Tea – Composition and Health Implications
Tea arrived in Europe in the 18th century, having been drunk elsewhere for centuries, for example by the Chinese for probably 5000 years. Tea was initially sold in coffee houses but became more popular than coffee, perhaps because of early Royal patronage, and by the 1750’s tea houses and tea gardens were common in and around London. The popularity of tea was such that from the latter part of the 18th to the beginning of the 19th century a transition took place in the drink of the laboring class as tea replaced beer and gin.
Composition of teas
Tea leaves contain, in quantities equivalent to 20–30% of the dry weight, a range of polyphenols, particularly the flavan-3-ols, namely (+)-catechin and (–)-epicatechin and their gallate derivatives.
The types and proportion of flavan-3-ols in the tea leaf varies with season, the age of the leaf, climate, and horticultural practices. In addition, major changes occur during processing. In the rolling and crushing used to manufacture oolong and black tea, polyphenol oxidase released from the leaf endoplasmic reticulum catalyses the condensation of the catechins to the yellow–orange colored theaflavins. These undergo oxidation and randomly polymerise to form high molecular weight thearubigins, which are major components and impart the red–brown color that characterizes black tea. These compounds are responsible not only for the colour of black tea but also for its astringent flavour. In comparison with green tea, black tea is low in monomeric catechins but high in oligomeric derivatives, most notably thearubigins (Haslam, 1998). Fermentation, however, does not affect the level or composition of flavonol glycosides, which are similar in black and green tea.
The polyphenols in tea can react with divalent ions such as iron. This could reduce the bioavailability of iron but this effect is not definitively associated with any increased morbidity. The presence of catechins indicates that tea extracts have considerable antioxidant activity. In certain model systems, such antioxidant activity compares favourably with the antioxidant nutrient vitamin E.
Consumption of tea probably provides a negligible intake of macronutrients. For example, less than 2% of the hot-water-soluble solids of black tea are proteins, 4–5% are carbohydrates and 2–3% are lipids, mainly linoleic and linolenic acids. However, macronutrient content is markedly increased by the UK habit of adding milk and sugar.
Tea also contains a range of micronutrients including manganese, potassium, niacin, riboflavin, folate and zinc. However, the contribution of tea to the overall vitamin and mineral intake in the UK has declined in recent years. According to MAFF’s National Food Survey statistics, tea consumption has decreased by more than 50% in the last 20 years to about 3.5 cups per person per day (one cup in the UK is typically 200 mL). In addition, the tea drunk today may be less rich in micronutrients than in previous decades as infusion times have decreased from 5–6 min to 40–60 s.
All types of tea also contain significant quantities of the purine alkaloid caffeine together with smaller amounts of theobromine. About 80% of the purine alkaloids are extracted into the water-soluble phase during brewing. Typically, a 200 mL cup of black tea contains 50–100 mg of caffeine.
Health implications of tea
There are suggestions that tea and tea catechins may have a variety of health effects in man. Most interest has been focused on the possibility that tea may inhibit the development of heart disease and cancers, diseases that account for the majority of premature mortality in developed countries.
Tea and heart disease
A number of epidemiological studies have suggested that consumption of tea per se or diets rich in the polyphenols found in tea are associated with decreased risk of heart disease and associated conditions. A prospective study in the Netherlands (Geleijnse et al., 1999) of 3454 men and women aged 55 years found a significant inverse association between tea intake and radiographically quantified aortic atherosclerosis. In addition, green tea consumption appears to be associated with about a 30% decrease in aortic lesion formation in hyper-cholesterolaemic rabbits (Tijburg et al., 1997).
A recent meta-analysis of 10 cohort and seven case-control studies serves to emphasize the hetrogeneity in reported effects. The authors estimated an 11% decrease in the rate of myocardial infarction with an increase in tea consumption of around three cups a day. However, the possibility of publication bias and evidence of geographical differences (a decreased risk was consistently found in continental Europe but studies in the USA and UK showed little or no effect, or even an increased risk of CHD) urges caution in the interpretation of this finding (Peters et al., 2001). Another Dutch study (Arts et al., 2001a) used data from the Zutphen Elderly Study (a prospective cohort of 806 men aged 65–84 years at baseline) to assess whether total catechin intake, which was 72 ± 47.8 mg/day at baseline, mainly from tea, apples and chocolate, influenced the risk of death from ischaemic heart disease or incidence or death from stroke. They report that catechins, whether from tea or other sources, may reduce the risk of ischaemic heart disease death but not stroke.
One possible mechanism by which tea may reduce heart disease risk is via the ability of catechins to prevent the oxidation of low-density lipoprotein cholesterol to an atherogenic form. In vitro, the oxidation of low-density lipoprotein by endothelial cells, macrophages and Cu2+ can be inhibited by a wide range of polyphenols and polyphenol-rich extracts (Duthie et al., 2000). Such effects may be due to the direct scavenging by the polyphenols of the oxidising species or may result from the polyphenol-mediated regeneration of vitamin E in the low-density lipoprotein. However, whether such in vitro antioxidant effects also occur in vivo and translate into reducing the risk of developing heart disease is unclear at present. The consumption of tea may also prevent atherosclerosis by mechanisms that do not necessarily involve the antioxidant properties of catechins (Duthie et al., 2000). For example, in vitro studies suggest that tea extracts may prevent platelet adhesion and aggregation by inhibiting the cyclooxygenase pathway and reducing the cyclic 3′,5′-adenosine monophosphate (AMP) response of platelets to prostaglandin I2.
Moreover, reported vasodilatory effects of tea extracts and polyphenols may be due to their affecting enhanced nitric oxide generation, cyclic 3′,5′-guanosine monophosphate (GMP) accumulation and other endothelium-dependent relaxation factors. The relevance of these findings to normal human diets still needs to be established. Caffeine in tea may reduce blood coagulation by inhibiting thrombin-stimulated thromboxane for- mation. It has also been suggested that reported hypocholesterolaemic effects may reflect reduced cholesterol absorption from the intestine, caused by flavan-3-ol esters reducing the solubility of cholesterol in mixed micelles.
Tea and cancer
Epidemiological studies of black tea consumption and cancer have produced mixed results. For green tea consumption, a review published in 1998 found that out of five studies on the incidence of colon cancer, three found an inverse association, one reported a positive association and one found no statistically significant association. For rectal cancer, of four studies one reported an inverse association and two reported an increased risk (Bushman, 1998). The inconclusive findings of these and other studies may reflect problems of measurement error (recall bias in case-control studies) and specificity of exposure (duration and amount not specified), or lack of control of potentially important cofounders. At the present time the epidemiological evidence is at best inconclusive as to whether there is any benefit from either black or green tea. In contrast to human epidemiological studies, many in vitro and animal studies have demonstrated the inhibition of chemically induced cancer by tea and tea polyphenols. For example, rats fed a diet containing 1% green tea catechins have a significantly reduced mortality from mammary tumors following treatment with a chemical carcinogen compared with rats given the carcinogen alone (Hirose et al., 1994). Hamsters fed green tea polyphenols display fewer hyperplastic pancreatic duct lesions after treatment with N– nitrosobis(2-oxopropyl)amine (Majima et al., 1998). In a comprehensive study, Yang et al. describe the ability of both green and black tea infusions to inhibit N-nitrosodiethylamine-induced lung carcinogenesis in knockout mice (Yang et al., 1998). Tea extract also significantly reduced the progression of chemically induced, non-malignant adenomas to malignant adenocarcinomas. Furthermore, the spontaneous formation of lung tumors and rhabdosarcomas was inhibited 50% in rats fed either black or green tea infusions.
There are a number of mechanisms by which tea may potentially influence both carcinogenic initiation and promotion (Duthie et al., 2000). In brief, these include direct and indirect antioxidant protection of DNA, the modulation of enzyme systems such as cytochrome P450 complexes that metabolise carcinogens or pro- carcinogens to genotoxins and the modulation of malignant transformation, apoptosis and gene expression. Gut flora profiles could also be altered to decrease the formation of potentially carcinogenic compounds such as ammonia and amines.
Bioavailability of tea catechins
Consumption of a single dose of green tea extract results in a small but significant increase in plasma concentrations of total catechins. The effect is less for black tea arguably because of the potentially poor absorption of high molecular weight thearubigens and theaflavins, which predominate in black tea. The presence of milk does not appear to affect the absorption of catechins from tea (van het Hof et al., 1999). In addition, radiolabelling studies with primates suggest that significant quantities of (–)-epigallocatechin gallate or its metabolites can be found in tissues indicating that they may arrive at target sites within cells in peripheral tissues (Suganuma et al., 1998). This is an essential requirement if consumption of the catechins found in tea is to exert effects similar to that observed in cell culture.
In many biological systems, extracts of tea have a wide range of effects with potential health benefits. In cell culture and animal models they may inhibit oxidation of low density lipoproteins, prevent mutations and be anti-carcinogens. Much of this activity is ascribed to the polyphenolic components of tea, in particular the catechin-derivatives. More studies are required to establish the bioavailability of these compounds and their effects in vivo before any health benefits can be established with certainty. The impact of these compounds when consumed as part of a normal diet and in amounts normally consumed by humans is not yet clear.