having the highest intake of dietary flavonoids at baseline (Commenges et al. 2000). In an American population, drinking two or more cups of tea per day was associated with a reduced risk of Parkinson’s disease, independently of smoking or coffee drinking (Macready et al. 2009). In Chinese adults aged 55 or over, tea (mainly black or oolong) consumption at baseline was clearly associated with lower cognitive impairment or decline 1–2 years later (Ng et al. 2008). Finally, in 70–74-year-old Norwegians, habitual intake of tea, wine, and chocolate (all of which are rich in flavonoids including catechins) was dose-dependently and additively associated with better cognitive performance (Nurk et al. 2009). Taken together, these findings support beneficial effects on brain function from habitual consumption of tea.

41.7 Conclusions It is becoming increasingly clear that tea, the beverage made from infusions of leaves from C. sinensis, can have both physiological and psychological effects that may benefit health. Tea contains plant chemicals known as flavonoids that have antioxidant properties and have been shown to benefit indicators of cardiovascular health in laboratory studies measuring effects on inflammation and vascular function, and in clinical trials administering fixed doses of tea or tea extracts. Population-based studies also, on balance, support a positive relationship between tea drinking and cardiovascular health. Laboratory studies suggest that tea and its components, especially catechins, can have effects on cellular processes that might reduce the risk of developing cancer. However, results from populationbased studies have been inconsistent, and currently it is not possible to conclude that tea reliably reduces cancer risk. There is some evidence that high doses of catechins from green tea may promote weight loss, potentially by stimulation of fat oxidation. In terms of psychological effects, tea can improve cognitive function acutely, and to some extent independently of its caffeine content. One reason may be due to effects on the brain of the amino acid, theanine, which has been shown to alter brain electrical activity. Theanine seems to improve relaxation and aspects of attention, without overstimulation. Possibly related, tea can also ameliorate physiological responses to stress and help poststress relaxation. This might explain population evidence that links tea drinking with resistance to depression. Finally, laboratory and animal studies suggest that tea and its components, both flavonoids and theanine, can have neuroprotective effects, suggesting resistance to neurodegeneration. This possibility is further supported by prospective epidemiological evidence indicating that populations drinking tea regularly show slower declines in cognitive function, or less risk of dementia with aging, while cross-sectional studies have found a positive association between tea consumption and cognitive function in the elderly. In conclusion, there have been a large number of scientific studies investigating physiological and psychological effects of drinking tea. Overall, a number of health benefits may arise from tea, but the strongest evidence is for cardiovascular and neurological health.

41.8 Applications to Other Areas of Health and Disease Gardner et al. (2007) have recently reviewed areas of health that may be affected by drinking tea, in addition to those discussed before.

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41.8.1 Dental Health The tea plant naturally accumulates fluoride from the soil, and so drinking tea can contribute to the beneficial effect that fluoride can have on preventing or treating damage to teeth enamel caused by dental caries, for example. The high levels of catechins in green tea may also protect against caries by inhibiting growth of oral bacteria.

41.8.2 Bone Health It has been suggested that compounds in tea including fluoride, phytoestrogens, and caffeine may influence bone mineral density, especially in older people. There is limited evidence that drinking four or more cups per day may increase bone mineral density, and reduce the risk of hip fractures, independently of whether milk is added – although adding milk clearly contributes a significant amount of calcium in regular drinkers.

41.8.3 Hydration Although high doses of caffeine can be diuretic, i.e. stimulating the kidneys to increase secretion of water, eventually leading to dehydration, there is no evidence that such an effect occurs at the levels of caffeine normally drunk in tea. On the contrary, it has been demonstrated that tea has a beneficial effect on hydration.

41.8.4 Iron Status Polyphenols in tea can inhibit the absorption of iron from non-heme sources (i.e., plants). This appears only to be of concern to those who may already be at risk from low iron status: in such cases, drinking tea should be avoided within 1 h of meals.

Summary Points • From a health point of view, the key components of tea are likely the flavonoid group of chemicals, including catechins, such as EGCG (high in green tea), their metabolites the theaflavins and thearubins (high in black tea), and the flavonols, as well as the amino acid, theanine. • The flavonoids have antioxidant, as well as other biochemical effects, and these reduce inflammation and improve blood vessel dilatation, which may lead to better cardiovascular health. • Despite laboratory evidence suggestive of cancer preventive properties of tea components, findings from population-based studies have been inconsistent, so it is not clear whether chronic tea drinking reliably reduces cancer risk. • Green tea catechins, at least at high doses, appear to aid weight loss in overweight people, probably by increasing oxidation of fat. • Tea can relieve effects of stress, and help with poststress recovery, both physiologically and psychologically.


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• Tea can benefit acute cognitive performance, possibly by the action of theanine in the brain, where it appears to aid relaxation but also improves some forms of attention. • Tea may protect the brain against degeneration: tea components are neuroprotective in the laboratory, and tea drinking is associated with less dementia in the elderly.

Key Terms Polyphenol antioxidants: A group of chemicals based on a polyphenolic substructure, widely available in fruits and vegetables. These compounds show strong antioxidant activity in laboratory tests, although the extent to which dietary sources reach target sites in sufficient concentrations is debated. They may benefit health by deactivating reactive oxygen species (free radicals), so reducing inflammation and cell damage. Flavonoids: A subclass of polyphenol antioxidants, some of which are common in tea, especially the catechins (strictly, flavonols) such as EGCG. Reactive oxygen species: Often called free radicals, these are small molecules that are highly reactive due to the presence of oxygen ions with unpaired (free) electrons. They are a normal byproduct of oxygen metabolism in cells, but if levels rise (a state known as oxidative stress), reactive oxygen species can damage cell structures including DNA. Electroencephalography: Recording of brain surface electrical activity through electrodes placed around the scalp. The electrical potentials reflect the sum of activity of millions of neurones, and result in rhythmic activity (brain waves) with frequencies between 1 and 20 Hz; these are classified (in increasing frequencies) as delta, theta, alpha, beta, and gamma waves, reflecting different brain states. Cortisol: A steroid hormone produced in the cortex (outer shell) of the adrenal glands, in response to activation of the hypothalamic–pituitary–adrenal–neuroendocrine axis. Cortisol levels in blood or saliva rise rapidly in response to psychological stress, although they also display an underlying circadian rhythm (high in the early morning and low in the evening).

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