January 2011 Issue
Hot & Cold — Despite Tea’s Popularity Worldwide, Research on Its Health Benefits Remains Inconsistent
By Densie Webb, PhD, RD
Vol. 13 No. 1 P. 32
People all over the globe have the hots for black and green—tea, that is. But is drinking it doing them any good in terms of preventing chronic disease?
Originating in China about 2,000 years ago, tea is currently the most consumed beverage in the world after water and well ahead of coffee, beer, wine, and carbonated soft drinks.1 It ranks fourth in the United States, with the average American consuming 155 cups per year.1 Of the tea consumed worldwide, 78% is black, which is typically consumed in North America and Europe; 20% is green, which Asian countries favor; and 2% is oolong, commonly consumed in China and Taiwan.2
All tea is brewed from the dried leaves of the Camellia sinensis bush and can be categorized into four types, depending on the level of oxidation: white (nonoxidized buds), green (nonoxidized leaves), oolong (partially oxidized leaves), and black (oxidized leaves). Tea’s manufacturing process is designed to either allow antioxidant compounds present in tea leaves (polyphenols) to be oxidized by naturally occurring polyphenol oxidase enzymes, which are also present, or prevent them from being oxidized. Green tea is manufactured by inactivating the enzymes in the leaves either by firing or steaming.3 The composition of tea varies with climate, horticultural practices, and the variety and age of the leaves.4
Tea polyphenols, the main group of which are flavonoids known as catechins, are key components with many biological functions, including anti-inflammatory, antioxidative, and anticarcinogenic effects.5 All teas (excluding herbal teas) naturally contain between 100 and 300 mg of flavonoids per serving. During the production of black tea, other flavonoids called theaflavins and thearubigins are formed. Because green tea undergoes little processing, most of the catechin content is unchanged and is found in the form of epigallocatechin-3-gallate (EGCG), epigallocatechin, and epicatechin. Among tea catechins, EGCG is the most effective at eliminating free radicals.4
Bottled teas are not equivalent to brewed teas. Recent research presented at the 2010 national meeting of the American Chemical Society in Boston revealed that many 16-oz bottled teas contain fewer polyphenols than one cup of brewed tea.
According to National Health and Nutrition Examination Survey 1999-2002 data, 65.4% of flavonoids consumed by U.S. adults come from tea. Green tea is 20% to 45% polyphenols by weight, of which 60% to 80% are catechins. EGCG is the most prevalent and most researched catechin in tea.1 The catechin content of black tea is only 20% to 30%, whereas theaflavins and thearubigins represent about 10% and 50% to 60% of total flavonoids, respectively.3
The potential health effects of catechins depend not only on the amount consumed but also on their bioavailability. Earlier research published in a 1997 issue of The American Journal of Clinical Nutrition suggested that the addition of milk to tea interferes with catechin absorption. However, several more recent studies, including one published in the Journal of Agricultural and Food Chemistry in 2007, found that milk does not affect catechin absorption.
Tea leaves contain three main components that have metabolic effects: xanthic bases (caffeine and theophylline), essential oils, and polyphenolic compounds. Caffeine acts mainly on the central nervous system. Some effects of caffeine are enhanced by theophylline, which induces psychoactive activity, has a vasodilator effect, and has a much stronger diuretic effect than caffeine. Theophylline also has a relaxation effect on bronchial smooth muscle. Essential oils, known for aiding digestion, are volatile and evaporate from tea during extended brewing time. Green tea has a higher percentage of essential oils than black tea.
Potential Health Benefits
All of the compounds in tea are believed to offer health benefits. According to Chung Yang, PhD, director of the Center for Cancer Prevention Research at Rutgers University, “Tea and cancer prevention is an area that has been most extensively studied for the past 20 years, but there are also interesting results on the prevention of cardiovascular diseases, obesity, metabolic syndrome, diabetes, and neurodegenerative diseases.”
Although most of the research has been conducted with green tea, the overwhelming majority of tea consumed in the United States and the world is black. Research has yet to carefully investigate the health effects of white tea.
Extracts of tea, especially green tea, and tea polyphenols have been shown to inhibit the formation and development of tumors at different organ sites in animals and in cells studied in the laboratory, including skin, lung, esophagus, stomach, liver, small intestine, pancreas, colon, bladder, prostate, and mammary glands. Tea and tea components have also been shown to inhibit carcinogen-induced DNA damage in a number of cell studies.2
Many mechanisms have been proposed for the inhibiting action of tea against cancer cell growth, but polyphenols, especially EGCG, have received the most attention. Tea polyphenols have been shown to inhibit the expression of carcinogen-activating enzymes and increase the levels of enzymes that detoxify carcinogens.2 However, the inhibitory activity of caffeine, which is also present in tea, has also been demonstrated in lung and skin carcinogenesis models.2
Epidemiological reports on tea and cancer, the first of which was published in 1966, have been less promising.2,6 The lack of a preventive effect of tea consumption against cancer observed in many studies may be due to the low quantities of tea consumed. However, a review of epidemiological studies found a small protective association, particularly among never-smokers, between tea consumption and lung cancer.6 Overall, cancer risk reduction is observed more frequently in studies with green tea than in those with black tea, probably because many polyphenols in black tea are poorly absorbed.4 But even polyphenols that are not well absorbed may have preventive effects when they come in direct contact with the digestive tract.
The amount of tea required for cancer-preventive effects has varied widely in studies—from two to three cups of green tea per day to 10 cups per day.4 Human clinical trials are either ongoing or planned to test the cancer-preventive effects of tea preparations.2
In 2005, the FDA denied a petition from a tea company requesting to use a health claim stating the connection between green tea consumption and cancer. In its review of the evidence, the FDA concluded that “there is no credible evidence to support qualified health claims for green tea consumption and a reduced risk of gastric, lung, colon/rectal, esophageal, pancreatic, ovarian, and combined cancers.”
Epidemiological evidence suggests that tea consumption is associated with cardiovascular benefits.7 Much of the research has focused on the catechins present in green tea.4 Catechins exert vascular protective effects by acting as antioxidants, antihypertensives, anti-inflammatories, antiproliferatives, antithrombotic agents, and lipid-lowering agents.3 However, the findings from clinical trials have yielded inconsistent results concerning green tea consumption’s effects on lipid levels, blood pressure, and coronary artery disease.1
Consuming large quantities of green tea—up to 10 cups per day—has been found to decrease serum concentrations of total cholesterol, LDL cholesterol, and triglycerides and increase HDL cholesterol concentrations. Consuming four cups per day resulted in no change in lipid levels.3 A clinical study found that regular daily intake of black tea decreased levels of serum C-reactive protein and increased glutathione levels in people with type 2 diabetes.8 The Boston-Area Health Study found that subjects who drank one or more cups of black tea per day had about one half the risk of a heart attack compared with those who did not drink tea. More recently, a meta-analysis of green and black tea consumption found that people who consumed three or more cups of green or black tea per day had a 21% lower risk of ischemic stroke than those consuming less than one cup per day.9
In 2006, the FDA rejected a petition from tea makers to allow tea labels to claim that green tea reduces the risk of heart disease. However, in its rejection letter, the agency stated that it had “determined that the evidence is supportive, but not conclusive, for this claim.”
Some evidence suggests that tea may also affect glucose metabolism and insulin signaling. Several studies have examined the relationship between tea consumption and the risk of type 2 diabetes, but the findings have been inconsistent.10 Findings have ranged from adults who consumed at least four cups of black tea or at least six cups of green tea per day and had a 30% to 33% lower risk of developing type 2 diabetes than did those who consumed no tea10 to studies that found tea consumption was not associated with the risk of type 2 diabetes at all. A recent meta-analysis of coffee and tea consumption found that people who drank more than three or four cups of tea per day had an 18% lower risk of developing diabetes.11
More recently, a well-controlled study of oolong tea and oolong tea with added polyphenols found that consuming 1.4 L (about five to six cups) of tea per day for five days did not improve glucose metabolism in healthy male volunteers.12 David Baer, PhD, research leader at the USDA Agricultural Research Service and one of the researchers with the study, said, “At this point, for diabetes, the evidence is less convincing than it was just two years ago.”
Several small clinical trials have investigated green tea’s and green tea supplements’ effect on weight loss.1 While some trials suggest benefits from green tea, including reductions in body weight, body mass index, body fat ratio, body fat mass, and waist and hip circumference, none of the studies have demonstrated consistent weight-loss effects. “The effects of green tea are minimal at best,” says Diane L. McKay, PhD, of the Antioxidants Research Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston.
Other Possible Benefits to Tea Time
A longer list of lesser-studied potential benefits to drinking tea include a reduction in oral bacteria that can cause halitosis and dental caries, a possible protective effect against Alzheimer’s and Parkinson’s diseases, protection against the damage of the UV rays from the sun, increased bone density, and improvements in the intestinal microflora.
Is There a Downside to Tea?
A handful of epidemiological studies have found a connection between tea consumption and bladder cancer and an increase in blood pressure. Additionally, drinking extremely hot tea (or any extremely hot beverage) increases the risk of esophageal cancer. Although research has shown that compounds in tea can interfere with drug-metabolizing enzymes, tea is not generally recognized as a food that people need to avoid while taking specific medications. The polyphenols in tea can, however, interfere with the absorption of nonheme iron. “Still,” says Baer, “it’s hard to imagine there are any real harmful effects, given the number of people in the world who drink tea.”
“Compared to sugar-sweetened, calorie-laden beverages like sodas and fruit drinks, either green or black tea is the better choice. They both provide zero calories, contribute to your hydration needs, and boost your intake of health-promoting phytochemicals,” says McKay.
The inconsistent findings regarding tea’s potential health benefits may be due in part to the inability to collect accurate data on the type and amount of tea consumed, including tea cup size, how the tea was prepared, and ingredients added to tea, such as milk, sugar, and honey.
“Together, the evidence suggests that there may be something there to benefit health, but it has just not [been] proven yet,” says Baer. “We still need to figure out just how strong the evidence is and uncover the mechanisms behind any health benefits.”
— Densie Webb, PhD, RD, is a freelance writer, editor, and industry consultant based in Austin, Tex.
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