Third International Scientific Symposium on Tea and Human Health: Role of Flavonoids in the Diet
Research Summary
TEA AND HEALTH
-- An Overview of Research on the Potential Health Benefits of Tea --
Introduction
Tea is an ancient beverage steeped in history and romance and loved by many. In fact, so popular is tea that it is the most commonly consumed beverage in the world after water. Although tea had a modest beginning (it was discovered by accident), its popularity spread from its origins in China to Western Europe and the Americas. Throughout history, tea has been believed by many to aid the liver, destroy the typhoid germ, purify the body and preserve mental equilibrium. Over the past few decades, scientists have taken a closer look at the potential health benefits of tea and have discovered that much of the folklore about tea may actually be true.
How Tea Works in the Body
Tea contains flavonoids, naturally occurring compounds that are believed to have antioxidant properties. Antioxidants work to neutralize free radicals, which scientists believe, over time, damage elements in the body, such as genetic material and lipids, and contribute to chronic disease.
Recent research has explored the potential health attributes of tea through studies in humans, in animal models and through in vitro laboratory research. For the most part, studies conducted on green and black tea, which are both from the Camellia sinensis plant, have yielded similar results. One recent scientific paper suggests that tea and tea flavonoids may operate through the following mechanisms (1):
- The antioxidant properties of tea flavonoids may play a role in reducing the risk of cardiovascular disease by decreasing lipid oxidation, and may lower the risk of certain cancers by inhibiting the oxidative changes in DNA brought upon by some carcinogens.
- Tea may trigger Phase I and II metabolic enzymes, which can lead to increased formation of detoxified metabolites of carcinogens.
- Tea may inhibit the rate of cell division, thereby decreasing the growth of abnormal cells.
- ·Tea selectively modifies intestinal bacterial flora that may provide beneficial metabolic actions.
Tea’s Role in Cardiovascular Health
Population studies have shown that tea consumption is associated with the reduction of the risk of heart disease and strokes. For example:
- The Zutphen study, conducted in the Netherlands, assessed 805 male subjects over a period of five years. After correcting for confounding factors, the researchers found that the incidence of fatal and nonfatal first myocardial infarction and mortality from stroke decreased significantly as intake of flavonoids, derived mainly from tea, increased in a dose-dependent manner (2). A follow-up to this study found that high intake of flavonoids significantly lowered the risk of stroke in study participants compared to low intake (3).
- A recent study, conducted by Dr. Howard Sesso et al. at Brigham and Women’s Hospital and Harvard Medical School in Boston and published in the American Journal of Epidemiology, examined 340 men and women who had suffered heart attacks and found that those who drank a cup or more of black tea daily had a 44 percent reduction in heart attack risk compared to non-tea drinkers (4).
- Dutch researchers found that study participants who drank one to two cups of black tea daily had a 46 percent lower risk of severe aortic atherosclerosis, a strong indicator of cardiovascular disease. Those who drank more than four cups of tea a day had a 69 percent lower risk. The study, published in October 1999 in the Archives of Internal Medicine, examined the association of tea intake and the severity of aortic atherosclerosis in 3,454 subjects who were free of cardiovascular disease at baseline (5).
- A recent meta-analysis discovered that the incidence rate of myocardial infarction, or heart attack, had an estimated decrease of 11 percent with consumption of three cups of tea per day (6).
- A study presented at the 2001 Congress of Epidemiology in Toronto found that consumption of black tea was associated with a lower prevalence of cardiovascular disease as well as decreased serum cholesterol and triglyceride concentrations (7).
One preliminary clinical study showed that short-term and long-term consumption of black tea reversed the impairment of endothelial function in subjects with coronary artery disease (8). In the study, function of the endothelium was examined two hours after administration of the first tea or water dose (450 ml or two cups) and again after drinking 900 ml of tea or water per day (four cups per day) for four weeks. Another clinical study found that regular ingestion of tea resulted in a significant and consistent increase in endothelium-dependent and endothelium-independent blood vessel dilation (9). Subjects with mild elevations in serum cholesterol or triglyceride concentrations consumed either five cups of black tea per day for four weeks or hot water. The researchers hypothesized that one mechanism for the apparent beneficial effects of tea on cardiovascular health could be this improved vasodilator function.
Furthermore, in vitro studies have shown that tea flavonoids protect low-density lipoprotein from oxidation, inhibit plasma lipid peroxidation, platelet aggregation and thromboxane formation -- factors that are important for maintaining a healthy circulatory system (10,11). Studies in animals are conflicting and more research is needed to determine if these effects occur in people.
Tea’s Role in Cancer Risk Reduction
Preliminary research suggests that the antioxidants in tea might play a role in human cancer risk reduction. Leading scientists worldwide are actively studying this possibility. Clinical trials and population studies are underway, but more evidence is needed before any definitive conclusions can be drawn. Recent research includes:
- The Iowa Women’s Study, which followed post-menopausal women between the ages of 55 and 69 for eight years, found that participants who drank two or more cups of tea per day had a 32 and 60 percent reduced risk of developing digestive and urinary tract cancers, respectively (12).
- A human intervention trial that examined the effect of treating superficial precancerous lesions (leukoplakia) in the mucosal lining of the mouth with a mixed tea product. After the six-month trial, partial regression of the lesions was observed in 37.9 percent of the group treated with tea as compared to only 10 percent of those treated with a placebo (13).
- Studies comparing groups of mice treated with a tobacco-specific carcinogen and receiving either water or water enriched with tea-derived antioxidants found that the tea-fed mice developed 24 percent fewer lung tumors and the average size of the tumors was 38 percent smaller as compared to the water-fed mice (14,15).
- Experiments that show that administration of green tea, black tea or specific flavonoids in tea inhibited the growth of established nonmalignant and malignant skin tumors in tumor-bearing mice. In addition, oral administration of black tea inhibited DNA synthesis and enhanced cell death (apoptosis) in both nonmalignant and malignant tumors in tumor-bearing mice (16).
- An abstract presented during the 2001 Congress of Epidemiology in Toronto revealed an association between tea consumption and a lower risk of rectal cancer in Moscow women (17).
Tea’s Role in Oral Health
Tea may also contribute to oral health. The flavonoids in tea may inhibit the plaque-forming ability of oral bacteria and the fluoride in tea may support healthy tooth enamel (18, 19).
Tea and Reduced Risk of Kidney Stones
A recent study that followed 81,093 women for five years suggests that consumption of beverages may significantly reduce the risk of developing kidney stones. The study found that for each cup of tea consumed by participants, the risk of developing kidney stones appeared to be lowered by eight percent (20). An earlier study of 45,289 men reported a similar relationship with a 14 percent decreased risk of developing kidney stones after tea consumption (21).
Tea and Reduced Risk of Osteoporosis
Although high caffeine intake is reportedly a risk factor for reduced bone mineral density (BMD), a study published recently in the American Journal of Clinical Nutrition found that older women who drank tea had higher BMD measurements than those who did not drink tea. The researchers concluded that the flavonoids in tea might influence bone mass and that tea drinking may reduce the risk of osteoporosis (22). Recent studies in adolescent (23) and postmenopausal women (24) found no relationship between caffeine intake and bone health.
References:
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2. Hertog MGL, Feskens EJM, Hollman PCH, et al. Dietary antioxidant flavonoids and risk of coronary disease: the Zutphen Elderly Study. Lancet 1993;342:1007-11.
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