Tea is a drink made from the dried leaves of Camellia sinensis; it is said to be the second most popular drink in the world after water. All tea starts as green but if the rolled and cut leaves are allowed to stand and ferment for 1-3 days before drying it becomes black. In green tea the enzyme that causes the blackening is inactivated by heat treatment which prevents blackening. Oolong tea is fermented for a shorter period and its colour and taste are between green and black tea. Tea leaves contain high quantities of polyphenols, which make up 20-30% of their dry weight. When tea leaves are rolled and crushed during processing, the enzyme polyphenol oxidase converts catechins (categorised earlier as flavonols) to polymeric forms, which give the fermented oolong and black teas their characteristic colours. Black tea is the form usually consumed in the UK although green tea is available and extracts of green tea in tablet form are marketed. Tea contains some essential nutrients but these probably provide only a tiny fraction of the adult requirement for these nutrients. Tea also contains the alkaloid caffeine and smaller amounts of theobromine which are responsible for the stimulating effect of the beverage.
• An antioxidant effect including the ability to prevent oxidation of LDL
• Anti-mutagenic effect and reduced tumour cell proliferation in vitro and prevention of chemically induced cancer in animal models
• Reduced platelet aggregation by effects on the cyclooxygenase pathway
• It may also reduce blood cholesterol because flavonols reduce the absorption of cholesterol in the intestine.
There is some epidemiological evidence consistent with an association between high tea consumption and reduced rates of heart disease and cancer, but this evidence is very inconsistent and sometimes even suggests a negative effect of tea. Animal studies consistently demonstrate the ability of green tea polyphenols to reduce chemically induced cancers in several animal models but human epidemiological studies have produced mixed results, some suggesting a protective effect, some no effect and some a worsening of risk. The epidemiological methods are probably too insensitive and too subject to confounding variables to be able to determine whether tea has disease-preventing properties or not.
Birt et al. (1999) have summed up the evidence of a cancer-preventing effect of tea extracts. These authors conclude that there are numerous experimental studies with animals which suggest that tea extracts can reduce the incidence of chemically induced cancers and that oral or topical application of tea extracts can reduce the rate of skin cancer induced by exposure of animals to ultraviolet light. However, they also conclude that epidemiology provides no clear evidence for a relationship between tea consumption and human cancer; some suggest an increased risk associated with tea, some suggest no effect and some suggest a protective effect. The two factors listed below complicate this analysis.
• There is evidence that regular consumption of hot liquids, including tea, may increase the risk of oesophageal cancer.
• Green tea has higher concentrations of polyphenols than black tea, which is most commonly consumed in western countries. Green tea drinking is more prevalent in areas where micronutrient deficiencies are more common and many of the epidemiological studies suggesting a beneficial effect of tea drinking were performed in these areas. Is there a difference between black and green tea? Is a protective effect of tea more likely in people who are micronutrient deficient?
Birt et al. (1999) quote a Dutch prospective study which found that black tea consumption was positively associated with breast cancer, not associated with risk of colorectal cancer and negatively associated (protective) with lung and stomach cancer. However, tea drinkers tended to smoke less than non-drinkers and also tended to eat more fruit and vegetables and when the results were corrected for this the apparent protective effects of tea disappeared.
At present, there is a lot of experimental data, using animals and in vitro systems, which point towards the potential for tea and particularly green tea to have protective effects against cancer and heart disease. Whilst such experiments may be of great value in generating hypotheses about the protective potential of agents in tea, they are not sufficient to make firm conclusions about the long-term benefits of these chemicals in people or to make public health recommendations. There is little substantial or consistent corroborating evidence from human studies that this theoretical potential actually translates into real benefits. It seems reasonable to say that if you are a regular tea drinker there is some conflicting evidence that you may get some benefit from it provided you don’t drink it too hot. It seems unreasonable on the basis of the evidence available to encourage people to drink more tea for health reasons or to take extracts of tea as a supplement. Those who enjoy tea can continue to enjoy it in the knowledge that it just may also have some long-term beneficial effects. Theoretically it might be expected that there would be substantial differences between green and black tea, with most of the animal and in vitro studies suggesting that green tea is more likely to have beneficial effects.
The dose of tea extract is not clearly established but 200-400 mg of extract is commonly used. These extracts should be standardised to contain a high level of polyphenols (up to 95%) with 40% of this as the catechin, epigallocatechin gallate. This is the equivalent of drinking four to six cups of green tea per day.