Tea, one of the most popular beverages in the world, is an infusion of the leaves of the Camellia sinensis plant. Although it has been consumed for centuries, tea is currently touted as a beverage that can promote health and help prevent a number of diseases. While scientists are very interested in the potential for tea and its constituents, particularly polyphenols, to prevent diseases, evidence that tea consumption actually prevents chronic disease in humans is limited.
Different Types of Tea Contain Different Mixtures of Polyphenols
There are several major categories of tea, which are distinguished by different processing methods and, consequently, different concentrations of specific tea polyphenols. Fresh tea leaves are rich in polyphenolic compounds known as catechins. When tea leaves are intentionally broken or rolled during processing, catechins become oxidized through the action of polyphenol oxidase enzymes present in the tea leaves. The oxidation of catechins, known as fermentation in the tea industry, causes them to polymerize and to form larger, more complex polyphenols known as theaflavins and thearubigins.
White teas are unfermented teas made from very young tea leaves or buds that are steamed immediately after harvest to inactivate polyphenol oxidase and then dried. Consequently, white teas usually contain higher concentrations of catechins than other teas. Tea leaves that are destined to become green teas are withered by air drying prior to heat inactivation of polyphenol oxidase. Although still rich in catechins, green teas may have slightly lower catechin concentrations than white teas. During the processing of black teas, tea leaves are rolled and allowed to oxidize or ferment fully, resulting in high concentrations of theaflavins and thearubigins and relatively low catechin concentrations. Oolong teas are only partially fermented—they are allowed to oxidize for shorter periods than black teas. Consequently, oolong teas fall between green and black teas with respect to their catechin concentrations. Since different categories of tea contain different amounts of catechins, theaflavins, and thearubigins, it is important to distinguish between the consumption of different categories of tea when examining studies of tea consumption and chronic disease risk.
Tea and Cardiovascular Disease
Coronary heart disease
While epidemiologic studies do not provide conclusive evidence that increased tea consumption offers protection from coronary heart disease, several well-designed studies have demonstrated significant risk reductions in consumers of green and black tea. After almost 6 years of follow up in a study of 4,807 Dutch men and women, those who drank more than 1.6 cups (one cup is equal to 8 fluid ounces) of black tea had a risk of heart attack only 68% that of non-tea drinkers. A study that followed 8,522 Japanese men and women for 12 years found that men who drank at least 4 cups/day of green tea had a risk of death from coronary heart disease that was only 58% that of men who drank 1.3 cups/day or less. However, no protective effect of green tea consumption on coronary heart disease mortality was found in women. In a meta-analysis, statistical techniques are used to pool the results of a number of similar studies in order to provide an overall estimate of the effect or risk of interest. The authors of a recent meta-analysis, based on ten prospective cohort studies and seven case-control studies, concluded that an increase in daily tea consumption of 3 cups decreased the risk of heart attack by 11%.
Stroke
Two prospective cohort studies have observed a protective effect of tea against stroke. One study followed 5,910 Japanese women who neither smoked nor drank alcohol for 4 years. The incidence of stroke was significantly lower in women who consumed at least 1.5 cups/day of green tea. In a study of 552 elderly Dutch men followed for 15 years, the risk of stroke for those who drank more than 2.5 cups/day of black tea was only 31% that of men who drank less than 1.4 cups/day. Two other prospective cohort studies failed to find a significant association between black tea consumption and stroke incidence or mortality, but tea consumption was relatively low in both cohorts.
Endothelial function (blood vessel dilation)
Normal function of the inner lining of blood vessels, known as the endothelium, plays an important role in preventing cardiovascular disease. Atherosclerosis impairs endothelial function, compromising the ability of blood vessels to relax (vasodilate). Endothelium-dependent vasodilation is known to be impaired in heart disease patients and people with elevated cholesterol levels. Two recent clinical trials suggest that black tea consumption helps to restore normal endothelial function. In heart disease patients, endothelium-dependent vasodilation was significantly improved 2 hours after drinking 1.9 cups of black tea or 3.8 cups of black tea daily for 4 weeks, compared to drinking equal quantities of water. Endothelium-dependent vasodilation was not affected 2 hours after 200 mg of caffeine, a dose equivalent to that in 1.9 cups of black tea, suggesting that tea polyphenols rather than caffeine might be responsible for the effect. In patients with elevated cholesterol levels, consumption of 5.3 cups of black tea daily for 4 weeks also improved endothelium-dependent vasodilation.
Cancer
In animals, tea and its components have been found to inhibit the development of cancer or precancerous lesions of the skin, lung, mouth, throat, esophagus, stomach, pancreas, liver, small intestine, colon, bladder, and breast. However, it is not clear whether tea or tea polyphenols will prove to be useful cancer chemopreventive agents in humans. Overall, observational studies on tea consumption and cancer in humans do not support the idea that increased tea consumption is protective. It is not known exactly how the consumption of tea and tea polyphenols inhibits the development of cancer in animals. Although tea polyphenols function as potent antioxidants in the test tube, their protective effects in animals may not be related to their ability to neutralize free radicals. Instead, the anticancer effect of tea polyphenols may be due to the increased activity of enzymes that function to detoxify carcinogens and free radicals and the inhibition of certain signaling pathways that play important roles in promoting the transformation of healthy cells to cancerous cells.
There are several possible reasons for the discrepancies between results from animal cancer models and epidemiologic studies in humans. Aside from potential species differences, it may be difficult for humans drinking tea to reach sufficient plasma and tissue levels of tea polyphenols to realize a protective effect. Although the concentration of tea used in animal studies is usually similar to that consumed by humans, the amount of tea consumed by animals, relative to body weight, is generally much higher than typical human consumption. The bioavailability of tea polyphenols, i.e. the amount absorbed into the blood, transported, and utilized, also appears to be relatively low in humans. Interestingly, a prospective study in Chinese men and women found that increased urinary concentrations of green tea catechins and their metabolites (biomarkers of catechin absorption) were associated with a significantly decreased risk of stomach cancer after adjusting for other risk factors.
Many observational studies of tea consumption in humans were not designed to specifically assess the effect of tea consumption on cancer risk. Some may not have adequately controlled for the effects of socioeconomic and lifestyle factors on cancer risk, while few obtained detailed information about the type and potency of the tea consumed. For example, a case-control study of older Arizona residents found a significant protective effect of hot, black tea consumption and the risk of skin cancer only after adjusting for tea brewing time. Longer brewing times (suggesting higher doses of tea polyphenols) were associated with decreased risk of squamous cell carcinoma of the skin. Yet few epidemiologic studies have recorded brewing time in their assessments of tea consumption. More detailed studies are needed before specific anticancer recommendations for tea consumption can be made.
Osteoporosis
A cross-sectional study of 1,276 older women in the U.K. found that black tea drinkers had significantly higher bone mineral density (BMD) at the lumbar spine and hip than non-drinkers. These findings were independent of other factors known to affect BMD (e.g., smoking and hormone replacement therapy) and whether milk was added to the tea. More recently, a cross-sectional study of 1,037 Taiwanese men and women over the age of 30 found that longer duration of habitual tea consumption (mostly oolong or green tea) was associated with higher BMD in men and women. However, an earlier study designed to examine the effect of caffeine on BMD found black tea consumption to be associated with slightly decreased BMD in 281 perimenopausal women. Two large case-control studies in Mediterranean countries found low tea consumption to be significantly associated with increased risk of hip fractures in men and women over 50 years of age, while a prospective study of more than 80,000 nurses did not find a significant relationship between tea consumption and the risk of hip fracture over a 6-year period. The mechanisms for a beneficial effect of tea consumption on BMD are not clear, although it has been hypothesized that fluoride in tea and/or tea polyphenols may play a role. Tea polyphenols could preserve BMD through estrogenic effects, by decreasing oxidative stress, or by altering gene expression. Further study is required to determine whether tea consumption affects the development of osteoporosis or the risk of osteoporotic fracture in a meaningful way.
Summary
Summary
Although numerous observational studies have examined the relationships between tea consumption and the risks of cardiovascular disease and cancer, there is no conclusive evidence that high intakes of black or green tea are protective in humans. The results of several recent epidemiologic studies indicate that black or green tea consumption may be of modest benefit in preventing myocardial infarction or stroke, while two recent clinical trials suggest that relatively high levels of black tea intake may improve impaired endothelial function. Optimistic results in animal studies warrant continued investigation of the mechanisms by which tea or its components prevent or inhibit the progression of specific types of cancer, but the implications for humans are currently unclear. Evidence that tea consumption reduces the risk of other health problems, such as osteoporosis, is limited but points to the need for well-designed studies that provide more detailed information about individual exposure to the bioactive components of tea, such as catechins and other tea polyphenols. Although the polyphenols in tea are thought to play important roles in conferring potential health benefits, other bioactive components in tea include caffeine, fluoride, and manganese. For more information about tea and its bioactive components, see the Linus Pauling Institute’s Micronutrient Information Center.
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