An Overview of Research on the Potential Health Benefits of Tea
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 have been shown 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 many chronic diseases. Recent research has explored the potential health attributes of tea through studies in humans and 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.
An examination of recently released dietary intake data from the National Health and Nutrition Examination Survey (NHANES) reported that differences in total flavonoid intake among subgroups are principally associated with the percentage of tea consumers and their prevalence of tea consumption.1 Valuable new information has also been recently reported regarding the bioavailability and metabolism of tea flavanols using novel approaches with in vitro digestion models as well as in human studies.2,3 These categories of data are important as individuals consider what foods to include to increase bioavailable and bioactive phytochemicals.
Recent research suggests that tea and tea flavonoids may play important roles in various areas of health and may operate through a number of different mechanisms still being explored. Recent findings about tea and health include:
- A recent review of tea and health in the elderly suggested that there is compelling evidence for the efficacy of tea in benefiting cardiovascular disease.4 The antioxidant properties of tea flavonoids may play a role in reducing the risk of cardiovascular disease by decreasing lipid oxidation,5 reducing the instances of heart attacks and stroke,6,7 and may beneficially impact blood vessel function,8
- Tea flavonoids may lower the risk of certain cancers by inhibiting the oxidative changes in DNA from free radicals and some carcinogens. an important indicator of cardiovascular health. 5 Tea may also promote programmed cell death, or apoptosis,9 5 and inhibit the rate of cell division, thereby decreasing the growth of abnormal cells.
- Tea polyphenols are bioavailable to the brain and can act via antioxidant, iron-chelation, signal transduction modulation, and other mechanisms to effect neuroprotective and/or neurorescue action, with potential implications for age-related dementia, Alzheimer’s and Parkinson’s diseases.10
- A unique tea amino acid, L-theanine ( -glutamylethylamide), plays a role in attentional processing in synergy with caffeine.11
- Tea-drinking has been associated with oral health12 and bone health.13
- Compounds in tea other than flavonoids have been shown to support the human immune system.14
- Due to the substantial data documenting tea’s health benefits, recently published Healthy Beverage Guidelines suggest water, tea and coffee should provide the majority of daily fluid intake.
- Unsweetened Tea (up to eight servings per day) is recommended because it is virtually calorie-free, delivers antioxidant phytonutrients and has less caffeine than coffee (about 40 mg per serving).15
TEA’S ROLE IN CARDIOVASCULAR HEALTH
Human population studies have found that people who regularly consume three or more cups of Black Tea per day have a reduced risk of heart disease and stroke. Clinical studies suggest that the risk reduction associated with Black Tea consumption may be due to improvement in some risk factors for cardiovascular disease, including blood vessel function, platelet function and a reduction in oxidative damage.
While researchers are still examining the various mechanisms by which tea flavonoids function, some studies suggest multifunctional mechanisms, meaning that several mechanisms work in tandem to collectively improve markers for cardiovascular health. Important areas of tea and cardiovascular health research include blood vessel and endothelial function, or the ability of the blood vessels to dilate to allow for proper blood flow, serum cholesterol levels and Low Density Lipoprotein (LDL) cholesterol oxidation. Each of these factors impact the risk of myocardial infarctions (heart attacks), stroke and cardiovascular disease. Study findings in the area of tea and the reduction in cardiovascular disease risk include the following:
- A total of 3,430 men and women aged 30-70 years from the Saudi Coronary Artery Disease Study were examined and 6.3 percent were found to have indications of coronary heart disease (CHD). The researchers found that those who drank more than six cups of tea per day (>480 mL) had significantly lower prevalence of CHD than non-tea drinkers, even after adjustment for risk factors like age and smoking.16
- The Zutphen study, which assessed 805 male subjects over a period of five years, 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. The researchers also found that drinking six or more cups of Black Tea per day was associated with decreased serum cholesterol and triglyceride concentrations. 6 A follow-up to this study found that high intake of flavonoids significantly lowered the risk of stroke in study participants.7
- A Harvard study examined 340 men and women who had suffered heart attacks and compared them to matched control subjects. They found that those who drank a cup or more of Black Tea daily had a 44 percent reduction in the risk of heart attack compared to non-tea drinkers.17
- Another recent Harvard study of 1,900 people found that those who consumed tea during the year prior to a heart attack were up to 44 percent more likely to survive over the three to four years following the event. Those who consumed fewer than 14 cups of tea per week experienced a 28 percent reduced death rate, and those who consumed more than 14 cups of tea per week were found to have a 44 percent reduced death rate, as compared to non-tea drinkers.18
- Dutch researchers assessed 4,807 subjects aged 55 years or older without prior history of heart attack. After a four to seven year follow up period, the researchers determined that those who drank three or more cups of tea per day (375mL) were 43 percent less likely to develop myocardial infarction and 70 percent less likely to die from myocardial infarction than non-tea drinkers.19
- A recent meta-analysis discovered that consumption of three cups of tea per day was associated with an estimated decrease of 11 percent in the incidence of myocardial infarction (heart attack).20
- A large Japanese population study of over 40,000 middle-aged Japanese reported that, among men and women, those who drank just over two cups (about 17 ounces) of Green Tea per day reduced their risk of death from cardiovascular disease by 22 to 33 percent, compared to those who drank less than a half-cup (3.5 ounces) of Green Tea daily.21
- Researchers from the United States Department of Agriculture (USDA) studied the effect of tea on 15 mildly hypercholesterolemic adult participants following a “Step I” type diet moderately low in fat and cholesterol, as described by the American Heart Association and the National Cholesterol Education Program. After three weeks, researchers found that five servings of Black Tea per day reduced LDL (“bad”) cholesterol by 11.1 percent and total cholesterol (TC) by 6.5 percent compared to placebo beverages.22
- The mechanism behind the blood cholesterol lowering effects of tea may be rooted in the effect of theaflavins, through interfering with the formation of dietary mixed micelles, which could result in reduced intestinal cholesterol absorption. Theaflavin-treated micelles/particles were analyzed and theaflavins were shown to have a dose-dependent inhibitory effect on the incorporation of cholesterol into micelles. The primary theaflavin identified for its effects was theaflavin-3-gallate.23